Wrong, that was addressed by the factory for both the full hemi AMA heads and the short stroke heads. It was all part of the kit
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Wrong, that was addressed by the factory for both the full hemi AMA heads and the short stroke heads. It was all part of the kit
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The people I know that had these heads couldn't get the CR high enough without milling the head and cutting and weakening the pistons - inviting failure at the enlarged valve pocket. The domed power max pistons disappeared long ago leaving customers with flat topped pistons to work with. The 77mm flat topped pistons had low compression height and low CR.
As far as I know there is only one supplier currently offering fully domed pistons (special order) for those fully hemisphered AMA short stroke heads and the CR is still going to be lower compared to a head with a stock combustion chamber (both heads being milled the same).
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Do you have a photo of the AMA pistons - or Mics copies? It would take a lot of dome to get 11 to 1 with a fully hemisphered head and a 73mm bore.
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lcrken
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That may be true, but Jim is right in pointing out that anyone doing a big valve, full hemi conversion today needs to be careful not to increase the combustion chamber volume any more than necessary. And the factory short stroke heads are a good example of why. The combustion chamber volume in a factory short stroke head is larger than in a standard 850 Commando. That means to get a high CR, they will need to use something besides a normal flat top piston. The problem gets even worse if you are using a big valve head modified for even larger valves and full hemi combustion chamber. The factory specs for combustion chamber volume are 50 cc for a standard RH4 850 head, and 53 cc for an RH7 head. Some of the RH7 heads that the factory modified for their race bikes have even larger combustion chambers. The one on the ex-factory flat track bike that I bought measured out at 57.3 cc. Even with the factory kit Omega pistons, it only had a measured CR of 9.2. I had to mill the head, re-contour the piston dome, and open up the valve pockets to get the CR up to 10.2, and that was as high as I could get it with those pistons. Even then, the exhaust valve pocket had to be cut so deep that it eventually burned through to the top ring groove. Of course, that's the extreme case, considering the modified head and that it had a lumpy cam (Sifton 460), but it is a good illustration of the point Jim was making.
You can minimize the increase in combustion chamber volume when doing the big valve conversion by keeping the squish bands, as some tuners did (Steve Maney for one), instead of opening the combustion chamber up to a full hemi.
Back in 2009 I compiled a list of combustion chamber volumes for several heads that I had worked on at different times. It may be a little off topic, and I might have posted it here in the past, but I'm including it anyway, just in case someone might find it of interest. I need to update it to include other heads I've used in the last 20 years, but haven't got around to that yet.
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Jeff Law’s 750 head converted to 920. Milled and counterbore machined – 44.6 cc.
Steve Shivers’s 750 head converted to 920. Milled and counterbore machined – 45.8 cc.
Head from Marc Field’s Dunstall 810 engine. 850 converted to 750 bolt pattern, milled. – 42.4 cc.
850 head from Jim Schmidt’s race engine. Milled. – 42.4 cc.
Axtell 30 mm port 750 head – 51.0 cc.
Axtell 32 mm port 750 head – 48.8 cc.
Stock Mk3 850 head – 51.6 cc.
Mk3 850 head milled .060” – 43.2 cc.
Stock Mk2 850 head – 51.8 cc.
Short Stroke 750 head – 55.2 cc.
Another Short Stroke 750 head – 52 cc.
Ex-factory Flat Track Short Stroke 750 head as removed from John Hately’s bike – 57.3 cc.
Ex-factory Flat Track Short Stroke 750 head after milling – 50.7 cc.
Nominal factory spec for standard RH4 850 head is 50 cc, and for RH7 short stroke head is 53 cc.
Ken
Fast Eddie
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Ken, out of interest, when you say combustion chamber volume, are you measuring by filling up to the squish band face or the gasket face ?
lcrken
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Gasket face. It's all part of the combustion chamber, and that's pretty much the standard way of measuring combustion chamber volume throughout the automotive industry.
Ken
Fast Eddie
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Cool. Just wanted to be able to compare apples to apples with your data.
lcrken
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Not sure which part of this thread you are responding to, but assuming it has to do with the AMA heads, I'm posting the part from N3/12 that shows the cylinder head and associated bits. For those not familiar with it, it was released as a Service Release titled "750 Racer and 750 Formula Racer Conversion Kits and Components", and also in brochure form as a Technical Release titled "750 RACER AND 750 FORMULA RACER CONVERSION KITS", both numbered N3/12. This is a snip of the cylinder head parts list and picture. The cylinder head part number is the one Norton referred to in other documentation as the "AMA Racer" head. There was also another AMA head, part number 061391, with different valve sizes, and referred to by Norton as the "AMA Homologated Combat 750" head. Both were marked RH2, and not to be confused with the "Wolverhampton Combat 750" head, which had standard Commando valves, was skimmed .040", and was stamped RH3, and usually had the letter C also stamped on it. That's the one you got if you bought a standard Combat Commando. Confused yet?
For those not familiar with these notes, I've attached images of the cover sheets from both. I think the documents may have been posted somewhere on this forum before, but not sure where.
Ken
SteveA
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Hopefully the pictures show how Norton achieved their objective with pistons for the fully hemisphered heads. These pistons were cast by Omega. This particular one is from a pair of 77mm pistons where the gudgeon/piston pin was machined for the standard pin size of standard alloy rods, not the larger pin size of the short stroke rods. So they were in my 89mm stroke 850 that was fitted with an ex Thruxton short stroke head from late '75. They were probably cast in '74.
It isn't a domed piston as such. At the top it is flat. It has a very much raised crown height and is then 'chamfered' at two angles. So the outer section goes into the fully sphered chamber and I think delivers some squish.
As far as compression goes they delivered about 10.25:1 with no material skimmed from the head or barrel face. So not what some of you expect. But it was enough to work with available pump fuel and more importantly it was enough when you had to bump start the thing for a road race start!
It isn't a domed piston as such. At the top it is flat. It has a very much raised crown height and is then 'chamfered' at two angles. So the outer section goes into the fully sphered chamber and I think delivers some squish.
As far as compression goes they delivered about 10.25:1 with no material skimmed from the head or barrel face. So not what some of you expect. But it was enough to work with available pump fuel and more importantly it was enough when you had to bump start the thing for a road race start!
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Fast Eddie
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Opening the combustion chamber out to a full hemisphere just seems like such a backward step to me.
Tuners of other makes are busy welding combustion chambers up to get a squish band !
Steve, are those pistons as heavy as they look?
Tuners of other makes are busy welding combustion chambers up to get a squish band !
Steve, are those pistons as heavy as they look?
lcrken
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This is how I had to modify those Omega pistons to get up to 10.2 CR in the engine from the flat track bike. These are the ones that broke through the exhaust valve pocket into the back of the ring groove. But to be fair, I had to modify them drastically only because the head had been modified for even larger valves, both intake and exhaust, and increased the combustion chamber volume significantly in the process. With a stock short stroke head you wouldn't have to go to such extremes. The clearance between the dome and the combustion chamber is close enough to act as a significant squish band.
When I ran out of Omega pistons, I had JE make these pistons with the same design, but with the top ring groove lowered to avoid breaking through.
We seem to be hi-jacking this thread a bit from John's cylinder head production progress. If there's interest in more short stroke conversation, maybe we should start another thread.
EDIT: Small mistake. The picture above is for some long rod short stroke pistons I had made. The ones designed for the standard steel short stroke rods are shown below.
Ken
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lcrken
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The problem with the fully hemisphered head is that those pistons are no longer available.
As Fast Eddie mentioned above - Smaller chambers are the opposite direction some builders are going today (Herb Becker).
Truth. I sold a few sets of the ones I had JE make, but didn't see enough demand to pay for another batch.
SteveA
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Thruxton got their best 750 power figures with these pistons!
And from Ken's post I think this is important: 'The clearance between the dome and the combustion chamber is close enough to act as a significant squish band.'
No, I could put it on a scale, but being cast, quite light!
I am just sharing history. I am not advocating anybody do different to what they are now. There is a lot about that bike I didn't do second time around including the 34mm inlet ports and 36mm carbs people say can't possibly work!
However, faster riders than me commented how quick it was at the time! And Croxford has said recently that the space frame with an 850 motor like mine was the fastest thing around Brands Indy! Certainly made mid range like it was going out of fashion!
SteveA
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I don't think we need to do that Ken, we have covered enough to consider how the Fullauto head could be developed in different directions, based on history we know. So personally I think we can consider this subject done.
I have been meaning to take photos of those pistons for at least a couple of years, this was as good an excuse as any.
lcrken
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The ones I measured many years ago weighed 325 grams for the bare piston, 84 grams for the pin, 27 grams for the rings, and 1 gram for the retainers, giving 437 grams for the complete piston, rings, pin, and retainers.
Ken
Fast Eddie
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No then!
At 325g they’re not as heavy as they look, not much heavier than stock 850 pistons.
Steve, I don’t think I suggested they didn‘t work! Just seems to me that removing all that metal, only to basically add some of it back onto the piston, seems like unnecessary work when you can get just as good squish, that’s a lot easier to set up (flat squish band + flat piston = very easy to fine tune) without all the work, and have a slightly lighter piston as a bonus!
At 325g they’re not as heavy as they look, not much heavier than stock 850 pistons.
Steve, I don’t think I suggested they didn‘t work! Just seems to me that removing all that metal, only to basically add some of it back onto the piston, seems like unnecessary work when you can get just as good squish, that’s a lot easier to set up (flat squish band + flat piston = very easy to fine tune) without all the work, and have a slightly lighter piston as a bonus!
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