Originally Posted by
alias5000
.....
It's as if the middle ring teeth are too close to the large ring
This is easy to fix with some homemade paper shims.
Start with a sheet of paper and punch or cut 10mm holes into it. Then cut out the holes to make shims. FWIW- I've done this often enough that I made a punch from a steel chainring bolt ---- sharpened to the outer edge by holding against a conical grinding burr. Once you've made the shims, soak them in oil (any oil) so they cannot later absorb water and they'll outlast the crank,'
From the description, I suspect that you'll need something a bit thicker, ie. a postcard, cereal box, etc.