Originally Posted by
JohnDThompson
If possible, buy two front calipers. Drill out the back of the fork as FBinNY suggests, and use the other on the rear with a standard nut instead of the allen head -- no drilling needed!
This is good advice because drilling out the rear is next to impossible. Using the front brake as the rear solves the problem nicely for the cost of a regular 6,, nut and washer.
If you have to pay much more for 2 fronts vs. a normal pair, you can usually get by with a pair, and use the rear up front. The bolt will be just long enough to clear one wall of classic steel forks, (measure the crown wall thickness from the bottom and confirm that the rear bolt is at least 6mm longer than that). You'll need a longer nut to reach, and the length will be so critical that you'll probably need to buy an overly long one and saw or file it to a custom length.