The very, very small ittsy, bitsy hole for the pin is .040 inch in diameter. The pin itself is larger for a very tight press fit. I just pulled one off of a No 1 MKIII rear sight for put on one that has a significant number of stops on the windage wheel. The original has 4 stops total. I made one that has 16 stops total. I believe the rear sight for the No.1 MKIII is the best ramp rear sight ever put on a military rifle. I just put this one on my K98Mauser which I assembling in .22LR using a .22LR Springfield M2 front bolt section.
--fjruple