I want to fit a Shimano 10spd chainring onto a square taper crank. However, the rounded plastic corners around the chainring holes:

foul on the cranks.

Has anyone ever flattened them off with a file? Is the metal underneath curved to the same profile?

I’ve filed cranks to fit rings where they shouldn’t go… you’ll be fine taking some off those, how much you thinking of removing?

yes, well 9 speed SLX but yes there’s metal underneath, get on with it.
Done with lots of ‘normal’ deores too.

how much you thinking of removing?

A quick measure suggests about 3mm (which is pretty much, but not quite, where the plastic covering ends)

Yes it’ll be fine. But please file the rings, not the cranks!

Cool. I’ll crack the file out then.

*wishes he had a proper vice…*

But please file the rings, not the cranks!

Just wondering why?

The older cranks spider arms have a small ‘lip’ for the chainring to sit on, but the newer ones that take the composite rings don’t have this. So my thinking is that if the newer ones have done away with it, what’s the harm in filing it off the old ones? And, surely its better to file the cranks once, than the ring each time you replace them?

I filed it, it worked 🙂 Turns out it was pretty much all plastic that needed to be removed so it was pretty easy.

Cranks are generally more expensive to replace than rings (and last longer). Therefore if Im going to remove metal from something, I’ll do it on the cheaper item, shorter lifespan item.

but you only nerd to do it once if you do the cranks.