what is a good compression figure?

[PCMAX]

Roughly how many PSI should a new cylinder/piston put out? Conversely below what PSI should you start to consider a rebore or new set of rings?

[Nathan S]

You won't get a good answer with that little info.

4T or a man's bike?
Trail-bike or MXer?
Big bore or 125?

In general terms:
4Ts will run higher figures than 2Ts.
Higher performance motors with more aggressive porting/cams will have lower cranking compression than mild motors. This will usually be offset to some extent by higher compression ratios.
Small bores will have less compression than big bores (assuming similar comp ratios).

[PCMAX]

You won't get a good answer with that little info.

4T or a man's bike?
Trail-bike or MXer?
Big bore or 125?

In general terms:
4Ts will run higher figures than 2Ts.
Higher performance motors with more aggressive porting/cams will have lower cranking compression than mild motors. This will usually be offset to some extent by higher compression ratios.
Small bores will have less compression than big bores (assuming similar comp ratios).

Standard set up 2T 125mx & 250mx

[Montynut]

Small bores will have less compression than big bores (assuming similar comp ratios).

Are you sure about that Nathan? If they had the same compression ratio I would have thought the pressure would have been the same but the volume would have been greater in the big bore.

[bazza]

145 ??

[Lozza]

Roughly how many PSI should a new cylinder/piston put out? Conversely below what PSI should you start to consider a rebore or new set of rings?

Can vary so much with just the type of gauge and the length of the hose, checking the installed head volume and a leak down test is far more reliable.

[Nathan S]

Small bores will have less compression than big bores (assuming similar comp ratios).

Are you sure about that Nathan? If they had the same compression ratio I would have thought the pressure would have been the same but the volume would have been greater in the big bore.

The inevitable leakage is a larger portion of a small motor's compression - think of the way a worn out 500 will start and run far better than a work out 125.
Plus the bit where small motors are typically in a higher state of tune than their bigger counterparts - ports/valves that close later for high RPM performance, that cost compression at cranking speed.