I've got Shimano BR5800 dual pivot calipers with recessed mounts. I need to put a 3-4mm bracket between my brake and fork. Is it safe to not have all the threads completely covered by the receptacle bolt when installed? I would have to back it off by about half. Is that safe? Btw, I will be using the space to mount a bracket for a friction drive, with top speed of 20mph, and regen braking, if that matters. TYIA for what I hope are yes replies =).

You can get longer recessed nuts long enough to cover the threads. However, I wonder if the brake mounting bolt is strong enough to absorb the force of that friction drive and if it's a suitable mounting point. At the very least get a long enough recessed nut to engage all of the bolt's threads.

If you have recessed pivot bolt nYou want to get a longer recessed pivot bolt nut. They come in different lengths

Ahh.....thank you. I didn't know they came in different lengths. That's the perfect solution, rather than trusting fewer threads. And, yes, it can withstand the motor; it's a commercial unit that has been fully tested. Thanks!!!

The usual amount of overlapping threads minimum is around 4 to 6. So as long as your nut has that many threads engaged on the bolt the mechanical strength should be good, of course having your safety system depend on the minimum might be questioned. BTW these nuts are made in varying lengths.

Now using this bolt, your brake mount, as the reaction arm fixing is another question. Andy

The motor mount also fixes to the forks and one other place, I can't remember. The brake bolt won't be stressed much. I learn so much on this forum, what a great resource! I just ordered a 30mm which should give me the extra room I need.