This is really frustrating. My outer chainring is a 110BCD. My middle chainring is a 110BCD. I am using a MTB triple crankset on a fixed bike with one ring. I am trying to improve my chainline, by moving the outer ring to the middle position, but it won't fit.
Both rings use a BCD diameter of 110. Shimano has proprietary rings such that a MTB outer 110BCD ring is not the same shape as a MTB middle 110BCD ring. Therefore, a 110BCD outer ring will not fit in the middle position both rings are 110BCD. Hypothetically, you could go out and buy a 42T outer ring for a Shimano MTB crank and try to use it in the middle position, only to find that it won't fit in the middle position.
Is it safe to file down a small bit of the chainring near the bolt hole so that it fits on the crank? For example, I want to file the parts of the spider furthest to the inside of the chainring, but being careful not to file any part of the hole.