Need to order new bolts for disk break calipers – trying to work out the right length. the old ones I had were wrong length (spaced with lots of washers) and are now rusty.

Does anyone know the depth of the thread on post mounts? Is it uniform for all forks and frame?

Assuming you don’t need an adaptor, go into your LBS & talk to them nicely, they’ll more than likely have loads sat redundant in the workshop

Different for everything, i’d aim for about 10mm thread engagement, possibly a small amount more, but wouldn’t want much less.