Real quick dumb question
 

Ok, so I'm not the most knowledgable about threadings and stuff - here's my question:

I have exactly 4 crank arm bolts (the ones that hold the crank to the BB) between three bikes. One bike is a MTB, one a road bike (fixie conversion), and one a track bike. I know that I can use the bolts from the track bike on the road, which is what I'm going to be doing tonight. However, I'd like to be able to keep the track bike built up for better weather days over the winter. Later this week I'll get an extra set of bolts for whichever bike doesn't have them after tonight...

But can I use the bolts out of the MTB on either the other bikes? I'm probably going to try it, but I know that sometimes bolts can thread OK but be just a <b><i>little</i></b> bit off and end up stripping things down the road.

Any thread gurus would be welcome. I believe that they're all cheaper Shimano sealed cartridge bearings...

Thanks,
hereNT

If they screw in easily by hand you should not be concerned. A very lite wrenching might raise a frisson of anxiety but if it goes easily with a finger or two assisting the wrench I would not be concerned. Thread incompatibility will manifest with marked increase in resistance within 2-3 turns. Just don't over torque it at the end, those bolts can stretch and break, especially older low grade crank fixers. There is some variance in head hex sizes which can cause interference when the head goes into the crank puller thread area as you need a socket wrench to tighten and if the hex head is too large the socket wrench has no clearance. This is uncommon but I have experienced it.
Steve

All of the ones that I've seen have been the same thread. There might be some obscure BB's around somewhere.

Those bolts require a good amount of torque, typically 30 to 35 ft/lbs. Imagine a 30 pound weight hanging on the end of a foot long wrench. I always use a torque wrench because, it's a bike part that left to my own instinct, I would undertorque. I suspect that lots of other guys do too. If you scan this forum for a while you will read many posts about crank arms coming loose and being ruined due to undertorqued bolts.

Shimano uses two crank bolt sizes and they are dramatically different in diameter. The older square taper bottom brackets all use 8mm x 1.0 mm bolts. Newer splined cranks (Octalink) use a 15 mm diameter bolt in the hollow axle. There is no mistaking one for the other.

If the bolts you have look anything alike and come even close to looking like they match the holes, they are the same.

FWIW Octalink fixing bolts interchange adequately with FSA ISIS fixing bolts. Just found this out last week when an ISIS bolt disappeared somewhere on the first 8 miles of a ride. Eight miles back one legged is a trip.
Steve

FSA uses the same 15 mm fixing bolt diameter and threading as Shimano on their steel ISIS bottom brackets. Some of FSA's Ti bottom brackets use a 14 mm fixing bolt.

The threads are all the same unless they're very obviously different sizes. If they even look like they might possibly fit, they will.

By the way, when starting a new thread, it is considered good form to use a descriptive subject heading. For instance this one could have been something like "Crank bolt threading" or "bolt interchangeability", rather than the meaningless generic heading you chose.

Sheldon "8 x 1" Brown