center pull canti brakes get stuck when tightening brake arm to brake boss
 

i have been tuning up a bunch of old mountain bikes. i take the parts off, clean them extensively and put the bike back together.

every time i try to put the brakes back on and tighten the bolt down to secure the brake arm to the boss post the arm seizes and wont move when i pass a certain torque. this is confounding because it is always far less torque than i was able to apply before cleaning. its also much less torque than i apply to newer brakes on canti posts. in some cases the bolt has to remain loose to the point that it seems likely to unthread in order to prevent seizing.

this keeps happening to me with so many of these older brakes. the only solution that i have found seems to lie in the washer that presses between the tiny bit of boss post that sticks up beyond the brake and the securing bolt. it appears that the washer gets grooved by the post and crushed and then is ruined. flipping the washer helps but doesn't seem to restore the potential for torque that is necessary.

do i need to get titanium washers or something!?!? am i just applying too much torque? any help is much appreciated :)

you may have over tightened the bolt,
the brakes bushing can be made thinner,
or the boss made longer by filing the shoulder back, without
removing any of the smaller part the brake rotates around

a file with no teeth on the edge will be key for that..

or you can clean up the threads to be grease free, inside the frame boss
and the bolt, apply loc-tite, and rely on its bonding to hold the bolt in
without overtightening it.

Is the return spring installed correctly -- both in the arm and the boss?

i think the assembly is correct because it works perfectly when the fastening bolt is on but loose.

if i overtightened the bolt, which i too think i may have done, what would have been likely damaged that would cause this malfunction?

this is happening to me repeatedly on lots of these shimano early 90's center pull brakes and every time i start to tighten them to a good torque they seize or the return gets heavily dampened. maybe the washers are old and junk?

what part of the brake is the bushing? and how would you make it thinner?

one of the brakes is the shimano br m650. all others are very similar.

You could flip over the end washer so the "good" side was pressing against the post end. These pivot boss bolts don't require much tightening torque. there's almost no force trying to unthread or pull/push on them. You can sand down/file down the post shaft a touch.

I've seen the top ends of the shaft slightly bulged before, lots of corrosion build up also. I often wrap sand paper around the shaft and shoe shine it clean. I also have experienced the arm tightening on the shaft with bolt tightening, usually sanding and careful tightening (one of the few places I'll use thread locker compound) solve the problem well enough. Andy.

Center pull and cantilever are two different brake types. You are talking about cantilevers.

First, if something repeatedly happens on a part that was previously working then one would have to conclude that it is your action that caused it. Over-tightening would by far be the most likely cause.

Unlike bolts that secure parts in place and prevent rotation, the cantilever pivot bolts have purpose but to keep the arms from coming off while allowing free rotation. Therefore the bolt should never be tightened down too much, as it will cause the bushing (the brass sleeve around the post) to mushroom or bulge outward, causing binding. The proper tightening is just enough to hold it in position against the end of the bushing. Once the bushing is damaged it takes very little pressure to cause the problem. I think Andy's suggestion of thread locker, as well as more care in the future, is a good solution.

You just need to carefully sand down the bushing until the arm works smoothly when the bolt is secure.

the part that is directly in contact with the frame boss.. front to back thickness of the brake

how skilled are you with a File? know 1 type from another?

sanding reduces the boss diameter, not altering , the length.

grease only the outside of the frame boss

The cleaning the grease and oil off, inside, thread lock app on the bolt
is probably the easiest to explain over the web.

sorry i got to that last..

instead of son of a machinist options, I've used before..

thanks for help. fixed as you guys directed: i sanded the top of the bushing just a bit and did not over tighten the bolt. i also sanded down one side of the washer that would make contact with the post so that it was perfectly smooth. the bolts tightened up enough that i don't think ill need lock tight and the brakes return perfectly.

foodger- Good job! I just did the same repair today at work. Three of the four bosses were rusty. The frustrating part was not the job, not the communication with the customer to aproval extra work. It was watching my coworkers taking it easy when we had a full load to do. Fun and talk is great when you have a beer in hand and have clocked out... Andy.