should i fit a big valve head?????

Ok Jim, turn about is fair play - so I can't make much sense of your summary - but that to get the most out of bigger valves other things need to be juggled, which still leaves me thinking bigger valves [w/o anything else done] can only help some but not hurt power in street level engine and usage. So what do ya predict bigger valves would do in dragster engine 920 Peel with hogged out Combat head w/o the belt on Drouin? i think with a longer belt I can rig up an on/off belt engagement like Mad Max movie showed. What would be your advice on the valve size effect of the original poster?

I thought Jim's summary was pretty simple to understand, but let me attempt to rewrite it in Hobot langauge:

"Big valve power spreads to gravel flying and kids crying with no change to port, full snort in every galaxy. Hold on till it really hits a 4 k then rear wheel spin rolls up asphalt until the end of the road smacks you down, on and on you go past ballon tired corner cripples and triple A fuel dragsters until the speed of light becomes a hindrance to pull you down, swallows both you and your bike only to catapult both into a much faster but parallel universe."

After reading this thru again I think I have to have one, a want has become a need. :mrgreen:

Glen

I know all the theories about short stroke, dramatic cams, huge ports and billet cranks and stronger cases, and I've raced a bike with the worst kind of top end motor for 12 years. My Seeley 850 is set up to pull like a train, and I always use a close ratio box with it. I've ridden quite a few racing bikes however this one is exceptional. It spins up extremely quickly on methanol and from experience I know it is fast enough to win races, and it is extremely easy to ride. It's only drawback is that it is difficult to gear properly. It is already very high geared, and taking it to a really big circuit like Phillip Island is probably not an option. I wouldn't get the most out of it and when it ran out of puff at the ends of the straights, it would be very frustrating. I have finally found 23 and 24 tooth engine sprockets (in the mail) which will make it OK for short circuits, however I only have one extra gear box sprocket one tooth bigger, and the rear sprocket is already very small. I still haven't used the 6 speed box with that very high overall gearing. I suggest that one of the most important things about the cylinder head is the way the valve seats are cut and matched to the inlet port etc. My inlet ports are tapered from 34mm at the carb back to the standard 30mm, and I would not change that. Anything which increases the need to use the top end of the rev range makes the motor more likely to destroy itself. My motor often sees 7,500 RPM on gear changes and I don't like that. Using the right crankshaft balance factor is critical if you don't want the big bang.

Well Glen what's fantasy to most was real life to hobot on Ms Peel wiping sneers off sports riders where and when it mattered most, handling turns accelerating on whimpy mere 750 Combat power. Poor things wisely had to slow way down for turns and then wheelied to easy to beat Peel into an entry up to 90. Trying hard as I could on moderns I could not pull off the G's I'm addicted to on Peel so gave up on selling her off to get a balloon tire buzzy corner cripple, even if they can beat Peel in IOM and Dayton tracts, its only the relaxing straights they'd get to show off hp over torque advantage. Catch on up to me till what Jim wrote may make as much sense as does to me, which is bigger valves help any engine across the range but not that much until other things improved too and then run up to level they come into play. BTW to pull off what I describe on remarkable Combat power road work means you don't allow rev's to get below 5500, so only 2000 rpm spread to get the job done in time. Peel with standard port head un milled with 2-1 megaphone 1st shocked the shit out of me in my drive way snapping throttle about 1800 rpm to end up on tail lens, couldn't believe I wasn't already hanging on well enough, so no grinning at that point -just pissed me off thinking it'd been caught off guard so hunkered down with white knuckled locked arms to try it again to be even More Shit Shocked as still ended up on reverse slope of seat - just short of tail light mount that time. Then "THE Grin" touched my pined back ears so tried it a 3rd time a bit smoother on throttle to stay in seat and the rest is history. Sorry if your C'do ain't making you as crazy as me.

Here's some head details to entertain from the recently late Ken Augustine of World's Straightest Commando fame after I quizzed him on Combat head issues, later might post Ken's race pilots describing same shock not able to stay in seat in low gear snaps........


H,

Take a small port head, do the Hemmings mod on the intake and that should eat all the clutches you want. I have done that modification and know what it entails but don't remember the exact angle or diameter involved. The part that I don't like about the Hemmings mod is that the inlet valve guide bore needs to be increased to .625" diameter to clean up on the original .500" guide bore when the inlet guide angle is straightened up. In more than just my opinion, the Norton head casting doesn't have enough material to properly support the larger diameter guide and the guide should be made smaller to give a higher caliber fitted ratio (length/dia). Unfortunately, welding the guide bore down inside the valve spring region is very difficult and the necessarily used , porous castings are full of oil. After any welding is done, the guide location needs to be determined, the hole drilled, bored or reamed then roller or ball burnished or the guide will loosen and fall out. The valve seats need to be removed, the odd squish band on the back of the combustion chamber filled, the combustion chamber resphered, head gasket surface recut, valve guides made, installed and fitted to new valves which need to be made, the valve seat bores need to be recut from the guide centers, valve seats and installation tools made then installed, seats cut, ports blended, springs shimmed and installed and pistons be repocketed.

Possibly Hemmings will make one for you or you can find a factory short stroke 750 head which comes that way. You will still need to repocket or find pistons but the original valve train geometry is for a 250cc cylinder from the Model 88. That is why the 750cc head has the odd crescent in the back of the combustion chamber. The 750cc head had to be moved forward or the pushrod tube bores would have encroached on the cylinder bores and the rocker geometry moved with the head but was not corrected until possibly the short stroke 750. Possibly but only possibly.

I have my own angles, locations and linear plus radius dimensions in a head which is almost done but I have $12,000 worth of time in it and am not giving it away. However, it is for sale at that price as I need to do other things and haven't time to build a decent short stroke bottom end for a Norton.

I hope all that helps but don't forget that the Commando chassis is a mess and needs work first or it will wobble or weave more badly than they normally do with the stock power. The Featherbed frame, of course, is worse.


KA

Wow, we had a hobot- now we got a glenbot too. :shock:

I suggest that if you want a performance boost, the first thing to try is raising the gearing. If I had a road bike I'd buy a racing close ratio gear cluster and fit the standard commando first gear pair to it. Then raise the overall gearing by dropping a tooth on the rear sprocket. I tried racing with the standard commando gear box and it is useless, no gear change was ever smooth enough, and it was too slow coming up through the gears. With the arrangement I've suggested use the low first gear to get the bike mobile, then never go below second gear unless the bike is stationary.

Glenbot,
' the odd squish band on the back of the combustion chamber filled, the combustion chamber resphered, head gasket surface recut, valve guides made, installed and fitted to new valves which need to be made, the valve seat bores need to be recut from the guide centers, valve seats and installation tools made then installed, seats cut, ports blended, springs shimmed and installed and pistons be repocketed.'

Why are Triumph twin motors of the same capacity as Norton commando motors usually slower ? And another question - why wasn't the short stroke commando motor faster than the long stroke motor? Have you ever heard of someone tuning a bike to a standstill ?

I know, that of all the modifications I have done to a Norton motor except raising the compression ratio, increasing the intake valve size was the one that required the least amount of tuning and messing around to get it to work.

It does require checking and modifying piston valve reliefs and usually requires jetting changes. It works well with the stock cam and exhaust system. It does not change the original rpm range of the engine. Jim

I got a very good result by recutting the valve seats to the three angles in my short stroke triumph. Then stuffed it by using cast iron valve guides. A small piece if the guide came away and went through the motor. I noticed it slow, and started looking at everything else to find the reason. After about 5 race meetings, I finally noticed the mark on the piston and found the dent in the valve seat. Looking down the inlet port I could not see where the piece had broken away.

comnoz said:

Wow, we had a hobot- now we got a glenbot too. :shock:

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It's early days with this and my head hurts just from that little attempt. Might try again someday, I got away with just a gentle Cosmic Spanking from The Hobot. :mrgreen:

Glen(bot)

The rest of the bike is ok brakes gearbox etc. all rebuilt engine an swinging arm centered in frame iso mounts all lined up wm3 rims set central with 2 laser levels to frame an each other. It doesn't weave no matter what happens an after riding modern sport bikes I must say I am impress by the handling if not the brakes. On Monday me and a mate (he likes triumphs foolish fella ) are having a lads day out first stop Dave Norrish (for one of mate triumphs cranks) then more importantly from my point of view to Mike Hemmings to drop off my engine parts and take his advise as to what should work best. I don't want to raise the rpm but do want a motor with guts an pulling power. I plan on fitting a Maney outrigger even though it will mean cutting a piece out of the Alton Ekit inner primary cover but I will just have to take a depth breath an keep telling myself its a good idea as I cut into it (nervously). It looks like 2014 is going to start off a bit costly but we all have to have a hobby don't we

acotrel said:

Using the right crankshaft balance factor is critical if you don't want the big bang.

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You keep repeating this myth ?

The balance factor employed in the motor is largely to keep the rider comfortable.
Within reason, it makes very little difference to the functioning or reliability of the engine.

With a large heavy flywheel, and an out-of-balance weight to keep the vibes tolerable (to the rider)
the bearing loads actually INCREASE (slightly) as higher balance factors are employed.

Quote one source which says anything different ???
But we diverge. As usual....

Toppy wrote "....I am impressed by the handling if not the brakes"



Toppy, this 320mm disc kit will give you more brake than you could ever use. The kit includes a new fork bottom, so no untidy spacer bracket. Also a very light six pot caliper and correctly sized Master cylinder that fits the stock pre MK 3 switchgear. I made some changes to things and managed to fit the kit to my MK3
Madass here on the site makes these, price is good too.

It has a master cylinder an hi friction pads and rear shoes. I have a 13 inch disc to fit an will be buying the large bore AP Lockheed calipar to go with it an keep the hydraulic advantage of sleeved master cylinder. There are lots of other mods here an there to but am trying to keep it looking as stock as possible as I like the black roadster look (yes I know its a common colour) so I was hoping not to change the standard disc an calipar but I think stopping is more important after trying the standard kit. Hopefully the 13 inch disc an big bore AP Lockheed calipar will do the job but still look like a "period" after market mod.

toppy said:

It has a master cylinder an hi friction pads and rear shoes. I have a 13 inch disc to fit an will be buying the large bore AP Lockheed calipar to go with it an keep the hydraulic advantage of sleeved master cylinder. There are lots of other mods here an there to but am trying to keep it looking as stock as possible as I like the black roadster look (yes I know its a common colour) so I was hoping not to change the standard disc an calipar but I think stopping is more important after trying the standard kit. Hopefully the 13 inch disc an big bore AP Lockheed calipar will do the job but still look like a "period" after market mod.

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The biggest single factor for an effective front disc brake is the ratio between the master cylinder and caliper. According to Vintage Brake, the optimal ratio is 27:1, caliper area to master cylinder are for a single disc. I have that ratio with a Grimeca master and Grimeca caliper and a 12" floater. Great stopping power. Brand of component make little difference, but a ratio much different than the one Vintage Brake recommend will make even the most expensive brake components perform poorly. That's the problem with the stock setup. I think the ratio is around 9:1. You just can't get enough force on the caliper with that ratio.

You can make the stock setup easy to apply and even make it quite powerful for an initial hard stop or maybe two, then it needs time to cool.
This is OK for some riding but does not cut it for fast riding in the mountains, the tiny stock pads superheat (Vintage brake or other) and render the brake useless at very inopportune times.

The brake in the photo will not overheat in those same conditions.

Norton Lockheed factory ratio is 17:1, 13 mm ~19:, 12mm ~21-22. Moderns 25-27.
With the restrictor hole punched out at least the hi grip effort can over come the tire definitively below 60 and squeal dam loud above that. I've tried to over heat factory brake in Mt. road but only time I induced fade was d/t some air layer on parts which just leaving lever pinned back over an extra night solved that for full faith and credit in factory set up as harsh as I'll ever need it. I do like the bigger lighter brakes for a higher performance Commando so Peel is mimicking the big single rotor sets up shown and sold. Still I mainly expect like sprung and spun mass to be the main benefit I'll feel.

Roadholders gave Norton the Unapproachable label and isloastics are ingenious set up but for gosh sakes even the elite moderns get wobble/weave onset when pressed on even with fork dampers so don't think you have pressed your Commando [or moderns] to know just how close you may be getting to THE Big Surprise. There is a reason un-tamed isolastics only raced in the essentially pure stock classes and only one doing well in higher classes against solid Seeley's has three point robust swash plates to tame the isolastic/frame rebounding. It took me a while before I ran into it but now I'm familiar with un-tammed Commando's I am weary not to fast power through unknown turns even sweet looking mild sweepers. You sound like a hot rodder so enjoy yourself but THE Hinge is lurking. Try some low air practice to induce wobble and imprint on that sense of control reversing to recover. If surface good and turn banked a bit and wind calm you can lay over a stocker and nail it to step out the rear w/o much bother. I've done it up to 80 on stocker in a freeway like section but its risky to keep smoothly upping speed thinking there ain't a transition awaiting near apex and extra shocker is letting off increases the flap as well as speeding up, ugh.

worntorn said:

You can make the stock setup easy to apply and even make it quite powerful for an initial hard stop or maybe two, then it needs time to cool.
This is OK for some riding but does not cut it for fast riding in the mountains, the tiny stock pads superheat (Vintage brake or other) and render the brake useless at very inopportune times.

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Boy you got that one right. As I learned the hard way on Monarch pass. Jim

I stand corrected. I just measured a stock caliper. The bore is 44mm. This gives a ratio of 15.36:1, with the master cylinder bore being a stock 15.9mm. This is still far from the optimal 27:1 ratio. Removing any restriction between the master cylinder and the caliper will not change the bore size difference between master cylinder and caliper. The pressure applied to the caliper is determined by the ratio, all other things being equal.

worntorn said:

Toppy wrote "....I am impressed by the handling if not the brakes"



Toppy, this 320mm disc kit will give you more brake than you could ever use. The kit includes a new fork bottom, so no untidy spacer bracket. Also a very light six pot caliper and correctly sized Master cylinder that fits the stock pre MK 3 switchgear. I made some changes to things and managed to fit the kit to my MK3
Madass here on the site makes these, price is good too.

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Doesn't look that untidy to me. I like how the centers look like old hot rod wheels.