Building new short stroke engine

Kvinnhering · Oct 4, 2012

Thanks to all of you who contributed to the discussion and give good advice. I appreciate these responses.

And Steve, it's very interesting to follow your build and it's great that you get your views and choices in this thread. It appears that we are on the right track.

You must excuse me for the bad language, I blame Google Translate. :lol:

acotrel · Oct 4, 2012

@Dances 'a near 50% balance factor is the most kind to the crankshaft and crankcases' should specify across what rev range, it is certainly untrue about a big twin revving at 8,000 RPM ! As I've said - I believe with the commando motor, you must choose what range you intend it to mainly operate over, and set it up accordingly. It is just as good to keep the revs low and work on developing more torque. To move the rev range upwards becomes expensive, and the whole package can end up being less rideable anyway. If you want the experience, just ride a hot Triumph 650 which has has the lot done to it to develop top end power. Short stroke helps you move the rev range higher, and doing it with the Norton head has advantages over the Triumph which uses the same stroke. The main reason you would buy a short stroke crank in the first place is because you intend to fang the motor. The only other reason would have to do with reliability if you intend to ride the bike slowly. A standard commando which is not thrashed is reliable anyway.

acotrel · Oct 4, 2012

Sorry Guys, I think I must be too old for this forum and I don't know why I am still playing with this old garbage anyway. I should have grown out of it years ago. I spent twelve years as a kid in the 60s and 70s trying to figure out how to make the old crap go faster, and between about four of us we had a measure of success. The best way to get a Triumph engine going quicker is to give it a good dose of nitro. That's what our Australian best ever used to get done to it.

Dances with Shrapnel · Oct 4, 2012

acotrel said:
@Dances 'a near 50% balance factor is the most kind to the crankshaft and crankcases' should specify across what rev range, it is certainly untrue about a big twin revving at 8,000 RPM ! As I've said - I believe with the commando motor, you must choose what range you intend it to mainly operate over, and set it up accordingly. It is just as good to keep the revs low and work on developing more torque. To move the rev range upwards becomes expensive, and the whole package can end up being less rideable anyway. If you want the experience, just ride a hot Triumph 650 which has has the lot done to it to develop top end power. Short stroke helps you move the rev range higher, and doing it with the Norton head has advantages over the Triumph which uses the same stroke. The main reason you would buy a short stroke crank in the first place is because you intend to fang the motor. The only other reason would have to do with reliability if you intend to ride the bike slowly. A standard commando which is not thrashed is reliable anyway.

You have presented several topics here, much of which I agree with but I'll focus on the BF.

I think you are mixing apples and oranges here.

The imbalance force F= (mass X tangential velocity squared)/radius So imbalance forces increase with the square of the velocity so everything is greater with speed - kinda obvious eh? Double the speed, then four times the forces; triple the speed then nine times the forces. So at the higher rpm (race speed) as one moves towards a balance factors that create greater bending moments +60%, +70% +80% you are greatly exasperating the amplitude of the bending moments on the crankshaft. Again, see the example bending moments hobot was so kind to find for us.

Your terminology relates to the solid mounted engines where they had to compromise and find a rider comfort happy spot based on the service of the motor (race versus street for example). I agree and understand this but I sense you have made a leap in logic. I cannot find anything from history to support selection of various balance factors for the Norton Twin to optimize engine durability. All I have read and heard has been rider comfort and preventing frame failures. The general skuttlebutt is getting the vibration to vector more horizontal and less vertical as the horizontal was less perceptible.

What you are dealing with are harmonics as a result of the engine and frame and vectoring of the vibrations more horizontal and less vertical. So there's a practical limit to how high a BF you can go; an example would be 100% where most all of the out of balance force vector would be horizontal but the engine would have such a greater imbalance that it would be unrideable and probably shag the frame in unthinkable ways. As an example, a Norton engine with a 78% BF may work in a Seeley Mk2 frame but may not necessarily work as well in say a factory Featherbed. A balance factor that works well in a factroy featherbed may not work so well for say a McIntosh. This has to do with the stiffness and mass distribution about each frame which are different.

What I have been saying is that the near 50% BF for Norton twins is where the engine is least stressed throughout the conceivable rpm range. This is supported by the example moment diagrams as well as the fact that the loads increase with the square of the speed. If you start at a less than ideal BF then those forces will go up with the square of the speed - not a good thing. Have a look at the moment diagrams and see.

It is not a coincidence that the isolastic system allowed Norton to not compromise on BF.

SteveA · Oct 5, 2012

Dances with Shrapnel said:
Steve A and Kvinnhering,

Your on the right track. Although higher compression is a goal to improve mid range torque and efficiency it really becomes critical when you look at cam selections. Less overlap then higher compression not that critical to regain mid range torque, more overlap then more compression required to maintain mid range torque.

A short stroke has the potential to generate more power through 1.) higher rpm (mass flow) for a given mean piston speed, 2.) ability to fit a larger intake valve and to some extent, 3.) lower friction losses, especially with a longer rod to stroke ratio. Although mechanical leverage due to a short crank throw is not conducive to torque, the fact that the bore has increased increases forces to the crank to enhance torque - it is more or less wash.

For any given rpm you will be seeing the same mass flow (air & fuel) so same energy, as a result, for the most part power characteristics will not change much unless you change ports, valves and valve timing events. I run a highly modified 75mm short stroke Norton Seeley and mid range performance is as good as a well sorted out stock stroke race norton but it has another 1,500 rpm for more power.

Bathtub heads are the way to get higher compression for the following reasons:
Provides the most compact and efficient combustion chamber (very important)
Provides quench bands that are easier to set up (flat planes parrallel to deck versus curved surfaces along piston domes)
Reduces reciprocating mass of piston as the piston has less dome mass.
Minimal valve/port flow restrictions if done properly.

Disadvantages of a bathtub shaped combustion chamber are having to weld on the cylinder head which softens the aluminum.

As I say, I think you fellows are tracking just fine on the projects and will enjoy your results as the short stroke engines can be very trackable.

The first think to consider regarding why short stroke? Forgetting my emotional needs to link back to all the works shorts stroke parts I had in the bike in '75, the first point for me is to be able to enter races that are limited to 750cc, CRMC F750, Belgian and other european races at 750cc. If I was only interested in 4 races per weekend with CRMC or BHR I could go for 1007, 93mm stroke (not that I could afford or want that!), I would probably limit myself at 920, basically because that capacity was actually being raced in the late '70s, even if more popular on the dirt in sidecars.

Norton were trying to compete in F750. The road bikes provided a 77mm bore cylinder barrel that even offered greater structural integrity with teh bolt through design, reducing stroke gave them 750cc as required and offered potential beyond the 89mm 750. Big Valve hemispherical heads were simple enough to machine from suitable blanks, pistons were made for them by Omega to get them a compression ratio that also allowed you to push start the bike at the start of the race, pretty important at the time, they could produce custome cams in house and had dyno facilities. When you look at it even as a works team they were running on a privateer budget, and the extra power they developed destroyed bought in parts (Gearboxes) until they found solutions.

Being highly restricted in potential this was as much as they could do with a base road engine as originally dictated by F750, and they clearly had to do more with chassis, frontal area etc. And they did, and like most developments some of it worked, and with riders prepared to ride beyond sensible limits they had some success.

Now, having selected the same route, due to a self imposed capacity limit which allowa a greater choice of races to enter, and to be kinder to an original '75 frame never designed to last this long....And please remember that when starting this project I did not have any doner parts to work with, I am not throwing out long stroke parts, I never had any. When buying new you have a pretty wide choice today from 93, 89, 80.4 and 75, and 360 or 90 offset.

I think both Kvinnhering and I have heard enough about BF and would only want to revisit if our engines proved to have vibration problems in our respective rigid or iso mountings, he has asked, please no more, start another thread if there is really any more to be said on the subject this fall.

It is suggested that care is needed in cam selection and installed timing figures, very interested in that, certainly considering that we have two applications here, a fast road/track bike, and a race bike, solutions should differ, comments welcome.

We have discussed the need to get good if not super high compression, comments on achieving that welcomed.

Now, the head I have is an RH4 and it is going to need some work, I have a sneaky suspicition it has had some welding in one of the combustion chambers in the past, certainly I am considering the big valves that will make the short stroke work better, so some considerable rework will need to go into it, meaning more welding. So, should we try to reduce the combustion chamber from the potential 55cc of a standard RH7?

Matt Spencer · Oct 5, 2012

Somebody somewhere once said " Some feel the Squish Band is More efficent " ( Than the Hemi ) .

4S cam , 10 / 10 1/4 : 1 C.R. , High Comp gets more milage but extremes are small %ages . PROBLEM
is haveing to pull back advance , and be worse than where you started . Inflexability at limit too . . .

The Dreaded Tapered ports for Greater Charge Density / Chamber filling - > Gas Speed Past Valve .

Also the 1 5/8 intakes should go on Standard Valve Seats ?? , matching P.R. 750 size .

Being able to use 9 / 10 of the power 9 / 10 of the time might be better than 10 / 10 t 1 / 10 of the time .

All the fancy infernal components should give good longivity / durability . @ the Std for S.S. 8.000 RPMs .
alledgedly the 4S was run in the Short Strokes . Id be trying my patent exhaust , & Ear Plugs

. My
fairfully rowdy Bonneville wasnt as raucus as a remuffled T Bird triple I was subject too the other day .

Wassel still do the long Mega's , in the 1 3/4 in for a wee snotty revver . Be intresting to compare
Valve timing to the old Symetrical 61 / 62 Tri. T120 figures . Was no impediment to 8.000 + whatsoever .

Somepeople somewhere think " those late square cams ( lobes ) arnt all theyre made out to be " . AS IN
if you arnt at the peak output , theyre just absorbing horsepower , and even if you are they still do so .

Therefore if you elliminate losses before they occur your far faster quicker . ( lighter rods , pistons etc )
Dunno if Id risk ball races , but if pulled down regularly :?: replacements no issue & THEY have less drag
( transmit better H.P.

) so theres another Iota Gain .

AS FOR YOU ; a strict diet , and I belive Stanley Woods did TWENTY MILE RUNS ,Dayly when in Training .

:mrgreen:

SteveA · Oct 5, 2012

The works short stroke camshaft I had was simply stamped TX, when I acquired it it was described as similar to a 4S but with a wider power spread...

The 850 motor it was fitted too neither lacked mid range or top end.

I am personally inclined to think that suggests it was similar to today's PW3, guessing it comes near to PW's claim to have based the PW3 on the best of the works Norton cams updated....

acotrel · Oct 5, 2012

At one stage in the development of the commando engine, a worker was sent out to buy a Triumph racing cam for copying purposes. The timings achievable with the one piece Norton cams are much less radical than recommended for the E3134 race kit cams when used in 500cc and 650cc Triumph engines. Those have long been tried and tested in 81mm stroke racing Triumph engines with 38 inch long separate exhaust pipes. If you are thinking of using a Norton camshaft, and not a Triumph grind and timings, you are probably starting behind the game if you want maximum performance at high revs. I suggest you should read up on the Tiger 100 race kit from about 1954. The cams were used for more than twenty years in race kitted Triumphs, Bonnevilles from 1959 onwards, and late 60s street bonnevilles used E3134 on both inlet and exhaust, the exhaust cam with the points extension having a different number, but same grind.
There have been a number of cam grids which superseded the E3134, but none have really returned much better results in Triumph engines. I suggest you should think long and hard about what you are building. It would be easy to end up with something less than an equivalent capacity Triumph engine. I believe the squish band head is a major advantage . Regardless of everything, when our guys have raced the hottest Triumph 750s against the hot long stroke Norton 750s, the Nortons have usually been better.
If you never intend to give the motor a good thrashing, forget everything I've said.

SteveA · Oct 5, 2012

acotrel said:
At one stage in the development of the commando engine, a worker was sent out to buy a Triumph racing cam for copying purposes. The timings achievable with the one piece Norton cams are much less radical than recommended for the E3134 race kit cams when used in 500cc and 650cc Triumph engines. Those have long been tried and tested in 81mm stroke racing Triumph engines with 38 inch long separate exhaust pipes. If you are thinking of using a Norton camshaft, and not a Triumph grind and timings, you are probably starting behind the game if you want maximum performance at high revs. I suggest you should read up on the Tiger 100 race kit from about 1954. The cams were used for more than twenty years in race kitted Triumphs, Bonnevilles from 1959 onwards, and late 60s street bonnevilles used E3134 on both inlet and exhaust, the exhaust cam with the points extension having a different number, but same grind.
There have been a number of cam grids which superseded the E3134, but none have really returned much better results in Triumph engines. I suggest you should think long and hard about what you are building. It would be easy to end up with something less than an equivalent capacity Triumph engine. I believe the squish band head is a major advantage . Regardless of everything, when our guys have raced the hottest Triumph 750s against the hot long stroke Norton 750s, the Nortons have usually been better.
If you never intend to give the motor a good thrashing, forget everything I've said.

Alan, I find it hard to imagine that the combined intellect and experience behind Norton Cam development in the last 30 or so years has been unable to come up with something that works in a Norton, long or short stroke. There are some fairly rapid Norton saround of various capacities and strokes. Maney has built a lot of short stroke cranks, Nourish has built a few, we have reports of them being raced on this thread....sure you are trading off somehwere compared to along stroke, or I guess the likes of Kenny Cummings would not be sticking with his...but it is a trade, not all loss....

Don't forget that your own motor is 850, even more mid range punch than a long stroke 750....that is indeed what those engines do very well...

To be honest I don't care much about Triumphs! Never enjoyed riding one, never actually wanted one, and whilst there were a few around in the '70s, the Nortons, featherbed, Seeley, Rickman, Dunstall, Jim Lee or Commando were pretty much all faster. There is one rapid Triton being raced within CRMC at the moment, it is a 970 well developed by its owner (if a little scruffy), and there is another, a 650 which has some surprising places, but a very talented rider. More of concern to me will be Weslakes, and I expect to have little answer to these, nor will I worry about that too much, I can't afford one, nor do I want one, and several UK Weslake racers are fond of telling grenade stories....they all seem to have a lot to choose from...

I have tried do make sensible choices about suspension (Maxton) geometry, wheels, tyres and brakes, gearbox!, but in the end it will be down to me to ride it competitively....whatever its actual RWHP or characteristics.

I tried my best to thrash the living daylights out of an '85 GSXR750F over a few race days last year and this....I didn't mind seeing the 10500 on the tacho of that as it went into the rev limiter or the 13000 of the GSXR750 SRAD I also sold this year to help fund my Rickman....best I managed was 3 3rd places in a weekend, albeit a distance from the 2 place bike and in a limited class....

Of course we both want the best we can get from these motors.....

Dances with Shrapnel · Oct 5, 2012

My earlier statement "Bathtub heads are the way to get higher compression for the following reasons:" is a bit too broad and absolute; opinions will vary and one needs to look at the specific application. The pro's and con's I presented are still valid. I don't have any horse in this race but will share my views and experiences.

For a given displacement on a two valve hemi head, as the stroke is shortened, it makes attaining a suitable compression ratio more difficult; it's a matter of geometry; an analog to this would be trying to herd a set number of mice together from a much larger area. Furthermore, although a higher lift/longer duration cam may be a driving factor in wanting to achive a higher compression ratio, the necessary deeper piston valve pockets makes achieving a higher compression ratio with an efficient shape even more difficult - funny how things work against you sometimes. I've not worked with an 80.4mm stroke engine so maybe the factory spherical cylinder head dome and mating piston crown is one of the best solutions. I recall from a discussion with Norm White that he rehemisphered the head symetrical with the barrel and targeted 12.6:1 compression ratio.

Much thought should be given as to how to achieve a given compression ratio. So much effort is typically directed towards higher volumetric efficiency but so much of that effort can be so easily pissed away if it is compressed into a sloppy ineffective combustion chamber. Now when I say combustion chamber I am referring to the shape and volume between the piston crown and cylinder head chamber at TDC. My theoretical conceptualization of an ideal combustion chamber is a sphere with spark ignition in the center. The sphere has the least surface area to rob thermal energy, provides the shortest flame travel and has zero air/fuel residing in a quench area. All the mixture ignites the fastest with least loss of heat due to least surface area.

Generally, welding up the cylinder head should be avoided as it makes for more work and complexity. With a bathtub shaped cylinderhead you are now faced with balancing the cylinder head volumes, softened alloy and unless you have a CNC mill, custom sculpting piston crowns.

Herb Becker has delved into this with two of my race engines; a 75mm stroke 750 at at around 12.6:1 and a 59.6mm stroke 500 at12.36:1. The 750 has the bathtub cylinder head chamber; the most recent version of the 500 uses a dome piston. I recently did a silicone cast of the 500 combustion chamber and it is amazing how the volume is so spread out, even with good quench clearance around the dome of the piston. The effective combustion chamber (non quench) includes the volumes between the valve heads and piston valve pockets and a small trapezoid between the valves leading to the spark plug. It does not look very efficient for ignition propogation and ignition advance that the engine requires supports this. Looking at the silicone model, other than reduction of piston mass, I honestly cannot say whether a bathtub shaped head would be any better.

My sense of the whole thing is that for moderately high compression ratios for the 89mm and 80.4mm strokes one should stick with flat top pistons and work on machining the crowns to provide flat quench and quench between the dome and piston crown as needed. This should all be easier to achieve than working with a bathtub shape. Maybe a rehemisphereing the head would make it even easier as the piston crown machining would now be symetrical.

It would be interesting to hear how those who have raced methanol and taken advantage of the ability to go upwards of 15:1 compression ratio have gone about it.

Not sure if this is all much help as I have been struggling with what's best and the answer is - it depends balanced with practicality.

acotrel · Oct 5, 2012

The only engines I've seen which run methanol at really high comp have been Jawa speedway engines. My brother has one which has a longer stroke t o give 600cc with the same head used on a normal 500 cc motor. The comp ratio must be well over 16 to one. I've seen a 1000cc Triumph Trident motor in a solo which was up on 14 to one on methanol. The owner said he had trouble getting a three cylinder ignition system which would fire reliably at the high comp ratio, it had also pulled the back out of the crankcases. I've always used methanol during racing, but never at a comp higher than 10 to one. The fuel gives better power for two reasons - high latent heat of vaporisation causes a more dense charge, and because you use jets of twice the size, tuning errors are not so critical. Because it never knocks, it is easier on motors, and the heat build-up is less. I believe my 850 motor is quick enough without high compression, and moving t o domed pistons would be a backwards step. In my 500cc Triumph I alkways used 12 to one 650 pistons, and the crown on the side away from the plug was always black. Years ago there was a guy running Triumph engined drag bike here, and he'd managed to get a second plug into each combustion chamber fom the centre of the gap between the rocker boxes. It must have been difficult to know if all four plugs were firing. I've never seriously considered doing a big number on my 850 commando engine and getting more power at higher revs. I always try to get it t o develop even more torque, and concentrate on gearing and handling of the whole package. If I needed something which was really diabolical with a lot of snap to it, I would never try to develop a commando engine to get it. A short stroke nourish weslake four valve engine to suit a 750cc class would be a much better way to go. You start with puma cases, a billet crank, carillo rods, a head shape which allows very high comp, and ports with a much larger valve area. With the short stroke and titanium valves and smart cams, it would probably rev over 10,000 rpm safely. It would be a much cheaper way t o go than trying to make a silk purse out of a sows ear. I love my 850cc Commando engine, it is quick enough in a decent package to win against 4 cylinder 1100cc cb750s. It is built for torque, and with my new 6 speed cr box, it will be superb. It was great with the 4 speed racing box, but now it will be even better.
I know you guys love and believe in Nortons, but the design has limitations, and improving it brings you up against the law of diminishing returns. There was somebody else on here a couple of days ago who uses the same approach as myself - cheap nasty motor, but effective, building on the strengths of the design to give a better package bike, and outride the opposition with the handling advantages. The theory that 'big is better, point and squirt' often doesn't work so well in practice, especially if the race circuits are in any way interesting. A vicious bike can give you a hard time, and make you slower - been there, done that ! All is not as straight forward as it might seem.

The motor in this is 600cc running about 17 to one compression ratio, it has beaten 1000cc vincents on speedway:

http://www.youtube.com/watch?v=XoWlC8wzA7g

acotrel · Oct 5, 2012

There is one thing which can give a good result in hemi head engines, which you might try. If you can find the right size flat top forged piston with a much higher deck height, you can turn the sides of the crown to make an angled upwards squish band around about 80% of the circumference within the combustion chamber. You will end up with the plug a bit closer to the crown, but because there is no dome standing up in the middle the chamber characteristics are better. You even end up with a lighter piston, all things being equal. I once used 350 BSA gold star pistons in a 650cc triumph engine . It ended up on about 10 to one comp, and the motor had wings. You need to make a template of the combustion chamber shape, and use it when turning the sides of the piston crowns to get a tight squish band. It would be worth a trip to the wreckers looking at pistons out of Japanese single cylinder four stroke motors, or buy shorter barrels. If you are using 73mm bore, have a look at Honda Fireblade pistons - teflon coated, super light, really thin rings, and thick top suitable for machining, cost about $100 each, complete and brand new.
Have a look at the squish band in a two valve Jawa speedway engine.

Dances with Shrapnel · Oct 5, 2012

acotrel said:
The only engines I've seen which run methanol at really high comp have been Jawa speedway engines. My brother has one which has a longer stroke t o give 600cc with the same head used on a normal 500 cc motor. The comp ratio must be well over 16 to one. I've seen a 1000cc Triumph Trident motor in a solo which was up on 14 to one on methanol. The owner said he had trouble getting a three cylinder ignition system which would fire reliably at the high comp ratio, it had also pulled the back out of the crankcases. I've always used methanol during racing, but never at a comp higher than 10 to one. The fuel gives better power for two reasons - high latent heat of vaporisation causes a more dense charge, and because you use jets of twice the size, tuning errors are not so critical. Because it never knocks, it is easier on motors, and the heat build-up is less.

Also due to the knock resistance (octane rating) it allows much higher compression ratios. Return on thermal efficiency with greater compression ratios begins to diminish after somehwere around 9-10 or 11:1 compression ratios but power is to be had by much more compact combustion chambers and less ignition timing (if you can achieve the sensible compactness for reasons stated in my previous remark).

acotrel said:
I believe my 850 motor is quick enough without high compression, and moving t o domed pistons would be a backwards step.

Care to expound on that a bit - reasons why you feel it would be backwards?

acotrel said:
I always try to get it t o develop even more torque, and concentrate on gearing and handling of the whole package.

I am past the point of using high HP engines but occasionally I'll get a good rider who is fully tuned up and can use every bit of the higher HP with the six speed gear box. Had some nice successes with it.

acotrel said:
If I needed something which was really diabolical with a lot of snap to it, I would never try to develop a commando engine to get it. A short stroke nourish weslake four valve engine to suit a 750cc class would be a much better way to go. You start with puma cases, a billet crank, carillo rods, a head shape which allows very high comp, and ports with a much larger valve area. With the short stroke and titanium valves and smart cams, it would probably rev over 10,000 rpm safely. It would be a much cheaper way t o go than trying to make a silk purse out of a sows ear.

I would have been able to retire by now if I went that route. My initial choice for the 500 Seeley was a Nourish but I was talked out of it based on availability of Norton parts and Norton expertise. It has been quite the journey so the goal is not always the objective - well that's my sorry ass excuse for getting talked out of the 500 Nourish build. But in all seriousness, we learned a lot along the way. Currently going with a 750 Nourish on the latest Seeley build so that should be fun and I have not ruled out a short stroke version but Dave Nourish will not be building these forever so......

acotrel said:
I love my 850cc Commando engine, it is quick enough in a decent package to win against 4 cylinder 1100cc cb750s. It is built for torque, and with my new 6 speed cr box, it will be superb. It was great with the 4 speed racing box, but now it will be even better.

From personal experience a five speed is plenty adequate for an 89mm stroke Norton although now I recognize where it does come in handy with these motors is on long tracks where wind can be a factor. If you have a handle on where the wind is going to be and when it can save you a gearing change (ex calm in morning and 15 mph head wind in the afternoon). I ran a five speed with the 75mm Seeley and was pretty satisfied with it but on the urging of one rider and wanting to move the Nourish box to another build I opted for a six speed. It takes me some time to get used to it but it's very forgiving on downshifts as the ratio jumps are not that drastic at speed.

acotrel said:
I know you guys love and believe in Nortons, but the design has limitations, and improving it brings you up against the law of diminishing returns.

Love and belief; you are being kind, often times feels like the blight.

acotrel said:
There was somebody else on here a couple of days ago who uses the same approach as myself - cheap nasty motor, but effective, building on the strengths of the design to give a better package bike, and outride the opposition with the handling advantages. The theory that 'big is better, point and squirt' often doesn't work so well in practice, especially if the race circuits are in any way interesting.

I don't believe a well tuned Norton short stroke really gives up much to a standard stroke Norton; with proper tuning you can make comparable mid range torque and have the advantage of extra Hp at higher revs. What oftens suffers on modifications is reliability and the builder not recognizing that when you change one thing and it may adversely effect other things. We currently came to this realization with the 180 degree crank on the 500 and how it really challenged the cam drive.

acotrel · Oct 9, 2012

I've had the discussion about building a short stroke commando engine, with one of my mates on severa l occasions. I am well aware of the theory as to why it might be better. However as I've said before, I believe there is a choice to be made between a high revving top end power motor, and a low revving torquey motor. I raced the 63mm stroke 500cc Triumph engine in a featherbed frame for 12 years. The only reasons I persevered with it, was that it was almost impossible to blow it up, and it had a history of sorts. I sincerely wish I has never bought it and tried to race it. As a kid I was a fairly decent rider, but that bike made me into a real dud. These days I can still ride OK, but I don't crash easily. I don't have the mental attitude to tolerate a bike that is always out to grab me by the throat. The Seeley Norton Commando 850 with a CR box is a brilliant motorcycle. For me it is extremely easily to ride it very quickly. I don't like it on long straights - I always get the uncomfortable feeling it is going to hand grenade. I originally built the bike in about 1978 simply because it was a good thing to build. But I never raced it because of work commitments, and the fact that I simply did not believe in racing such a dangerous motor. In about 2003, I gave it to my mate to ride, as he still had a comp licence, then I raced it myself and won a couple of races. After I fitted the close box and the TZ350 fork yokes, it became rideable.
I can only say that I am bemused by the long stroke 850 motor, realistically it should not be that fast, but it certainly doesn't hang about once you get it rolling. I really wish I'd been aware of how good they are back in 1973 when I was still switched onto racing.
What I don't understand is why anyone would built a 750cc commando motor with the same stroke as a triumph 650 engine. The head is obviously a better design, but what else ? You would be handicapped by the one piece camshaft. I never limit myself to the prescribed factory timings, and I believe that the factories were conservative about what they sold the average idiot - often a compromise - It is worth experimenting, particularly in the case of the commando engine.

Matt Spencer · Oct 9, 2012

Its actually the SAME STROKE as a TRIUMP 500 Engine . The real one . The Pre Unit .

The wee 500 R with real race tune pulled from 5.000 like billyo . 1 1/16 carbs & ports on longer intakes .

NORTON 750 was renouned for " Breathing Efficency " or " DEEP BREATHING " as they put it .
If the induction is set up as per Accrotel & Moir , ( 32 Mk II or 34 wotsit ( works ran 33 ! )
with gas flowed ( NO inhibitants ) manifold , THAT IS contiuous internally to Int. Port .
Int Tapers 32 @ face to the 30.5 before / into bend . The CHARGE DENSITY should allow
maximum effecieny B.M.E.P.
=========

These wee things are wot we need . A Cylinder stuffed rather well and the valve shut rather early .
So the blankety thing can get on with it .

Allowing for the fact that the infernal components and cases are upgraded :?: :!:

BAR uncalled for wear for exceeding comfort zones ( usual on race engines ) AND if the Oiling System is updated ( some would say adequate allredy :!: )
My thoughts are 10.P.S.I. per 1.000 rpm's , SO , ' we ' need EIGHTY FIVE P.S.I. minimum . HOT . preferably NINETY P.S.I. at the feasable 8.500 D.N.E. RPMs

Which may call for a rather more capacious Oil Pump . Drilling of Oilways OverSize ( 5 mm or 3/16 Min. in real money )

AQllowing for the fact that we may well use the full 8.500 , and if we expect to produce as much output there as at 8.000 RPMs ,
' we ' may well want to go to 36 or even 40 mm at the Carburators , and perhaps comesurate porting / valves / manifolds .

Needle Roller Bearings throughout if possible . ( drag reduction ) perhaps the notorious ' Closer ' springs of ferrari on Cam followers & perhaps Rockers .
ENSUREING these components ' follow ' = stay in contact . rather than start jumping and danceing across the countryside , haveing escaped . :shock:

:lol:

TOP END lubrication would need thorough anaylisis . As to maintain Flow Rates . a metering jet , obviously . :?: Still even distribution to Ea. Rocker Arm required .
And internal pollishing for drainage . And Air Flow investigateing, breatherwise .

One top pommy piston Manufcturer . States " surpriseingly , Two Stroke pistons last longer ( RACEING ) than Four Stroke ; AS they are CUSHIONED at BOTH ENDS of the STROKE "
Four Stroke only being Cushioned ( pnematically ) at the top . ( Unless its a Pre Unit 360 Degree Triumph

* )

* Therefore , if youre prepared to configure for the Pressure Fluctuations ( External Oiling

:lol: 8) ( Rust Proofing :wink:

) )
The Smoothening / ness , of action , may be fundamental to the satisfactory functioning of the apparutus at these rates of revolution
LIKE A PRE UNIT BONNEVILLE .

:lol: 8) AHEM . Valve Float at 8.500 or thereabouts , depending on temperatures & conditions .
This is a intermittant reading ( temporary , though not through mechanical failure . Though Temperature Soak is probably relevant .)

NOW , according to Bruce , or whoever it was , CASTOR OIL is the ONLY LUBRICANT that RUNS TOO Heat , all the rest Diverge from thereabouts .
This explains why the T100 R ( Pre Unit :wink: Same Stroke

) ran like shit on pump gas & mineral oil

:!: )Check model aircraft garbage to verify this if required /
" Properties of lubricants " .

SO , there we are . Be telling you how to do the exhaust next . :shock: wasnt as obnoxious as the rackety muffler 95 Triple . 1-1 left haners eco , 2-1 Rt . :lol:

ALSO , ' I Belive ' our intrepid Drag Racer / development rider Rawlins ultimate stage f development on his 850 was 4 S Cam , 36 carbs ( Mk 1 ) and Short Stroke P.R.( Hemi
or Squish Band ( 750 P.R. head on 850 Bore for Squish ?? :?: ) , obviously Big Valve ( if you call those Big , bigger'n standard , anyway . This Spec warrants investigation .

ALSO on Std. Pipe Dias , WHAT effect did the Interstate ( LENGTH ) derive . Something , obviously , I should think . Didnt have any Rockets Fitted ! :?

http://thevintagent.blogspot.com.au/200 ... les-1.html

Kvinnhering · Nov 14, 2014

You must have a few more km on it now that summer is over......how is it going?

And what is the long winter plan?

SteveA

Hi Steve.
The bike is going very well. Coming on cam from 4500 up to 7000 rpm. It's also have good torque. I'll take it on the dyno bench to the spring and get it fine tuned and verify power and torque.
In winter I will mount FA head with larger inlet valves and Jim will also clean up the ports. I will also mount MSD ignition system and head steady from him. Carburetors will be replaced of PWK 34mm. I hope the gearbox will take the torque :?

I have fuel/air ratio sensor on each cylinder connected to two instrument. These will be replaced with wideband type for more accurate measurements. I use these is to adjust the carbs one the road.
I hope to change the first gear to the standard ratio. The first gear is to high ratio on close ratio cluster from RGM for my taste.

A case in the end, the engine will not idle below 1000-1100 RPM !! Is this normal for short stroke ??

Fast Eddie · Nov 14, 2014

Kvinnhering said:
You must have a few more km on it now that summer is over......how is it going?

And what is the long winter plan?

SteveA

Hi Steve.
The bike is going very well. Coming on cam from 4500 up to 7000 rpm. It's also have good torque. I'll take it on the dyno bench to the spring and get it fine tuned and verify power and torque.
In winter I will mount FA head with larger inlet valves and Jim will also clean up the ports. I will also mount MSD ignition system and head steady from him. Carburetors will be replaced of PWK 34mm. I hope the gearbox will take the torque :?

I have fuel/air ratio sensor on each cylinder connected to two instrument. These will be replaced with wideband type for more accurate measurements. I use these is to adjust the carbs one the road.
I hope to change the first gear to the standard ratio. The first gear is to high ratio on close ratio cluster from RGM for my taste.

A case in the end, the engine will not idle below 1000-1100 RPM !! Is this normal for short stroke ??

Why are you going for the big valves on a 750? Is it because its a short stroke?

Rohan · Nov 14, 2014

acotrel said:
I believe my 850 motor is quick enough without high compression, and moving to domed pistons would be a backwards step.

Folks should know by now that the 850 motors don't achieve higher compressions with domed pistons.
Nor did the Combat 750...

On petrol, anyway.

acotrel · Nov 14, 2014

Triumph motors use domed pistons and so do Jawa Speedway motors. I always have my doubts about flow through the combustion chamber around TDC when both valves are open. It is the same reservation I have about oval shaped Hi-comp combustion chambers.

A couple of years ago I saw a short stroke 500cc Nourish engine Seeley at Phillip island which the owner said was nasty'. The Triumph T100 Daytona racer seems to have been successful. In my own Triumph based 63mm stroke 500, I was still using the long rods, and I believe that was the reason the motor was all top end and poor midrange power. I'm glad I sold it, it was just bloody nasty - too hard to ride well and easy to crash.

Kvinnhering · Nov 14, 2014

Fast Eddie said:
Kvinnhering said:

You must have a few more km on it now that summer is
Why are you going for the big valves on a 750? Is it because its a short stroke?

Click to expand...

Yes. Factory short stroke also used the larger intake valves (some told me). Jim's C solution is to use larger intake valves, but with unchanged diameter of the inlet channel! This leads to higher flow and higher air velocity.

Building new short stroke engine

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