Pistons are cut on a barrel shape looking at them from the side as well as an oblong shape looking at the piston from the top. The area on the piston where it is the largest diameter is 2.100" down from the top of the piston. Use a dial caliper or electronic caliper to span across the diameter of the piston and find the number. Let's say that the piston measures 4.002". They're telling you to add 0.001" to that and then bore and hone the bore to a finished dimension of 4.003" for instance.
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