AJS & Matchless Owners Club

Hello,
I am confused about different piston shapes – new pistons for alu heads offered on the market show different piston heads: with and without valve pockets. Compression is listed 7.5 with pockets and 6.3 without pockets (see pictures below). Which is the correct one, or are both suitable (my engine number is 56/16MS28374)? My fear is that a wrong piston type may kiss the valves …
Thank you. Regards, Peter

I assume you have a G3 not sure which year. If you check your parts list pictures you should see the 6.3 cr no pockets piston is correct.

Rob

This from the original post, may have given the game away 56/16MS 28374. So 1956 350cc 16MS road based tourer.

For 56 the piston with the valve cutouts would be correct. Only the trial models continued with the earlier low comp piston all the road models went to 7.5 comp ratio.

Provided the cut out piston is specifically for an AMC engine then both types will fit but only the higher comp one is correct for year.

The CS short stroke scrambler had a totally different piston so forget that one.

ajscomboman: That is what I thought, because I found an article/book elsewhere, saying that they upgrade compression during production years.
The cut out piston in question is offered for 350cc AJS/Matchless, so I would have to trust in the dealer´s specifications...!? If you have any information/experience where to buy a cut out piston that meets all AMC specs, please let me know (until now I found "JP" as manufacturer).
Thank you.
Regards, Peter

tippi11 wrote: ↑Tue Sep 21, 2021 6:05 pm ajscomboman: That is what I thought, because I found an article/book elsewhere, saying that they upgrade compression during production years.
The cut out piston in question is offered for 350cc AJS/Matchless, so I would have to trust in the dealer´s specifications...!? If you have any information/experience where to buy a cut out piston that meets all AMC specs, please let me know (until now I found "JP" as manufacturer).
Thank you.
Regards, Peter

You'll be ok with the JP piston. Some people say they're heavier other say the same weight and have never had issues. Luck of the draw I suppose.

Thank you very much!
Regards, Peter