Bicycle Mechanics - will this work? Crankset replacement

Just after some assurance from those with deeper skills and understanding before I jump into this and start ordering parts.

I am replacing cranks/chainrings on a 1992 Trek 820. I was hoping not to replace the BB, but when I pulled the cranks to measure the spindle and BB shell it was obvious that the bearings are in rough shape. Not loose, but growly. Also the spindle is 122mm( I guess 122.5) and I don’t see anything that uses that for sale.

I am trying to maintain chainline with middle chainring 50mm from centerline.

I am replacing Shimano Biopace 48/38/28 and square taper BB with 122.5mm spindle in 68mm shell.

With

Shimano Alivio FC- M410, 42/32/22 square taper and recommended
Shimano BB UN 54 68/118.

I am assuming the recommended 118mm spindle length is to maintain typical 50mm center ring chainline. Am I right?

Are my assumptions correct? Is this going to work?

Am I crazy not replace cassette and chain at the same time? I think I can still get a 7 speed cassette to fit.

Thank you for your help. People on this board are amazing!

Just after some assurance from those with deeper skills and understanding before I jump into this and start ordering parts.

I am replacing cranks/chainrings on a 1992 Trek 820. I was hoping not to replace the BB, but when I pulled the cranks to measure the spindle and BB shell it was obvious that the bearings are in rough shape. Not loose, but growly. Also the spindle is 122mm( I guess 122.5) and I don’t see anything that uses that for sale.

I am trying to maintain chainline with middle chainring 50mm from centerline.

I am replacing Shimano Biopace 48/38/28 and square taper BB with 122.5mm spindle in 68mm shell.

With

Shimano Alivio FC- M410, 42/32/22 square taper and recommended
Shimano BB UN 54 68/118.

I am assuming the recommended 118mm spindle length is to maintain typical 50mm center ring chainline. Am I right?

Are my assumptions correct? Is this going to work?

Am I crazy not replace cassette and chain at the same time? I think I can still get a 7 speed cassette to fit.

Thank you for your help. People on this board are amazing!


Yes, shimano's spindle lengths for mountain cranks are for a 50mm chain line.

Yes, you'd be crazy to not replace the cassette and chain. There are still seven speed cassettes available, but they're not common.

Another thing, even if you find something slightly shorter than 118 like a 115 for example probably it will work anyways. The difference is not that big eventho it looks big, but 3 to 5 mm shorter BB axle are only 1.5 mm each side in a 3 mms shorter BB or 2.5 mm shorter at each side in a 5 mm shorter bb.

Yes, if you can dig out the Shimano tech doc for the FC-M410, it should list BB spindle length for 50mm (at least) chainline (I'm not sure why you're uncertain about this, as "recommended 118mm" implies you already have the tech doc). You'll want to make sure you get the correct crank arm length for your leg length (170mm or 175mm; 175mm seems to be by far the most common one). In the very unlikely case that you've got an E-type front derailleur mount,
you'll also want to make sure the BB is correctly sized for an E-type front-derailleur.

I'd certainly put in a new chain (8-speed; the SRAM ones have a master link included, unlike the Shimano chains), and a new cassette should be pretty cheap. If the current cassette is really worn, it'll be pretty obvious once you install the chain.