Short Stroke ~ High Compression

hobot said:

Gary do you also have to compensate for the cam and rocker geometry shift to center both bores? Hope it works are sweet.

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Steve,
I've been messing about with the push rod lengths for other reasons, but I don't think moving the head on the barrels by 0.030" or so will change the angle of the pushrods by more then 1/2 a degree, so I'm not expecting any noticeable effect.

Gary

Alrighty Gray that is so refreshing to hear when messing around customizing. I learned more geometry from your posts that I thought I wanted to know. While dealing with this seam area might consider counter sunk groove for copper wire mash sealing.

Here's an update on my cylinder head misalignment problem: -

First I made a couple of "pucks", stepped in diameter so one end is a snug fit in the cylinder bore and the other in the cylinder head counterbore: -



Relying on just the pucks to align the barrels and the head (all head studs and bolts removed) you can see in the next picture how the mounting hole patterns in the head and barrels are misaligned: -



The 21/64" drill bit is sitting in the threaded hole in the barrels and the misalignment is such that a 3/8" head bolt entered into this hole will not engage the thread in the barrels even though this hole has already been enlarged by 1/32"

Next I made a pair of brass bushes to fit in the counterbores of two of the barrel to crankcase socket head cap screws: -



With the pucks providing alignment the brass bushes locate the business end of a long 3/8" dill as I drill into the head from the underside of the barrels: -



The new 3/8" holes in the head are used to locate a couple of stepped brass plugs ( 3/8" x 5/8") with the 5/8" end locating in the fastener counterbores in the barrels above the heads of the socket head cap screws: -



Thus, hopefully, when head and barrels are assembled they are aligned as if the pucks were still in place. I'll still have to open up some of the fastener holes in the head and barrel a little bit more to allow them to be installed.

'Pistons and the loss of a bit of compression are cheaper than a Full Auto Head and another set of barrels....just avoid sharp edges and heat spots....of course the squish performance will potentially be lost too....'

I would make the same comment. In the past I fitted pistons from different bikes into Triumph motors. It is easy to make a mount with a draw bolt to hold the piston in a lathe. It would be only under extreme circumstances when I would machine the head or barrel. To my mind pistons are consumables, the major throw-away items in a race engine. It might be a pain if you have to machine every piston that you change, however machining the head or barrel can be irreversible and they are expensive to replace.

Re: High Compression Pistons

I have just experienced the same problem with damage to both JS Hi-Comp pistons using a Fullauto head. I am waiting to hear back from Jim regarding a solution but no doubt it will involve greater clearances and therefore lower compression!

Re: High Compression Pistons

bluepaul11 said:

I have just experienced the same problem with damage to both JS Hi-Comp pistons using a Fullauto head. I am waiting to hear back from Jim regarding a solution but no doubt it will involve greater clearances and therefore lower compression!

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I have a Full Auto head and JS High comp pistons...so I am interested....

Mine is 77.5mm bore and 80.4 Stroke, my first dry build did not indicate problems, I have skimmed 40thou off and had teh pistons modified as identified in Jim's drawing. I realy don't anticipate a problem with this setup, so I am really interested to know the root of your issues.

Are you using a larger bore?

Hi Steve.
I have my Short Stroke on road now and it also have 77.5mm bore and skimmed 40thou off RH4 head and 40tho thick coppergasket. I also had the pistons modified and have now 9.33:1in comp. Only issue was clearing between top of piston and head. Thats the reason for 9.33:1.

Yes I did with 10:1 pistons from JS, I carefully had the squish machined in both hemispheres without changing the greater diameter of either one and without going deeper into the existing squish area. As you say, almost as if the head was set up wrong in the original factory jig.
This was also on a 1973 RH10 head, I will look for pics , although I think I only have some afters not befores
Regards Mike

Kvinnhering said:

Hi Steve.
I have my Short Stroke on road now and it also have 77.5mm bore and skimmed 40thou off RH4 head and 40tho thick coppergasket. I also had the pistons modified and have now 9.33:1in comp. Only issue was clearing between top of piston and head. Thats the reason for 9.33:1.

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Understood Jan, and goodnews you are riding it

9.3 is a good ratio for a road bike. I need nearer 10 plus, with the high compression pistons and 40 thou of Jim calculates 10.3.....with .045 squish....

But I seem to recall you had isues with combustion chamber sizes?

Steve

Hi Steve.
You are quite right Steve. There was a difference of 0.4 cm3 or 0.07 in the differences in compression ratio between the cylinders.

Unfortunately / fortunately, so is there a difference (in my Engine) in how much pistons protruding over the top of the cylinder. The difference is 0.08 mm and this represents 0.38 cm3. Fortunately for me it was the right cylinder relative to the cylinder head and the two equalized each other almost.

So check length tolerances on the connecting rods and the height from pin hole to the top of the pistons. And assemble them so that they are as similar as possible when they are assembled.

It is also possible crank plays a role here.

Many people find that this is splitting hairs, but this is me and is a part of my hobby. :lol:

Peel is 'stuck' with 89mm stroke and searching long vs short stroke for boost & came across this which I want to believe... I see conflicting reports of speed/acceleration of piston decent in long/short strokes on ability to get intake going, generally see claims that short stroke jerk piston down faster so draws in better but tested physics show longer strokes jerk/accelerate pistons harsher per rpm than shorter stroke. These guys were discussing car V8's with 4"+ stroke and bores so longer stroke and Nortons.

With that huge stroke I would probably run lots of duration/overlap/lift actually for three reasons. First off the stroke would desensitize the engine to the big cam, second off the overlap would probably be a huge benefit to feeding the cubes with that teensy bore and lastly high lift would help deshroud the valves to the bore.

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Ain't found better thread to store this detailed calculated tested factors in bore stroke vs displacement and rpm down to percentage to expect this way on that. A treatise to study at length.

http://www.strokerengine.com/StrokedEngines.html

It pretty much goes like this: For a given cam duration, you'll get the same piston speed, regardless of stroke.
If you halved the stroke, you'd get twice the rpm and the same piston speed.

There would be much more stress on the engine with 1/2 the stroke and double rpm. Stress is proportional to the square of rpm, but only directly proportional to stroke, so the short stroke engine would have approximately double the stress at double the rpm.

Do not comprehind the cam durration part of the rpm stress, but do get the longer strroke increasing both rpm piston jerk stress and increased stroke friction stress at same rpm so short stroke Norton can deliver more power - mainly d/t less friction, as reports say they did not rev higher than potent longer strokes nor have any bigger bores or valves. For same displacement short stroke should beat a long stroke yet bigger engine displacement in same size package may make more power, up to its rpm friction loss level. Maybe my P!! was a short stroke as marked for 9k redline and did not have a power drop off sense as could float front level till after 6000 in 4th when air friction dulled the pull so front settled down to finish top out, which I never ever did. Plugged in mass & known 1/4m e.t.'s before me to get almost 80 hp result which agree's with Axtell and Woods type engines reports of same era. It was geared fairly high for 150 mph potential [I never dared reach] but had more down low grunt than my factory Combat or Peel Combat geared rather lower. I would ride it in 4th hand off throttle idle 600 rpm while plowing through deep sugar sand ruts throwing a wake of sand like a boat at about 15 mph yet could snap throttle to instantly rooster tail like a hill climber. It had open dual low headers which is another mystery factor for such impressive off idle response. P!! was a '68 so why can't we do similar or better now on Amals no less? If it wasn't built to sit 2" low in front its power would been un-applicable d/t wheelies even on street tire.

if the cylinder bores are not exactly at right angles to the centre line of the crank, you might have a friction problem even though the gudgeon pins can slide sideways.

X-file said:

There would be much more stress on the engine with 1/2 the stroke and double rpm. Stress is proportional to the square of rpm, but only directly proportional to stroke, so the short stroke engine would have approximately double the stress at double the rpm.

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The above assertion can be misleading.

In the context of Norton twins, in practice, if we are running a 75mm stroke the redline will be generally governed by the maximum recommended mean piston speed. So as a guide, if an 89mm redlines a 7,000 rpm, a 75mm would redline at around 8,310 rpm. Really the same maximum stress but roughly 19% greater rpm thus 19% greater mass flow. You also benefit from being able to fit larger valves in for better breathing.

Dang it Dances that implies Norton management stiffed us retaining the long in tooth strokes. The popular short cut way to view piston parting risk is going by piston speed but is not the crux of the piston-rpm issue > Acceleration Jerks at TDC & BDC. Shifting bores ahead of the crank axis also eases jerk down acceleration some. Ring flutter could still drop power w/o immediate piston failure. Alas its not piston rpm tolerance that limits Commandos its the jump rope crank shaft so if shorter stroke, besides less piston jerk there'd be less jerk to flex crank to bind rod shells too. There are charts that class piston speed ranges of various race engine classes but depressing Norton wise to view it.

https://www.google.com/?gws_rd=ssl#q=pi ... calculator

hobot said:

Alas its not piston rpm tolerance that limits Commandos its the jump rope crank shaft

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Who says that ??

Roadtests of old used to note what rpms valve float set in,
and Nortons weren't immune from this - going back in history.

Terrys (makers of valve springs) recommended that valve springs were replaced after every 2 race meets at least,
for optimum performance. And they were the biggest. And used by Nortons ??

On Manxes, this was easy, the valves springs (hairpins) being external !

And piston accelerations would have to have been a factor ?

We are talking a special engine in this thread. Valve trains are common-straight forward to upgrade not to loose control before something bigger goes wrong. We have piston/rods now that no one yet knows what rpm they can stand. We have crank cases no one breaks 1st anymore. We have gearboxes that are bullet proof. What is your evidence crank is not main rpm limiter.

hobot said:

What is your evidence crank is not main rpm limiter.

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A lack of blown up cranks in this 'special engine' ?
And where is this valve gear, and pistons, that can survive high rpm buzzing ? - if they are not involved with this skipping rope crank....