A10 compression ratio

[melvyndalby]

I recently aquired a BSA Rocket Goldstar (REP), it has +60 pistons in it,  can anyone tell me how I can find out the compression ratio,  thanks Mel

[bsa-bill]

With possible exceptions, flat top 7.5:1 ,  raised portion and then flat top 8.5:1 ,  Domed top 9.?:1
Could differ depending on make I would think

[RichardL]

Really tall peaked top,  10:1 (oooh,  very scary,  batten down the hatches,  wear the brace on your rightright. knee).

[muskrat]

G'day Mel, welcome to the forum.
                                             Also look inside the piston for make or casting #'s then google.
 The other way is a bit messy, but the one I use, is to cc the combustion chamber. With the piston on TDC comp stroke lean bike over till the spark plug hole is vertical, then using a syringe accurately measure how much oil it takes to reach the bottom of the spark plug thread. I do this to make sure both cylinders are the same. Swept vol of cyl + your cc / your cc = comp.
Cheers
 Just saw manosound's. I got 10.5:1 in the cafe, but I'm still a month away from 50.