1958 G11CS - Odd Pistons!

Information relating to the Matchless G11 or AJS Model 30 600cc twin
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Joined: Mon Dec 17, 2018 1:49 pm
Location: UK

1958 G11CS - Odd Pistons!

Postby DavidHulse » Sun Jul 05, 2020 3:17 pm

Welcome to my nightmare! I just finished rebuilding the bottom end after much uncertainty, sweat and tears; and turned my attention to the barrels and pistons to find yet another problem: the pistons that came with the basket case are two different compression ratios!

They both have the correct AMC part numbers according to my Plumstead workshop manual (023503 HC and 022226 LC), but their dimensions don't match the technical data in the same manual. My pistons don't have any oversize or standard markings, but their diameter (of both) is 2.803 inches, whereas the manual indicates it should be 2.833-2.834. The bore size on both barrels is 2.817, but the book says it should be 2.834-2.835.

I am confused over whether the 0.014 difference between my pistons and bores is acceptable, when the manual indicates a difference of only .0006? And why are they smaller than specified?

Secondly, which piston should I replace (assuming I can get either one)? It's a G11CS, so theoretically should have HC pistons; but would LC be better for an old machine like this?

The condition of the barrels is not great, with very little wear and a few scratches, but they are probably usable. But the outside diameter of the top spigot is only 3.000, so if I have them rebored even to +.020, making the new bore 2.854, it will only leave .146 of metal (.073 either side), which doesn't seem like a very safe margin.

Any suggestions would be most welcome. Thanks.

Member
Posts: 50
Joined: Mon Dec 17, 2018 1:49 pm
Location: UK

Re: 1958 G11CS - Odd Pistons!

Postby DavidHulse » Tue Jul 07, 2020 7:44 pm

Have found a solution to the problem. Have remeasured pistons and bores and concluded they are standard size. Getting a used standard compression piston, on its way to me now from AMC Classic Spares. Couldn't get an HC piston. So hopefully the replacement piston will match the standard one I've already got.

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