MTB bottom bracket and chainset problem

[John]

I have a Trek 4500 MTB from about 2003 that I've been servicing to use as a commuter bike. Changed all the cables, brake blocks and chain. Chainset was bent so bought a SHIMANO FC TY501 Square 6/7/8 Speed Chainset and replaced the original 73/113 square taper bottom bracket with a new Shimano UN55 73/113.
When I fitted the chainset to the BB, the chainset fouled the BB shell - turned out to be 4 protrusions on the inside face of the chainset, presumably designed to stop the chain falling between chainset and BB shell and jamming. So I flattened them with a hammer  and all is well.
Except.... the chainset now sits so far inboard that I can't get the (new) derailleur to go far enough in to stop the chain rubbing on the inner plate of the mech when in low/low gear. Seems that the BB spindle isn't long enough?
So - is it that the chainset isn't designed for use on a MTB? (turns out to be a "trekking" chainset) or is there some difference in the BB despite the measurements being the same as the one I took off? (I've checked the existing and it has the same 73/113 marked on it).
Do I need a new BB - 73/122.5 maybe - or do I replace the chainset with a MTB specific one?
Many thanks for any help!

[LittleWheelsandBig]

BB lengths should match the crankset, not the frame. Different cranksets need different length BBs. You need a longer BB. Check what Shimano recommends for that particular model crank.

https://www.treefortbikes.com/Shimano-Tourney-TY501-6-7-8-Speed-Crankset-28-38-48t suggests 122.5mm

I do not know if the chainring sizes are the same for your new and old cranksets. If there is a big difference in the big chainring sizes, then the existing front derailleur may not work even if the chainrings were in the correct position laterally.

[Davef]

I occasionally fit a mountain bike shimano hollotech chainset to my cx bike (to give me 36/22 chain rings for occasional silly stuff). I have to fit a couple of spacers (essentials large plastic washers) to get the alignment correct and take up the slack. This only applies to hollow tech style of bottom brackets. I was skeptical at first. It seems fine.


Sent from my iPad using Tapatalk

[Lightning Phil]

I occasionally fit a mountain bike shimano hollotech chainset to my cx bike (to give me 36/22 chain rings for occasional silly stuff). I have to fit a couple of spacers (essentials large plastic washers) to get the alignment correct and take up the slack. This only applies to hollow tech style of bottom brackets. I was skeptical at first. It seems fine.


Sent from my iPad using Tapatalk

Different type to square taper and inserting washers not relevant.

[rogerzilla]

Is it definitely a 73mm shell?  They are not all that common on 21st century bikes.

[Bolt]

I would measure the chainline with the original chainset fitted and work out what length of BB is required to achieve the same chainline with the new chainset fitted.

[Torslanda]

Or simply take it on trust, I fit A LOT of TY chainsets.

You need a 122 BB.