Bicycle Mechanics - Tapping brake boss threads, any tips?

Hi all, The rear brake bosses on my chro-moly frame are a problem. When I had to put the brakes on, the threads were so tight I had to turn the bolt a quarter turn and back until they were on and the bosses got so hot I was terrified they were going to crack off (I think they are cast chromoly). Removing the bolts reveals they are almost devoid of thread whilst the threads in the bosses look fine suggesting the threads in my bosses are clogged with something. On the way home I'm gonna pickup a M6 x 1 tap and a handle and re-do the threads.

My question is, what makes the tap follow the existing threads and not just cut a new set of it's own ? And, does anyone have any tips :) ?

Btw, the frame has recently had a stay replaced and been professionally blasted and given a multilayer proper bake on new paintjob so that could be why the threads are so clogged...

The tap is quite tapered and all one need do is keep it centered and take it slow. The tap will be so deep before it engages the thread that you'll know if it's not centered. Back it out a couple of times before driving it home to keep the spoil from fouling the thread and to check if you're actually removing anything.

A light oil wouldn't hurt.

How do the depth of the threads on the bolt/screw look. if they are distorted, get a new one(s).

Good luck.

I almost said something about how hot it is when a woman isn't afraid of tools, but I won't go there. ;)

Cheers Greg, The threads on the bike look cool but the first bolt I used had almost nothing on it whe I removed it. I guess this is because cast chromoly is massively harder than a generic hardware store bolt.

I was gonna use oil because the Park website says to but I'll probably use Finishline 'wet' or something I have hanging around :).

I would definitely use cutting oil on steel threads, and make sure to back the tap out about half a turn for every turn you give it, so the shavings can be cleared, otherwise it can clog and break the tap or wreck the threads you are cutting. Good Luck!:thumbup:

Just thought, I've got a few ounces of motor oil in the bottom of a bottle, I'll use that.

He he he - perfect ! Got a set of three taps, each more fully threaded than the previous one. Used Shell Helix 0/40w semi-synthetic motor oil cos I had some spare. Liberally slathered boss and tap with it before starting and then poured it down each flute on the tap after every revolution (very messy but VERY smooth job :) )

Perfect bosses :D.

Btw, cheers Greg, you were exactly right.

Originally posted by Astra
Btw, cheers Greg, you were exactly right.

About how hot it is when a woman's not afraid of tools? ;)