The BSA Workshop > Twins

A10 Pistons


Looking around the web, the lowest compression pistons I can find for the A10 (53 plunger) are 7.25:1. I believe the correct pistons for a GF are 6.5:1.
Does anyone have any comments about the advisability of using the higher compression pistons? They are flat, the lower compression ones are dished.
Any thoughts would be valued.

The 7.25;1 should be fine. If you think it too high then you can get a plate to fit between crankcase and cylinder base.

 Other than new old stock Hepolite and BSA you may find it hard to source 6.5:1.The later Golden Flash used the higher ratio. If you are thinking of using the new "hepolite" pistons being widely sold be aware that the rings supplied as standard have a very poor reputation and that some sellers remove them and supply better quality ones.

Dished pistons were no longer available back in the 1960s. It's hard to imagine there are still any Flashes that still have them. Flat top 7.5:1 are absolutely fine.

Thatís really useful.
Thanks both of you.
It looks like itís been rebuilt fairly recently. So if thereís  no wear in the barrel and the pistons are OK Iíll reuse them with new rings.
Iím just feeling the need to check everything in the engine is OK as the front wheel wasnít and the carburettor was in a poor state.


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