Bottom bracket drive side differences

[figbat]

I took the slightly loose square taper bottom bracket out of the venerable Trek today - I’d guess it’s the original from 1997 and it came out with no fuss at all. Good start. It is stamped Shimano BB-LP26, 73, 1.37x24.

Then I fitted the replacement, a Shimano BB-UN26, 73mm, 110mm spindle. So far, so good, but when I went to tighten the drive side crank arm it bottomed out on the body of the BB before getting fully tight. When I then measured the protrusion of the spindle from the body of the BB the new one is 2mm shorter than the old one. Both spindles measure at 111.6mm overall.

Is there any way to be sure of getting a BB with an appropriate drive side clearance? Is there a way of checking the tapered hole on the crank arm to see if that has deformed? Any other thoughts?

 

[Venod]

Try the non drive side crankarm to see if that tightens, if it does the taper in the drive side one may be worn.

If its worn a new one is the best bet, but you could use a shim to take up the slack or a run of weld on the taper and file back to size.

 

[figbat]

Ooh, good idea on swapping the cranks. They aren’t that old and have never been loose, but they were cheap too so could potentially have deformed.

 

[Cycleops]

Very unusual for the cranks to be worn but your shim suggestion should work.

 

[scotsbikester]

Shimano UNxx series bottom brackets are not necessarily symmetrical.

See this thread from another cycling forum:

https://forum.cyclinguk.org/viewtopic.php?t=105385