I can't tell you that...
I use a 'seat of the pants test' to determine what is the state of play..
If you are of average weight an M20 with good compression will just hold your weight on the kickstart whilst on the compression stroke, before slowly going 'over compression'..
Engines with high mileage on them are unlikely to do this.
Generally that is a good enough test...If the engine is weak you'll find out and having an actual figure doesn't really help...
Deterioration of the ex. valve face and seat is the most likely cause of compression loss in my experience, followed by bore and ring wear..
Frequently an engine which is down on compression will be transformed simply by lapping in the valves and seats...Ian
email (option): ian@wright52.plus.com
['You totally surprised me with your reply "I can't tell you " Your technical knowledge I thought you would be able to tell me how many times the plate in a M20 horn vibrates in a second and the different decibel achieved at variable frequencies']....
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That what keeps this hobby interesting, you can never know everything about everything...
I've had numerous BSA singles (M and B Series) since 1970 and owned various examples continuously for the last 37 years...During that time I've gone out of my way to learn a much as I can, but I'm still going to need more time....
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Regarding compression I've never worried too much about the figures..Just about what is good and works well, what doesn't and when to do something about it!....Ian
email (option): ian@wright52.plus.com
good pragmatic answer Ian.
It's inaccurate enough on something with an electric leg, let alone a real leg.
If it smokes (on a SV) it needs rings.
If the compression seems lower than normal (whatever that is) and it doesn't smoke - check valves.
Otherwise 'best' way to check is with a 'Leak-down tester' - pressurize the cylinder and measure how long it takes for the pressure to drop ...
Or don't bother, until it stops .........its usually evident afterwards what the problem was....