A little extra power

Working it out: BSA M20/21 basic engine alterations.
Cylinder capacity
 If you are contemplating altering an M20/21 to increase its capacity or compression ratio it is useful to be able to work out in advance the effect of the changes you are thinking of making. The most common alteration is to overbore the cylinder as this will produce quick results in terms of a bit more power. Increasing the bore size substantially will also bring the added benefit of a higher compression ratio. To calculate cylinder capacity in cc use the following formula:
3.142 x R x R x L where R is the radius of the bore and L is the stroke of the flywheels. So for a standard M20: 3.142 X 41 X 41 X 94 = 496cc. For an M21: 3.142 X 41 X 41 X  112 = 591cc.
Compression ratio
To calculate the compression ratio the volume of the cylinder head is required in cc. This is usually done by  filling the cylinder head space with a thin fluid, such as paraffin, from a graduated vessel containing a measured volume of fluid. Once the head has been filled it is a simple calculation to deduce head volume by deducting the amount of fluid left in the vessel from the original total. In the case of the M20  and M21, which have identical head volumes this figure is 127cc. To calculate compression ratio use the following formula:
V1 + V2 divided by V2  where V1 is the cylinder volume in cc and V2 is the cylinder head volume in cc.  So for a standard M20: 496 + 127 = 623 divided by 127 = 4.9~1. The same formula applied to the M21 gives a result of  5.6~1
These figures show the effect on compression ratio of the 95cc capacity difference between the M20 and M21 when the head volume remains unaltered but the cylinder capacity is increased.
M21 with a B33 piston
An easy alteration for the M21 is to fit a piston from the OHV B33 (of the correct type) This piston at the std. bore size is 85mm dia and can be fitted to a late un-linered M21 barrel simply by over-boring. Using the formula to calculate cylinder capacity the resulting volume is:
3.142 X 42.5 X 42.5 X 112 = 635cc. If this new capacity is now used with the formula to calculate compression ratio the result is: 635 + 127 = 762 divided by 127 = 6.0~1. so capacity has increased by 35cc and compression ratio by .4 of an atmosphere over standard
These un-linered barrels will take a bore size of up to 90mm quite safely so it is just a question of sourcing suitable pistons. From a practical point of view compression ratio in this type of engine should not exceed 7.1.
If the cylinder head joint face is machined to increase compression ratio the head volume must be re measured to ascertain its new volume before using the above calculations. The easiest way to do this is while the head is off the engine. Turn it upside down and set the gasket face level before filling with fluid as described  previously.