Loose crank...replace bottom bracket or the whole shabang???

[kaian]

Several months ago I picked up a 1986 Miyata 710 and started "restoring" it. I had a couple of friends help me take it all apart, re-lube and adjust everything on it. After doing this, I noticed that the crank was loose. We tightened it, and it loosened again. Upon closer inspection, it appears that the problem is with the bottom bracket. If you grab one of the crank arms and shimmy it, the whole thing (crank and bottom bracket) will rock back and forth. We tightened as much as we could (still allowing for it to turn) and it still has some play in it.

Would replacing just the bottom bracket fix this issue or is it possible the crank itself is damaged and the whole thing should be replaced?

[smilingcat]

1986 is getting to be bit old. Few things have changed since that time.

head set size changed, went from quill to threadles steerer tube+stems, to name a few.

Your bottom bracket might be in luck though. From the sound of things, it may be the races on the BB is worn out and so has the spindle. Your crank is attached to the spindle so the crank itself is probably okay.

Just make sure that when you replace the BB and the spindle make sure the width of the BB is compatible and the threading is also compatible (you have 50/50 chance on threading) and that the spindle is compatible with the crank. Spline on the spindle has changed over time as well. BB comes with a new spindle.

Send a PM to DebW, she knows a lot about bikes.

smilingcat

[kaian]

Yes, I sent a PM to DebW earlier today about another bike and figured that she might chime in on this posting, too.

I read some threads on bike forums that suggested that if you ride a loose crank that it can damage the crank itself, so...I was just concerned that maybe the small amount of riding I did (maybe 30 minutes total) on a bad BB could have damage the crank and now it will no longer stay tight. Of course buying a new BB would be a more simple solution, so I hope that's all it is!

[DebW]

Sorry for the delayed response. I've been out of town.

I would assume that this is a square-tapered spindle from 1986. Sounds like you might get away with replacing just the bottom bracket. These came in English, French, and Italian threading, so you need to match the cups with the frame dimensions and threading. Italian has right hand threads on both sides, English has left threads on the left side. Miyata is Japanese and I think they used English spec bottom brackets. Spindles also come in different lengths for double or triple and sometimes smaller increments to improve the chain line. Did you inspect the bottom bracket when you had it apart? Is there pitting or scoring on the bearing faces? Are the cups worn excessively? Are the outer flats where the crank attaches still flat and smooth?

If you simply told me that the crank had play, the most likely reason would be that the crank-spindle interface was damaged because somebody rode the bike without tightening the crank arms on properly. If the whole bottom bracket is moving, then that's not the (only) cause. Either this bottom bracket is badly worn or parts of it are simply the wrong size (and probably very damaged because of that). I'd suggest replacing the entire bottom bracket. You might end up with a sealed cartridge bottom bracket just because they are easier to find these days. If the flats of the old spindle and the cranks show no damage, you'll probably be fine with fitting the old cranks on the new spindle, but you'll have to try to be sure. If that doesn't work and you end up needing a new crank anyway, then you won't be restricted to square taper bottom brackets, but you'd want a crankset that matches a 6-8 speed cassette because chain widths and chainring spacing has changed over the years.