Hi graham
would be worth looking at the cylinders are they already anodised or raw alloy?
If they are raw uncoated then the anodising ads about 0.02mm to the surface if its hard ano finish. I think cosmetic (tarty colors) is thinner.
This usually has to be accounted for in the design, I am thinking the pistons may be tight with the ano finish after coating.
Better info here
GROWTH DUE TO ANODISING
The anodising process converts aluminium into aluminium oxide Al2O3. This species has a volume roughly twice that of the aluminium from which it was formed, this gives rise to an approximate growth of half the applied anodic film thickness.
For example a 50 micron film will use up 25 microns of aluminium in its formation, thus it will create growth of only 25 microns per face.
An internal diameter will decrease by 50 microns if such a film is applied, an external diameter will increase by the same amount.
This must be borne in mind when making allowances for growth:
Remember-growth is approximately equal to half the applied film thickness per face*
A special case that needs discussion is that of growth on threads:
Although the film grows on threads in exactly the same way as it does on anything else, the thread angle causes an effective growth, which means that a greater allowance needs to be made for threads.
The effective growth on diameter can be calculated by the following formula: