AJS & Matchless Owners Club

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

is there enough meat to open the groove a tad? however this will still leave you with loose comp rings which I suggest will fret your grooves in use, would it not be easier to have the barrel lined to standard or to whatever piston size you can readily obtain? kind regards

I would not think that 4 thou clearance on the side of a compression ring was anything to worry about - but then you would worry about something that I wouldn't even notice! Regarding the Oil control ring, if all else fails can you find an AMC short stroke one (86mm) and file the gap? Ring me if you want to discus.

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

You could think about plating the compression rings top and bottom to bring them up to size.

Rob C

Steve, do you remember you opened two threads for the same topic?

Rob C

Yes Rob, maybe a bit naughty, but I think I got more response as a result.
Hope its not flouting the rules.
Steve

I believe the recommend gap btn ring and lands is 2 or 3 thou max, any more than that and the piston will pump oil up to the combustion chamber. Apart from wearing the ring grooves.

I know this is an old post , but having worked as a Automotive machinist , it is a simple job to fix , they make special spacers called ring inserts that are used to repair ring grooves , usually the top ring groove , Pistons are easily mounted into a lathe i:e pulled back on to the chuck via a eye bolt through the headstock or using end pressure from tailstock and trued, and machined , most machinists can sort this very quickly .cheers .