Are they interchangeable on older bikes? I have an old frame that I'm converting, want to know if I can swap the rear into the front, since the front's kind of shot. They're old Dia-compes from the eighties, I think.

The front bolt is usually longer to go through the fork.

What is "shot" on the front brake?
They probably just need some adjustment.
Enjoy

As mentioned, the front will have a longer bolt however the rear can be made to work. Simply use the recessed nut from the front and fasten it to the rear brake's bolt on the inside of steer tube. This leaves the brake attached to just one side of the fork.

I have one bike setup like this and it works fine.

or you could just take them apart and swap the front bolt into the rear brake.

that's what I did on mine.

Sorry; by "shot," I mean that it's corroded to hell. They both are coming from a frame/fork that I found in a junk heap. I'm just trying to salvage a front bake out of the two, rather than going and purchasing a new /new used one. Trying to do this on the cheap, as it were.

If you can't salvage the one you've got, give Bicycle Station over in Brooklyn a call

Really inexpensive but useful things you may find there.

QBP makes a problem solver nut that is longer than avarage which enables you to run a rear brake on the front with the nut and bolt going through both sides of the fork. I think it is about 5 bucks.

Like this? https://www.bikeusa.com/MERCHANT2/mer...ct_Code=B01500
It'll only work with recessed brake holes though. However, you could drill out the back of the fork to use a recessed nut.

Think this is what I was looking for. Now that I know I can, I'm gonna swap the mounting bolt. Cool beans.

Yup

What he said... worked for me- although it doesn't engage the threads more than maybe a third on an inch... but it should hold.