I'm looking to put a new Shimano rear brake caliper on an old Moto frame from the 70s. The pivot bolt doesn't fit through the frame. Does anyone know any tricks as to how to do this. The guy at the local bike shop said to drill a larger hole in the frame, but before I attempt that I'd like to know if there are any other options.

Its probably a bolt on brake. Also, it *may* be long reach. Drilling holes is something people do to attach allen bolt type brakes to older frames. I've heard people on the C&V section do that a lot. With my luck, I'll probably break the bridge

It is an Allen bolt type brake, and modern. It does concern me that the bridge might break, although I'm not sure what else I'd want to do. I really don't want to put a center pull brake on it.

So what exactly is wrong with the old one? (a picture is worth a thousand words )
You probably have trouble with centerpulls as well cause it won't have a centering cable guide somewhere.
You can probably get an older bolt on type brake somewhere. Have you tried loosescrews or Niagra cycles? They have weird stuff on stock.

Can you exchange the rear caliper for a front? That way you'd have a longer bolt that would reach all the way through the brake bridge without having to drill anything.

I mounted modern dual pivots to my 70's Moto a few months ago. I ended up mounting the rear brake up front and the front brake in the rear. This allowed the longer bolt to stick through the brake bridge for easy mounting (in the back). Since the front brake now has a shorter bolt, you simply install the nut throught the bottom of the fork crown. Once it's threaded you can tighten it through the hole in the rear as you normally would. The last trick is to swap brake cartridges so that they are oriented in the right direction. You wouldn't want the pads to slip out under heavy braking

And you will need long reach brakes, no doubt.

The problem that I have found with modern Shimano brakes is that they have the same length screw for both front and back; the extra length for the front comes from a longer bolt, which will not fit inside the frame hole. I suppose I could try to go with another caliper brand, although I have already bought a Shimano. What kind of calipers did you put on your Moto rvdaddy? In answer to triplebutted's question, I really just don't want the extra weight, the unsighlyness and the hassle of working with a centerpull. They're terrible to set up adjust and they, in my experience, are not nearly as reliable.

When the C&V people (including yours truly) do this we drill out the fork for a recessed bolt. I've not heard of drilling the rear arch and would be quite leary of doing so.

That's the way to do it.

As always the late and missed Sheldon Brown has the fix for us. I have used his technique on 2 older frames. It's as easy as he makes it sound.

https://sheldonbrown.com/gloss_ra-e.html#recessed

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