bent crank spider?
 

I got a used Shimano 105 group for a song, and everything's working great except that the SPIDER of the right crankarm is bent, so the chainrings wobble. I'm definitely sure that the chainrings themselves aren't bent, have laid them out on a flat table and there's no obvious problem, but when I put them on the spider and tighten 'em down, it doesn't lay flat...

Any tips on fixing this? I imagine I could bend it back into shape with a vise, but what part would I want to try bending? I dunno how to align it properly other than by trial and error, which could be very tedious...

I guess you know why it was so cheap... :(

can you take the crank and lay it on a flat surface as well, to check which spider arm is wonky? Also, double check the mating surface of the crank and the taper, to make sure they are going on perfectly straight. And while you are at it, check to see if the BB spindle is bent! It wouldn't take much of a bent spindle to see a mm of deviation at the top of the chainring.

I recommend finding something super-flat, like a granite counter top or something, it'll help.

peace,
sam

If the crank arm is bent, this can lead to knee trouble since your pedals will wobble on every rotation.