Using Enfield piston rings in a '55 18CS

[sjr]

Hello Gents,

I had a badly worn barrel (already +0.050" oversize with a Robbins piston in it).
Undaunted, I found an even larger oversize Robbins piston (+0.080"). - unfortunately without rings.
The +0.080" bore is 84.58mm
And a 500cc +0.020" Enfield Bullet bore is very close at 84.51mm - and Enfield rings are easy to get.

Now I have my new Enfield rings and my newly bored +0.080" Ajay cylinder, and they fit well together.
However, the fit of the rings in the piston is a different story:
- the Enfield oil control ring is much too high and won't even go into the groove (0.151" vs 0.121" in a 1/8" inch groove off the Ajay)
- the Enfield compression rings are a bit low and sloppy in the grooves (0.0585" vs 0.0625" in a 1/16" groove off the Ajay)

Perhaps I will re-use the old +0.050" oil control ring. It will have a massive gap, which is a bit worrying.
I'm not sure about whether to fit the Enfield compression rings. Is 0.004" of additional vertical clearance potentially troublesome?

Or on another tack, should I be searching for a different "donor" ring type?
I've looked everywhere for Robbins originals and drawn a complete blank. I don't think AJS made +0.080" oversize.

Many thanks in advance for any help or intelligence,

Steve

[DM2]

Well, you could get the piston machined to take the Enfield oil control ring.

[clive]

But that would still leave the compression rings loose in the grooves. I am no engineer but 4 thou clearance sounds too much.

[Radcap]

why dont you get a new piston with rings ,they are available ill look for the place i got mine from ???? cant remember but ive got the receipt somewhere , my gudgion pin came out and made a serious mess

[sjr]

NOS pistons are occasionally available but not with rings in this size, not where I have been looking anyway.
If you know of rings in +0.080" oversize I'll be most grateful!
I'll happily pay you a finder's fee!
I don't want to sleeve the barrel back, since its a CS (ally barrel with the reputedly cast-in liner) and I don't want to risk messing that up - see my other topic on this subject..
So I have to work with the +0.080" oversize bore and I like the look of the Robbins piston, it looks identical to a genuine Ajay.

I'm going to try surface grinding 0.015" off each side of the Enfield oil ring, which should make a good fit.
As for the Enfield compression rings I think I'll use them despite the warning about fretting. Phil Irving's "Tuning for Speed" suggests that there should be some height clearance so that gas pressure can get behind the ring and seal it more effectively against the cylinder wall - doesn't say how big the gap should be, though! Its my risk to take I guess.

Thanks
Steve

[saltbox alf]

I have Enfield rings on my +20 wire wound piston and a BSA oil control from an A10 +60 if i remember rightly, all I had at the time to build and test. Very small gap twix ring and land but enough for flutter I suppose.
Over all it seems ok after quite a few miles, burns a little oil, but then the barrel is pretty grotty too! Lots of oil retaining rust pits, although it was honed before putting it together is was never going to be good!!

The bike goes well enough and does G3 stuff as it should, so have a go with yon rings, it may well work satisfactoraly

[Don Madden]

Robbins was an American company, now defunct. The pistons they made were form & fit direct replacement for the AMC bikes. The size, weight & rings were close. The maker closed long ago but a large stock of new parts circulated among suppliers for some time. A couple of years ago, the remaining stock was purchased by the North American Section of AMOC & there may be a few bits left. Contact our secretary: John Diederich, cycletoys@aol.com. Cheers! Don

[sjr]

Brilliant! Thanks Don. I have written to John Diederich.
Best wishes
Steve

[sjr]

Unfortunately neither John Diederich nor his recommended alternative Perfect Circle could help.
The American club has disposed of its Robbins parts some time ago.
So I'm probably back to surface grinding for the oil control ring unless there are any equivalents out there.
The original oil control ring was 1/8" height, the diametral size is 82.5mm +0.080" = approx 84.5mm
Any ideas?
Steve

[SPRIDDLER]

Have you tried Thorntons? They have a vast range of NOS vintage pistons and rings.
http://www.fwthornton.co.uk/8.html