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I've got 27x1 1/4 wheels in a frame intended for (I think) some sort of 700C. Clearance with a Michelin World Tour at 75psi is around 2mm at the caliper and grit very occasionally gets into the gap - exciting noises and some definite braking effects as it gets dragged through, but no locking up yet - I'm guessing the size of grit or road dressing that might lock the wheel is either too large to be carried in the tread or it gets knocked off by the caliper.
The 'inside' of my rear caliper looks a bit hard used, but in terms of safety it's probably less of a problem that the fag paper clearances found at some fork crowns? Possibly use mudguard clearances as a guide if it worries you?
Alternatively, try reading this by way of reassurance:
https://www.lfgss.com/conversations/288796/
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MrE
That's the polite version! Butseriouslytho... bits of grit and stuff picked up off the road will obviously scratch up the brake and if I break a spoke it's more likely to rub than a caliper with more clearance, but other than that is there a safety issue? Locking up the back wheel if a really big stone got stuck in there would be bad, but not as bad as if it were the front wheel.