I have measured my BB shell width to be 73 and I am using a OEM isis bb and cranks from another bike. (113 spindle length)

Recently bought a second hand frame from here and there are loads of threads (although looking a little rusty). It gets to 3mm from sitting flush then doesn’t seem to want to tighten up any more.

Potentially a problem with the rust (steel frame) in there? I don’t really want to force it as I know naff all about threaded BB’s as my octalink BB seems to be indestructible and my bb30 bearings are removed with a hammer and installed with a g clamp. Currently only hand tightened with the tool.

Are you installing the drive side first? Its ok to partially thread in the nondrive side, run the drive side in tight, then finish the nondrive side.

The nondrive side does not need to sit flush; there are often a few mm of thread sticking out beyond the BB shell. The drive side is flanged and needs to be flush.

It could be rust in the threads if the new cup is longer than the one it replaces. If cleaning with a wire brush doesn’t work, a LBS should be able to chase the threads.

Use grease or antisieze on the threads!