Bicycle Mechanics - Reccesed allen head bolt question.

I know this is a rather mundane and maybe stupid question but anyway.

I picked up some used ultegra sl brakes and the recessed allen bolts are freakin long. Like 30mm long and they aren't very nice looking too.

Anyway I am mounting there on a 89 peugeot triathlon. The current bolts are a snug fit I am guessing that the mounting holes for brakes have been a pretty static size. I measured the length of the hole and it is 15mm(pretty sure). On loosescrews.com (http://www.loosescrews.com/index.cgi?c=Brake&sc=Recessed%20Nuts&id=38530042064) there is a 10mm recessed bolt. Will this 5mm make a difference or what???

I'm not sure what you're asking... you're referring to the tubular nuts that secure the brakes the the frame and fork, right?

AFAIK they're a uniform size; required length may occasionally vary a bit though... although the 30mm ones you mention sound like they're to enable fitting a rear brake to the front or something...

I'd say the bare minimum engagement for this fastener would be no less than three or four threads...

Hi Binxsy it sounds like the mounting bolts you got are for a carbon frame and fork. I do not think the 5mm will make a big difference.

Recessed mounting nuts (They are nuts, not bolts. The "bolt" is part of the caliper assembly.) come in a variety of lengths from 10 to 30 mm to fit forks with differing crown thicknesses and rear mounting. As long as you have adequate thread engagement (as noted at least 3 or 4 full turns and 5 or 6 are better) you should be ok.

Here is a source for them:
http://www.loosescrews.com/index.cgi?c=Brake&sc=Recessed%20Nuts&id=252707975

Recessed mounting nuts (They are nuts, not bolts. The "bolt" is part of the caliper assembly.) come in a variety of lengths from 10 to 30 mm to fit forks with differing crown thicknesses and rear mounting. As long as you have adequate thread engagement (as noted at least 3 or 4 full turns and 5 or 6 are better) you should be ok.

Here is a source for them:
http://www.loosescrews.com/index.cgi?c=Brake&sc=Recessed%20Nuts&id=252707975

Hill rider has it almost right.

recessed mounting nuts come in various lengths to reach the bolt on standard brakes with forks of varying thicknesses. It's a lot easier to do it this way than to provide different mounting bolts on the brakes.

One not so minor point I disagree with.
Screw threads don't reach maximum strength until the engagement depth equals about 75% of the diameter of the screw*, so for your 6x1 brake mounting bolt you want a minimum engagement of 5 full threads or 5 or more turns of the nut if at all possible. If it's on the borderline, use a longer rather than shorter nut provided it doesn't bottom out.

BTW- if yours is too long (bottoms out before it's tight) you can cut it down with a hacksaw. The cut edge will be inside the fork hidden from view.

* source click here (http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html)

Thanks! I think I might just cut the long ones I have. But yep thanks for the info!

Hill rider has it almost right.....

One not so minor point I disagree with.
Screw threads don't reach maximum strength until the engagement depth equals about 75% of the diameter of the screw*, so for your 6x1 brake mounting bolt you want a minimum engagement of 5 full threads or 5 or more turns of the nut if at all possible. If it's on the borderline, use a longer rather than shorter nut provided it doesn't bottom out.
Yeah, right after I posted I did the calculation and realized a minimum of 5 turns are needed to get adequate engagement on an M6x1 bolt and 6 full turns are needed to get 100% of the bolt diameter inside the nut. So my "5 or 6 are better" should have been more certain and said 5 or 6 full turns are required.