My daughter has an old MTB that she's using in college as her beater bike (the "Bix Beiderbecke Beater Bike" for you jazz history buffs). I want to upgrade the crank because the stock triple crank is junk and I can't get the BB bearings adjusted properly in any event. I don't think that's going to be a problem. I have Bontraeger triple with BB taken off an '08 Trek 1.2 that I'd like to put in there.

But I'm not sure if it's compatible with the 7-speed cluster that's currently on the rear wheel of the beater bike. Since the derailer is a Suntour, I'm assuming that the cluster is also a Suntour. The Bonty worked with an 8-speed Shimano cassette. If they are compatible, would the chains also be interchangeable?

I know I didn't explain this very well, but I'm hoping the answer is obvious to someone.

Changing the crank should have no effect on how the rear derailleur works. Assuming that the rings are the same, you would be able to reuse the chain, but I would not. An inexpensive SRAM chain with its master link makes it simple to install (after sizing), and should give good service. If the chainrings are not the same size, you might need to replace the chain anyway, so why not just 'bite the bullet' and do it anyhow?

This all presupposes that the BB coming out and the one going back are both the 'standard' English threading, of course. If the large chainring is more than 2 teeth larger/smaller, you will need to raise/lower the front derailleur for the best shifting.

Thanks. I'm sure they're both English threads. Both made by American companies. The chainrings on the old bike are 48-39 and I'm guessing 30. The newer Bontrager crank is 50-39-30. So sounds close enough. The shifters are actually Shimano, so I don't know what's up with the Suntour RD, but I'm going to give it a whirl and see how it works. I have that chain that was on the Trek. It has less than 300 miles on it, so I'm guessing that's good to go as well.