Harvey anwered Peter: <snip>...simple jig to do this...Now, I'd do it differently: 12" pipe set on flange screwed to board- hardware store items. Brass washer on top to minimize marring the crank arm. hope this is clear.
In the early and mid-70s, when I improvised a lot, I changed out crank lengths twice a year by using a large C-clamp and a 10 or 12mm socket with bandage tape on the socket and the thread-end of the pin. It always worked and never damaged a crank, pin, or socket; although, it may be why I have arthritus in my hands today.
Tom Donahue Melbourne, FL
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