I have a 1963 A65 Star Twin I have the barrels off at the moment to check if they need a rebore. The barrels at present are standard, I have carefully measured the wear in the bore and the maximum wear which is at the top is.003"this is the difference between the bottom unworn part of the bore and the top. The manual
states that if wear is over.005" then a rebore is required, as mine are under that it should not need a rebore at present. I have also checked the piston clearances top middle and bottom this also confirms .003" maximum wear. I have a new set of standard piston rings but on trying them in the bore I find that in the unworn part of the bore I have a piston gap .015" and in the worn part of the bore it is .018". The manual
states that the piston gap should be.008" to .013" so the new rings are above that recommendation is this normal. I have also measured the cylinder bore in the unworn part and it measures 2.9525" the manual
standard bore size states 2.9521"-2.953"
Just to give some background on this bike, the pistons that are in now are the original pistons that it left the factory with in October 1962 it is a 1963 model and was not first registered until June 1963 these pistons have BSA cast into recess where the gudgeon pins go and the number on the otherside is 68-251. The bike has done approx 13,500 miles from new. when I bought it some 6 years ago it had stood dismantled for over 30 years,it only did 4,000 miles in the first 12 years of it's life.