threads dont just strip unless something its screwing into causes it,
ive used my echo SL bolts on loads of different types of BB all being 15mm and removed and re-fitted them loads of times and theyve allways gone straight in,
do the bolts not screw in to either side of the bb or just 1 side?
if its just 1 side it might be that thread on the bb
if its both sides its probably the bolts being wrong size thread
try the 'process of elimination'
but ive uve striped 2 sets of bolts its probably the BB
just a thought