Thanks L18 but I've already got the pistons and am running out of time and money to keep returning new parts. I wish I'd known this a couple of weeks ago!!!

Here's my compression ratio calculation. I just took a guess on the space between the top of the piston and top of the block at 0.5cm.


Original Pistons:
7.6 x 7.7cm => 349.3cc displacement per cylinder

Compression Ratio = 8.5
displacement volume + combustion chamber volume
= 8.5 x combustion chamber volume

dv + ccv = 8.5ccv
dv = 8.5ccv - ccv = 7.5ccv
ccv = dv/7.5 = 46.6cc per cylinder

original combustion chamber volume = 46.6cc

New Pistons:
7.7 x 7.7cm = 358.5cc displacement per cylinder
dish is approx 5.7cm across, .12cm deeper than stock
added combustion chamber volume due to dish depth is
3.06cc

Added bored area due to oversize = 1.21cm^2 (need to know
space between top of piston and top of block to calculate
added volume due to this area difference)
WAG of 0.5cm space so bore volumem above piston = 0.61cc

so new ccv (combustion chamber volume) is
original ccv + deeper dish volume + added bore volume above piston

46.6cc + 3.06cc + 0.61cc = 50.27cc

so new Compression Ratio = (50.27 + 358.5)/50.27
= 8.13

Ignoring that little volume (0.61cc) due to the space between the top of the piston and the block it would be around 8.22

That is way too low - I'd rather be closer to 9:1. Crap.