What ring gap for GPM 73mm pistons?
Posted: 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 sizeâ€.
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
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 sizeâ€.
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