Tumbleweed,
Beezermacc clearly knows his stuff, no debate there. Knowledge of the numbers and markings inside the pistons is foreign to me and, I'd guess, to most. The problem I'm reading into this discussion is that, with your measurement method and accuracy, it is really impossible to give good advice. Using your flat ruler or scale, you've managed to correctly measure the bore diameters to within 1 mm. Not too bad, accuracy-wise, with what you have. But for the pistons you are off by over 2 mm using the same method across flat-top or dished pistons, per Beezermacc (meaning, no dome to go over).
The only reason I'm pointing these things out is that you appear to have now leapt to a decision to rebore and get new pistons. In my opinion (which, with the addition of 1-1/2 quid will get you a cup of coffee at Starbucks), you should get some really good measurements before making that leap. Of course, your machinist will provide that info when you go for the bore, but by that point you've probably committed yourself. At 40-over on the pistons, if the fit is actually right, you would not want to go up to 60 over, not just for the money, but because it is the last practical rebore value. Having spewed all that, my guess would be that your pistons are too loose in the bores, but it should be validated with good measurements.
Richard L.
