A 1966 18GB 5 Main Piston, if original from BMC will have a number 1
thru 5 stamped inside a diamond. This symbol denotes the cylinder wall
machining tolerances to a matching piston. In addition, if the block
hasn't been decked, there should be a cooresponding number in a diamond
on the block deck adjacent to the bore. There can be as much as a few
thousandths difference between diamond 1 to a diamond 5. In some cases a
standard piston can be used if you have the lowest toleance daimond 1.
Suggest you have the cylinder bore and upper ridge inspected to
determine total wear and out of round. If machining is required, a good
machinist will match the correct bore size to the piston. If your bore
is slightly worn, I know a dealer who has NOS original BMC +0.010
pistons for approx $ 150. I hope he still has them.
Luck: Rich Boris 67B