Short Stroke ~ High Compression

I think there are at least a couple of forum members who have built short stroke 750 Commando motors using Jim Schmidt's Piston and 6.57" long rods. I have hit a problem with this combination of parts and wonder if anyone else has the same issues and if so how they have fixed them. The problem is not with any of Jim's parts but with the Norton Cylinder head and the fact that the counter bore that creates the squish area has been machined out of register with the mounting hole patterns in the head and in the cylinder block; at least it is on the two RH10 heads that I have. That's not a big deal with standard pistons and rods since the pistons don't protrude above the top surface of the block, but the high compression pistons do and because of the out of register squish region they hit the head. I have, so far, very gingerly opened up the mounting holes from about 13/32" to 7/16", but I'm not quite there yet.

I guess this applies to any kind of high compression piston regardless of whether the engine is a short stroke or not. Has anyone else experienced and tackled this problem?

If I understand what your pistons ran into then the solution is boring into head chamber to clear pistons but remains uncentered in head so parts of the rim still wider that others. Combat pistons stick up .050" inch w/o the base plate installed. Ken Canaga has solved similar situation by milling piston to match the relived dome of the hemi chambers so squish area was angled not flat on. I don't know how much material in that area but seems adequate. Squish effect does not seem ot have much benefit in Nortons as plenty race examples w/o any.

ggryder said:

I think there are at least a couple of forum members who have built short stroke 750 Commando motors using Jim Schmidt's Piston and 6.57" long rods. I have hit a problem with this combination of parts and wonder if anyone else has the same issues and if so how they have fixed them. The problem is not with any of Jim's parts but with the Norton Cylinder head and the fact that the counter bore that creates the squish area has been machined out of register with the mounting hole patterns in the head and in the cylinder block; at least it is on the two RH10 heads that I have. That's not a big deal with standard pistons and rods since the pistons don't protrude above the top surface of the block, but the high compression pistons do and because of the out of register squish region they hit the head. I have, so far, very gingerly opened up the mounting holes from about 13/32" to 7/16", but I'm not quite there yet.

I guess this applies to any kind of high compression piston regardless of whether the engine is a short stroke or not. Has anyone else experienced and tackled this problem?

Click to expand...

The problem I found with my stock RH10 head was that the squish band area seemed to have been machined with a blunt piece of brick , it was pretty much all over the place. So I got it machined to a uniform depth all round and ensured its nice and square. Wasn't an issue when the pistons were so far away as standard, just came to light when measuring squish and trying to get to .050 ish.

Without looking up the head chart, were RH10 heads intended for 750's ?
Thought they were 850...

I've only built short stroke 750 engines using the stock full hemi RH7 heads (no squish bands) and matching Omega pistons, so haven't experienced the problem you describe. I assume from your description that you still have the squish bands in the RH10 head, and are using flat top pistons in stock 850 cylinders. If I understand your description properly, your piston is hitting the deck surface of the head at the edges of the counterbore, because the counterbore is not concentric with the bore. Is that correct? I've built several 750 and 850 race engines using standard heads with flat top pistons that projected up into the counterbores, and never had a problem with them not lining up. Maybe I've been lucky, or maybe you've been unlucky in getting a poorly machined head. In any case, I wouldn't think enlarging the bolt holes to move the head around would be a great plan. The cylinder head seems to be mostly located by the tight fit on the two 5/16" studs at the front of the cylinder, so if you enlarge them along with the holes for the 3/8" fasteners, you might find it hard to keep the head in the proper location, unless you are planning to fit locating dowels. A better option would be to bolt the head to the cylinder, and then mark the location of the bore on the head from the bottom. Next stop would be a milling machine to cut the counterbores in the right location. That's what I do when I open up a head to take 920 pistons. I clean the head, spray it with blue Dyken, bolt it to the cylinders, and use a sharp scribe to mark the location of the bore, and then take the head to the mill and open it up with a boring tool.

I'm not sure how high a CR you get with a stock 850 head and short stroke crankshaft. With the original full hemi heads it was pretty difficult, because the combustion chamber volume was larger than in the stock squish heads. As hobot mentioned, I had to machine the piston a lot to get it pushed up far enough into the head to get a decent CR. That shouldn't be as much of a problem with the stock heads. If you are starting with flat top pistons, and want a high CR, like in the 12s, you'll have to machine the piston to a stepped contour to get it. That's what Steve Maney had to do with his short stroke and ultra short stroke race engines.

Hope this helps. Like I said, I never had your exact problem, so I'm just throwing out some comments here.

Ken

Rohan said:

Without looking up the head chart, were RH10 heads intended for 750's ?
Thought they were 850...

Click to expand...

Short stroke 750 used 850 cylinders (77 mm bore) and shorter stroke (80.4 mm) to get 750 cc. Heads were still 850 bolt pattern, but had larger intake valves in re-angled guides, and full hemi chambers with no squish band. Modern practice, if you don't have the original heads and pistons, is to use Stock 850 heads, like the RH10, with flat top pistons to build your own short stroke 750. Steve Maney built some that way, but with his head work, fitting larger valves and re-angled guides, but still with the squish band, and made some serious horsepower with them.

Ken

lcrken said:

If I understand your description properly, your piston is hitting the deck surface of the head at the edges of the counterbore, because the counterbore is not concentric with the bore. Is that correct?

Click to expand...

That's exactly it Ken.

Relative to the mounting hole pattern the counterbores (could be the whole combustion chamber I suppose) are offset to one side by about 0.020" and as a pair they are rotated by about 1/2 degree, so that one is 0.045" further forward than the other. I've had the head on the barrels and I've been able to turn the engine over without the piston snagging the head: sometimes.

As you noted, getting the compression ratio up there with a short stroke is a bit more challenging. The pistons are Jim Schmidt's finest with a slight dome, theoretically good for may be 12:1 although I think I'm in the 10 to 11 range, so I'm reluctant to enlarge the counterbores, although I think it would have minimal effect on the combustion chamber volume.

With the benefit of hindsight I don't really like the idea of the piston protruding from the cylinders at the bore diameter. Next time I'd consider using Jim's regular rods (6.4") and take just enough off the cylinder barrel so that only the dome of the piston extends above it then skim the head to all but remove the counterbores.

Generally I have found the head is usually bored right. IE the counterbores are where they should be in relation to the bolt pattern. The barrels however may be all over the place.
Even if they started out square when they were built [not likely] they may be off if someone has done a bore job and was not too careful with the setup. Jim

comnoz said:

Generally I have found the head is usually bored right. IE the counterbores are where they should be in relation to the bolt pattern. The barrels however may be all over the place.
Even if they started out square when they were built [not likely] they may be off if someone has done a bore job and was not too careful with the setup. Jim

Click to expand...

This is an interesting comment.....

I am am going to be building up my 750SS with the 6.57" rods shortly (honest...) and I have two sets of barrels at the borer now...... At least the guy I am working through, Norman White, knows what is needed so I guess his borer does!

We will see what comes out, one set of barrels will be sleeved 77mm, the other bored 77.5, not sure in which of these the problem is more likely to show up, if it does!

The head is a new Full Auto so I am not expecting misalignment there, just hope my cast iron barrels are up to it, or we shall be saving for Maney barrels to get some modern and unmolested accurancy!

However having said that, the problem you identify should'nt be unique to your build and could be apparent with Jims high compression flat or domed pistons in any 77mm build, SS or not!

As we have not seen the comment on this forum before as far as I know, it looks to me like you have been unlucky, and like Jim says, more likely with the barrels, you don't mention the history of those, if they are 77mm it would seem unlikely they are used and unhoned bores, are they new?, have they been honed? sleeved? or are you actually using an oversize?

I wanted to have my best set of barrels honed for the forged pistons, they check out pretty good but accurate measuring showed it would be difficult to get the minimum clearance and a good honed finish, hence the purchase of a set of 77.5 and reboring then honing them to clearance.

The other barrels needed sleeving to make them any use at all, so we will see which ones come out best on dry assembly!

Lookig at compression it is moot if you really need high 11s. I am shooting for low 10s in my first build. To do that I will have to machine the piston periphery anyway as per Jim Schmitts data. You could additionally machine the very outside of the piston to about 50thou in to be flush with head and gasket surface! which to be honest is most likely what I will do if this happens to me. 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....

Way back when in the '70s after a valve dropped and ruined the original domed Omegas I had to fit flat tops into my short stroke big valve hemi head, running with the 89mm crank (78mm pistons, full 850) .....I had to machine a 45 degree chamfer on the piston edge to clear the head....it wasn't very wide, can't remember exactly but 50 thou comes to mind....(after about 36 years :roll: )...

No squish band and it could be that the combustion chamber weld repair was less than perfectly concentric with the bore.... to maintain that low 10s compression the barrels were faced 60thou and the head 40thou.....but Mick hemming whose machinist did the rework thought it should have gone together no problem....just dry build to check the valve pockets were deep enough....they were...but the dry build revealed the piston to head interference problem....and was an easy fix...

Some times you are going to get the compression figure you are going to get, not the one you wanted!

FWIW the best CR I was able to get with the original short stroke full-hemi head and Omega pistons was 11.9, and to do that I had to shorten the cylinders a bit, skim the head a bit, and reshape the top of the piston. These are pics of the head and pistons I used.





This is what the pistons looked like when original.



One of the problems with this mod was that the top ring had very little protection from the direct combustion gases. When I had replacements made by JE with the same top profile, I had them move the ring pack down a bit to give a wider top land. That also eliminated a problem I had when using an ex-factory race head with really large valves, where the valve notches were so close to the back of the top ring that they eventually burned through.

Ken

I think I understand your problem with the short stroke 750, you are trying to use the 850 head and compensate for the lower cylinder volume ?
Have you actually enlarged the bolt holes in your Fullauto head ? I think I would sleeve them back to standard so you are certain about where the head is positioned. Then I would reshape/relieve the crowns of the pistons to get clearance and accept whatever comp. ratio you end up with. I've been considering using Jim Schmidt's long rods and 12 to one pistons in my 850, as I use methanol fuel however anything above 10 to one would be good. I never use petrol in a four stroke race motor, however I cannot see much advantage in going to very high comp. even with methanol, you need the good ignition system to fire it reliably. At present I use standard compression and lots of ignition advance, and the bike is quick enough. I suggest it doesn't matter what you do, you end up balancing comp. ratio, ignition advance and jetting to get a certain combustion characteristic, and the results from optimization at different comp ratios are probably much the same, unless you can get more out of the fuel with a higher comp. as might be the case with methanol.
Ken, when you fitted those very high crown pistons, did you go up much in jet sizes compared with standard ?

Here is the head with the offset counterbores. You might be able to convince yourself that the squish areas do not appear to be mirror images of each other, but to be honest, the offset is only 0.030" and probably isn't apparent in the pics. The counterbores have the right center distance (3.44") but as a pair they are shifted to the right (0.020") and rotated counterclockwise (1/2 deg). The diameter of the cylinder bore is 3.05" and the counterbore is 3.08", so a 0.030" displacement of the head causes the edge of the counterbore to intrude in to the path of the piston as it rises above the deck of the cylinder block. I've opened up most of the head-to-barrel mounting holes by 0.030" which just about allows things to be lined up, but I may need to go a bit larger here and there and, as Ken suggests, I should probably figure out a way to fix the relationship between the head and the block once corrected, to avoid trial-and-error assembly in future.

I good solution to your problem would be to use three eccentric sleeves on the studs of your cylinders and/or head to centralize the combustion chamber to the bore. I had to do something similar to this to tighten up the fit on one of my heads. I put sleeves to the two studs that thread into the head near the pushrods and the one head stud in the rear center. Or you could put sleeves in barrel bolt holes. Since you've already opened up the holes in the head - you'll have to figure out what works best. See one of my brass sleeves (not eccentric) on the stud below (.003" head gasket ring and copper wire around pushrod tunnels shown).



The compression of the Domed pistons varies according displacement and stroke. A short stroke 750 with the domed pistons gets around 11.2 C.R. But the same piston in a larger displacement long stroke 850 gets about 12.3 C.R. A 73mm long stroke with domed pistons will have around 11.3 C.R. These C.R. figures change if valves are sunk into the head etc. Each piston will have the same height in relation to the head at TDC. So I guess I need to add few more details in the jsmotorsport.com website. See bottom paragraph on 12 C.R. for a 750.

Domed lightweight piston below


To get 12:1+ C.R. on a 750 you could use the .003" headgasket ring and copper wire as shown above or mill the head and cut the piston periphery approx .040" as show on the diagram below (flat top piston shown, but the cut is similar). You have to be careful and measure up everything or you could end up with 12.5:1 C.R. or more. You could go with our .020" thick head gasket and cut the piston periphery the same.

This is a Combat head Ken fitted to 920 cylinder via boring out holes for eccentric drilled plugs and chamber counter bore emphasizing the eccentric squish rim and put Timeserts in the threads.

hobot said:

This is a Combat head Ken fitted to 920 cylinder via boring out holes for eccentric drilled plugs and chamber counter bore emphasizing the eccentric squish rim and put Timeserts in the threads.

Click to expand...

So the counterbores were made larger diameter to suit the 920 bore? and they were moved rearwards on the head? (hence the wider squish band at the rear?), but then they were brought back, concentric with the bores, by effectively moving the whole head forward on the cylinder block by relocating the mounting holes?

ggryder said:

hobot said:

This is a Combat head Ken fitted to 920 cylinder via boring out holes for eccentric drilled plugs and chamber counter bore emphasizing the eccentric squish rim and put Timeserts in the threads.

Click to expand...

So the counterbores were made larger diameter to suit the 920 bore? and they were moved rearwards on the head? (hence the wider squish band at the rear?), but then they were brought back, concentric with the bores, by effectively moving the whole head forward on the cylinder block by relocating the mounting holes?

Click to expand...

You can find pictures of the conversion process here

http://tinyurl.com/ysuvwb

I think I posted a thread about it back in 2010 with a little more info, but I haven't been able to find it so far.

I didn't move the counterbores in relation to the cylinder bores, I just enlarged them to match the 81 mm bore. I kept the cylinder head in the same relation to the bore centers, and just moved the four outer head bolt holes to match the 850 bolt pattern. The center head bolt and the two front cylinder studs are in the same location for 750 and 850 heads, so I used those as a reference. Hope that's not too confusing. Bottom line is that the converted 750 head ends up identical to an 850 head in terms of bolt patterns and combustion chamber location, with the counterbore concentric to the cylinder bores, but enlarged.

Ken

Ken

lcrken said:

FWIW the best CR I was able to get with the original short stroke full-hemi head and Omega pistons was 11.9, and to do that I had to shorten the cylinders a bit, skim the head a bit, and reshape the top of the piston. These are pics of the head and pistons I used.

This is what the pistons looked like when original.

One of the problems with this mod was that the top ring had very little protection from the direct combustion gases. When I had replacements made by JE with the same top profile, I had them move the ring pack down a bit to give a wider top land. That also eliminated a problem I had when using an ex-factory race head with really large valves, where the valve notches were so close to the back of the top ring that they eventually burned through.

Ken

Click to expand...

Thanks Ken, havent seen pistons like that since I took them out of mine in about '77...one with a valve head embedded in it :?

Gary
Last year after the big blow up with the crankcase breather, I found I had the same problem you are having.
I had to rebuilt my Steve Maney short stroke engine that he had used at Daytona in 2000.
What Steve had done when he created his own AMA style non squish band head, compounded my problems.
He had machined the head down to the combustion chamber squish level therefore creating a completely flat head.
So much had come off the head that a fin was missing!
As you stated, the Norton head combustion chamber is not concentric with the bore AND I also found it to be slightly bath tube shaped.
It is not a perfect hemisphere, it is tighter on the spark plug side of the combustion chamber by about 40 thou.
So even if you machine the piston crown eccentric to fit inside the combustion chamber, you also have to make the cut deeper to clear the bath tub sides.
This looses the correct squish clearance to the head & looses compression ratio as well.
I think the best solution would be to have the hemisphere in the head machined to a perfect hemisphere, but not sure how you would do that.
I don't think it's a good idea to open up the holding down bolt holes, what if the head moves in relationship to the barrel, it could cause contact with the piston & possibly a very expensive rebuild.
I ended up machining the new pistons from Ken Canaga (thank you) so that there was 40 thou clearance all round to the nearest combustion chamber wall & also had to machine the squish band around the piston crown to fit it into the head.
Call me or email me personally if I can help you more.
John

pouchy750 said:

As you stated, the Norton head combustion chamber is not concentric with the bore AND I also found it to be slightly bath tube shaped.
It is not a perfect hemisphere, it is tighter on the spark plug side of the combustion chamber by about 40 thou.

Click to expand...

John,
I don't know about the main part of the combustion chamber not being concentric with the bores; if they are not then that doesn't cause me any problems because my pistons do not have high domes.
In my case it's just the counter bored area of the head, and they are not intentionally offset they have just been machined out of register with the other features.

I made a pair of "pucks" one end of which fits snuggly in the cylinder bore, the other end fits snuggly in the cylinder head counter bore. With the pucks in place, and the head on the barrels, both are in proper alignment. Then you a can peer down the mounting holes in the head and see that they are misaligned with respect to the corresponding holes in the barrel. I have been able to fit 8 out of the 10 fasteners by opening up holes in the head or barrel a small amount, but in the case of the worst two holes the offset is 0.050" and I need someone with a milling machine to bore through the head off-center with those existing holes.

Other posters have said I will need to adjust the alignment permanently by fitting dowels and I agree, there's no way I will ever be able to assemble the head to the barrels accurately without something along those lines.

Gary

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