Well, here's some good news! While this project is going slowly, mainly because my hands seize up in the cold weather and it's been sub-zero in the mornings for the last week, I today finally got round to installing the crank and cc'ing the chambers.
Now all of the aftermarket 1400 conversions use different Pistons and I'd at first thought that this would have to be because of radical differences in the combustion chamber volume to deal with the extra swept volume. I'd done a rough dummy up with just a piston filled with oil and flat-surfacing the head and was a bit worried that that was the case a s the CR did calc out as too high butI wasn't willing to just 'Guestimate' it in such an amateurish way so today I did the full dummy up.
CCV came in at 78cc. Bore is 104mm, stroke is 81.2mm
To calculate the CR you use the formula SV+CCV divided by CCV
Swept volume is 52x52x3.412x81.2=749.156 cu/mm
Divide that by 1,000 to convert to cc's and add 78=827.2cc
Divide by 78=10.6.
Winner, winner! Chicken dinner!
It would seem that the reason the AM kits all use different pistons is down to correcting the balance factor, not because of a different combustion chamber design! Now this makes sense if you want to simply offer a 'Bolt On' kit that doesn't require disassembly of the bottom end but since we're using a 1400 crank as well as the B&P kits it's a complete non-issue. The 1400's are balanced really nicely at the factory so to my mind it's actually easier and probably not a lot more expensive to just buy a crank and do the bloody job nicely!
Oh, a 10.6 CR is right on the money for this project