AJS & Matchless Owners Club

Has anyone successfully run +.080 pistons on a 650. The cylinder must be pretty thin at that point but I see Mondial in Italy used to sell them !

Better to sleeve back to standard bore.
Dave.

of course that would be ideal but adds significantly to the cost of the rebuild

Not sure how thin the liners would be on a 650 as don't have a set of barrels to hand (someone here will though) . . .

But (and I had no idea when I acquired the beast as a project), a '53 'G9' engine had been taken out from the std 66mm to 70.5mm which translates as +180. Beyond simply 'misguided' one might well say! See pic of one of the pistons next to an original 66mm wire-wound one.

The barrel spigots down below were scarily thin, also not quite concentric due to whomever had rebored it. However . . . even at an average thickness of under 60thou, the thing ran extraordinarily well, with quite astonishing acceleration, and I thrashed it mercilessly until I investigated why it was so quick, saw what I had been risking in terms of potential for an almighty explosion - and very rapidly had the cylinders resleeved back to 'sensible'. That said, the lower spigots are well supported by the throat on the crankcase for most of their depth, so just maybe the PO who did it wasn't totally insane and just wanted a 'Q' bike for fun. The twin concentrics jauntily splayed, the lightened rocker arms and ported heads and the extra compression ratio all made for an 'interesting' object. (Part of me regrets having undone it all in the interests of peace of mind!)

The point being, that +80 on a G12 could well be perfectly OK??

g80csp11 wrote: ↑Tue Nov 23, 2021 10:01 am Has anyone successfully run +.080 pistons on a 650. The cylinder must be pretty thin at that point but I see Mondial in Italy used to sell them !

Is this for John's engine Peter?

ajscomboman wrote: ↑Tue Nov 23, 2021 5:30 pm Is this for John's engine Peter?

Does this influence any potential advice?

Mick D wrote: ↑Tue Nov 23, 2021 5:51 pm

ajscomboman wrote: ↑Tue Nov 23, 2021 5:30 pm Is this for John's engine Peter?

Does this influence any potential advice?

Nope only that we'd be talking about a 59 model with thinner spigots if it is.

Yes its Johns , 1959 with smaller spigot into crankcase for this year . bigger post 60

I used to race a 60 CSR 650 and ran a pair of +80 Mondial Pistons running at 10.25 to 1 comp . I had no problem with the pistons or bores , it was the crankshaft that eventually broke after 6 years of abuse.

I also have a pair of +100 if anyone wants to try them .