What ring gap for GPM 73mm pistons?

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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What ring gap for GPM 73mm pistons?

Postby Roderick » Tue Feb 12, 2019 1:20 pm

I recently bought 2 sets of GPM pistons for a 1960 G12 DL & a 1961 Model 31CSR twin from Club Spares.
The Oil Control rings for both sets of pistons came already gapped at 8 thou. which, traditionally, would have been judged acceptable.
Compression rings on one set of GPM pistons came gapped at 16 thou. which in the past would have been judged half – worn. On the other set of GPM pistons, the ends of the compression rings actually overlapped in the bore suggesting they could well be an over-size & unsuitable for fitting with these pistons.
Club Spares say they cannot advise on gapping GPM rings for the pistons they sell; they say they have asked GPM for advice about it but without success. Barry at T & L Engineering, Elstow cites 3 thou per 25mm as the rule for Hepolites, etc. in the past and also that as an extra thou. piston to cylinder clearance has to be allowed when fitting GPM pistons, rings gapped for a bore without this extra allowance would then probably report an 8 thou. gap when placed in a cylinder bored for a GPM piston.
Trawling around the Club Forum and the Internet looking for enlightment on this matter, I found a lot of people struggling with the same issue since GPM pistons are nowadays fitted to BSA s & Nortons as well as AMC engines. To summarise, the general view seems to be that GPM pistons themselves are fine pistons but, unfortunately, the maker for whatever reason, doesn’t take much trouble with the rings supplied with them which is a shame. Some people solve that problem by fitting quality, proprietary rings like Totalseal instead which improve quality/performance but adds to the total cost.
On ring gapping GPM pistons, I came across this site which suggests that the traditional rule(s) for gapping piston rings no longer apply with modern manufactured pistons & rings :-
http://www.britbike.com/forums/ubbthrea ... 714&page=1
To paraphrase:
“FIRSTLY, the gap width recommendation re the 1st and 2nd ring is reversed from what we used to do. Now, 2nd ring gaps are WIDER than first ring gaps instead of narrower because they found that the "old way" induced ring flutter on the top ring. The ring flutter causes poor sealing - much worse sealing than does even a far too wide (spec-wise) ring gap.

SECONDLY, ring gaps themselves are slightly wider than used to be recommended. .004" per inch of bore used to be standard. Now, .005/inch is the common recommendation for the top ring and .0053 for the 2nd ring and an oil ring gap of a MINIMUM of .015, regardless of bore size”.

For a 73mm piston that means that:-

- The TOP ring would be gapped at 0.014 inch.
- The 2nd ring “ “ “ “ 0.016 inch.
- And an oil ring gap would be a MINIMUM of .015 inch., (regardless of bore size).

I am stuck at the moment with 2 engines on the bench awaiting top half completion. Both have cost a lot in time & money to get so far & I don’t want to have to re-build them because of getting the piston ring gaps wrong.
Do you think the ring gaps on a 73mm should be?
Rod

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Re: What ring gap for GPM 73mm pistons?

Postby ajscomboman » Tue Feb 12, 2019 3:51 pm

Asking for opinions is opening a can of worms. Ultimately it's what you're going to be happy with. I've tried both old and new ways to fit the rings and I honestly can say I noticed no difference what so ever. No extra smoke or oil usage with the larger gaps, I've even had an oil ring go to 50 thou end gap and still no smoke.

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Re: What ring gap for GPM 73mm pistons?

Postby clanger9 » Tue Feb 12, 2019 4:05 pm

You might be over-thinking this. As long as the ring gap is sufficient for the rings not to bind, it’ll be fine. How fussy do you imagine the AMC factory was? The values at the end of your post look sensible.

Piston to bore clearance is rather more critical, but a little piston slap never hurt anyone 😉

PS: ring-to-groove gap is SUPER critical. I’d me more concerned about getting that fit exactly right if I was using different rings...
1966 AJS Model 14 250 CSR "cafe racer"
1993 Ducati 750SS

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Re: What ring gap for GPM 73mm pistons?

Postby g5wqian » Mon Feb 25, 2019 6:27 am

i am using 0.012" for top and second ring , .020" for oil ring .

i have .006" running clearance in the bore , ie i am on a +.040 overbore which measures 2.881" [2.875" + .006" ]

you may be better off with .004"-.005" running clearance in the bore if you dont want it as slack as mine is , i have a bit of piston slap when cold , but it doesnt seize and will rev flat out , although i rarely take it over 70mph and most of time i ride at 60mph max . .

im using thick base gaskets and standard composite head gaskets from club spares , have good compression 135psi and dont burn oil and the plugs are nice colour, i use champion N5C plugs .

running on sae 50 engine oil .

cheers , ian

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Re: What ring gap for GPM 73mm pistons?

Postby SPRIDDLER » Mon Feb 25, 2019 9:36 am

Roderick wrote:“FIRSTLY, the gap width recommendation re the 1st and 2nd ring is reversed from what we used to do. Now, 2nd ring gaps are WIDER than first ring gaps instead of narrower because they found that the "old way" induced ring flutter on the top ring. The ring flutter causes poor sealing - much worse sealing than does even a far too wide (spec-wise) ring gap.

Coincidentally, on the subject of the second ring gap being larger than the top ring I recently read the extract below, (although consideration of such marginal mechanical tweaks is not my scene)............:

You get better compression if the second ring gap is 30% greater than the top ring gap. They say it means the combustion pushes the piston down more effectively for more of the firing stroke, so increasing power at all rpm, as well as reducing crankcase pressure from blow-by.
It’s not intuitively easy to understand (or believe) the physics, but it’s been found to prevent pressure building up in the space between the top and second rings. The pressure build up above the top ring reduces the pressure difference between the space above the top ring and the space below it. That pressure difference is what gets pressure behind the top ring, forcing it out to seal against the bore.
I think this stuff was discovered by testing, less than fifty years ago and the people who found it out didn’t rush to tell their rivals.
I poke badgers with spoons.

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Re: What ring gap for GPM 73mm pistons?

Postby Roderick » Tue Mar 12, 2019 11:02 am

I decided to get another 2 sets of compression rings from Cox & Turner & will fit them along with the GPM oil control rings. Cox & Turner say use the 5 thou. per inch rule for gapping for old engines such as this.

Many thanks to all those who offered advice & help.

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