Originally Posted by
fietsbob
Buy new parts, you said ...
I can only get them used and it seems unlikely that the bolt was weak. My plan is that when the new, to me, bolt arrives is clean stem inside and roughen with sandpaper in horizontal direction and do the same to the handlebar clamp area, tighten and hope. These bolts are Whitworth. If this does not work I will go with the closest undersize grade 5 bolt and failing that grade 8. Since this is a vintage bike I hesitate to use English or Metric bolts but see no other choice.