The thread in one of the postmounts on my Rebas has stripped. Fortunately the other one is ok. So, will one bolt tight be fine to hold the caliper without causing problems and if not what can be done about it?

helicoil.

Local framebuilder – engineer will be able to sort it. Will come out good as new,

I wouldn’t trust one bolt.

Half dozen zip ties?

Can you get it tapped and drill the mount out a bit bigger?

Buy a helicoil set and put one in, will cost about £25′, or take it to bike shop who will do it , better than new

So, will one bolt tight be fine to hold the caliper without causing problems…

One bolt done up extra-tight will strip that thread too, just like you did with the first one !

Sensible answer – one bolt might get you home in an emergency but no, it’s not OK. That’s why there’s two bolts to begin with.

Helicoil is the solution, or IIRC there’s a similar product under a different name that’s better suited to blind holes. Can’t remember what it’s called though.

Google to the rescue – it’s a TIME-SERT.

Have you stripped ALL the threads, or are there some that a slightly longer bolt would engage with?

Permanent/Proper fix is a helicoil though.

Cost me £35 for both post mounts to be helicoiled at a local engineering company. LBS wouldn’t touch them, they wanted to sell me new lowers.
The engineer said that they should be done when manufacturered.

I helicoiled mine with a kit from machinemart. I had never done it before but it was easy and held tight.

I “inherited” a Helicoil kit from a workmate who was retiring last week. 😀
4, 5, 6 & 8 mm sizes.