What compression will this come out to..

more size will not give you more compression, it will burn more fuel to give you more power.

compression has nothing to do with the size of bore.

IT can get pretty complicated. When the volume of the head is provided it probably would be the volume that you would have if you bolted the head to a flat plate and then filled it with water through the spark plug hole. There may be an additional volume that must be factored in if the piston does not come to the very top of the clyinder or if the piston top is not flat.

The displacement is the bore times the stroke, which on a 125 cc might be something like 124.5 cc.

The compression ratio is the volume at bottom dead center divided by volume at top dead center. The volume at the bottom will be the displacement plus the head volume plus some unknown, while the volume at top will be the head volume plus the unknown.

So, the equation will look something like this: compression ratio = (124.5cc + HeadVolume + Unknown) / (HeadVolume + Unknown).

If you knew the compression ratio was 12:1, and the volume of the stock head was 7cc, then the equation becomes 12 = (124.5cc + 7cc + X) / (7cc + X)

Solving for X , X = 4.3 CC

Now, if you swap out the 7cc head for a 5 cc head the equation becomes

CR= (124.5cc + 5cc + 4.3cc) / (5cc + 4.3cc)

CR = 14.4 : 1

High octane race gas, for sure!

Note that all that was based on guesses for the actual stock compression ratio and stock head volume.

Rod

But to really complicate things, 2-stroke compression ratios are generally not calulated as static compressino ratios that use the BDC volume divided by the clearance volume. Instead, they are generally calculated with a dynamic compression ratio which takes the volume at the beggining of compression divided by the clearance volume. Compression doesn't begin until the exhaust port is covered by the piston.

Don't ask us, ask a pro.