I am looking to buy brake calipers for a frame. I have read and understand the measurements needed for brake reach but wondered whether the mounting bolts are a standardized diameter and length or if this varies and if so can be switched out?

Rear brake bridge width = 35mm
Front/Fork crown brake bridge width = 42mm
Brake reach = 45mm

I mock fitted some Shimano 105 (1050 series) to the frame whose reach was just adequate but the mounting bolts were much too short.

TIA!

EDIT: looking for recessed brake calipers

There are nutted brakes and recessed brakes, perhaps that's the problem. I think 1050 only came in recessed. If the frame is drilled for nutted brakes, you'll have to drill out the rear fork hole and the front bridge hole to get the brakes to fit. Got pics?

Ah, thanks for the clarifying, forgot to include that info. Recessed brakes I believe. Not the greatest pic but mounts are in carbon fork and stays.

A-ha! OK.
They do make different lengths of recessed nuts.

https://www.universalcycles.com/shop...s.php?id=51088

Awesome, thanks!

If you buying new brakes, would wait till these turn up before buying extra parts front brakes will ship with 3 (if Shimano) bolts, and you choose the appropriate one, rears should be a standard size fitting (for all modern bikes), so only one is shipped (this has been the case for all the Shimano brakes I have purchased in the last 10 years, retail boxed, or OEM packaging.

It may be more cost effective/less headache to just buy a new pair then. Good to know.

Sometimes, if your fork isn't too bulky, there's a cheap and easy work-around.

Use the front caliper on the back. That lets you use an exterior nut to secure the caliper.
Use an extra long recessed nut to secure the rear brake caliper on the front. New brake sets commonly come with varying length recessed nuts so, if you can find one that works, you're good.

Drilling out the front hole rear brake bridge isn't as hard to do as one might think. You can't get a drill in there but I've grabbed the drill bit with my vice grips and rotated it by hand to drill out the bridge. Only takes a couple of minutes.