Hi Nigel,
The measurements you have taken on the good piston seem OK,
Pistons generally are tapered, but they are oval as well, then they are stepped down in diamater between the rings nearing the crown.
In general terms the only relavent measurement that needs to be taken is at 90 degrees to the pin at the Bottom of the skirt,
This measurement should be the nominal bore size less the piston clearance, ie, 71mm minus 0.1mm(0.004in) for example
Stick the piston in the bore in the running orientation and an accurate measurement can be done with a feeler gauge.
With split skirt pistons the clearance is normally smaller than solid skirt, that said my Ariel 500 single 81.8mm bore has a split skirt piston and runs on 0.001-0.003in. clearance
My Super Rocket with solid skirt pistons is running fine with 0.003in (GPM pistons)
If you buy new pistons, the bore size will be marked on the box, not the piston size if you follow me, so have your engine shop finish the bore to that size.
Nigel You will have to believe Me and start looking at the oiling system in your bike ( I know the bore sizes appear or are different)
But the fact that the cam followers wore so badly and then its the left hand piston that siezed makes me very sure thats there is an underlying lubrication problem.
Triton Thrasher, its not an easy thing to attach an oil pressure gauge to an A7/10 engine, neccitating drilling into the crankcase to the oilway inside the pressure relief valve, its a bit easier on SRM type end fed conversion engines, by drilling in to access the end of the pressed in oil pipe, or into the outer cover
Regards
John O R