roccobike

I've got a bike with an old, cup and cone bottom bracket that is having a little problem keeping the left side crank attached. I would like to really lean on the tightening nut, but I'm not sure how much torque I can apply and don't want to risk breaking the BB. I searched BF and Sheldon Brown, but could not find the answer. Does anyone know how much torque I can apply to the retaining nut? If not a number, just a general how hard can I 'lean' on this thing?

silversmith

If you are tallking about keeping the crank attached to the spindle, then I'd worry that you need a new crank, at minimum. If the cranks are aluminum then its even more likely that you need a new crank.

The problem is that when a crank gets loose on a spindle the wobble wears/beats away the metal in the area ofthe crank that interfaces with the very hard steel of the spindle. It doesn't take long for a loose crank to eat itself up.

Retro Grouch 

The torque spec is around 30 ft/lbs. Imagine a 30 lb. weight hanging on the end of a foot long wrench.

Unfortunately, I agree with silversmith. I suspect that your crankarm is hosed. The good news is that your LBS can get you a replacement for around $20.00 if you aren't too picky about having a perfect match for the right side.

roccobike

Thanks, I may be wrong, but I don't think the crankarm is toast. For some reason the darn nut keeps backing off, then the crank becomes loose. When I first attach the crank, it's a tight fit. I think I'm not putting enough torque on the nut. But I didn't want to lean on it and snap the threads off the bottom bracket. I've got a torque wrench and will torque it down with some loctite on the threads and see if that works. Thanks, again.

silversmith

I may be wrong, too. But the crank fixing nut self-loosening is a classic sign of a crank whose spindle interface has become enlarged.

I bet if I look I've got a Sugino or SR left crank (may have cosmetic issues) in my parts bin. I'd have to check size and models. See what you need and myself or someone here may be able to help you out.

DannoXYZ 

Tighten the nut again, then remove it. See how deep down the hole the flat outer edge of the spindle is. If it's flush with the surface of the crank that the nut rests on, the crank-arm is hosed. It got overtightened and expanded in the past, or it was repeatedly wobbled loose and buggered itself on the spindle. What ends up happening is the wobbling crank behind the nut loosens the nut and it comes off...

With a perfectly clean interface on new spindle and crank, you can actually remove the bolt/nut completely after torquing to proper-spec and the friction will hold the parts together for a LONG time, weeks or months of use. Here's a list of torque-specs for common parts on a bike, crankarm bolt is typically 25-33lb*ft depending upon the model: ParkTool - torque specs.

Also when you tighten the bolt, grab the OPPOSITE crankarm, that way you don't cock or mis-align the mating surfaces on the crankarm you're installing.

roccobike

Good point Danno, I tried your recommendation. The arm was not flush with the BB surface. Apparently I needed a larger wrench with more leverage than I had been applying. I never had this happen before, but then, this is a low end crank.
Thanks for the advice, it is much appreciated.

San Rensho 

I would only add, grease the spindle liberally, make sure the threads are perfectly clean and oiled and oil the washer and bolt head.

You should be able to tighten it easily by hand. If as you are turning the wrench it comes to an abrupt halt, then you know the crank arm is gone.