Workshop Stories: Stripped Calliper Threads

[Johan Bornman]

Workshop Stories: Stripped Threads:<?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

 A fastidious customer lamented to me the other day about his nice XTR front calliper with a stripped mounting bolt. Purchased second-hand, this ?feature? was not declared by the seller. He simply made do with a smaller bolt and a nut on the other side of the calliper. Whilst this is perfectly functional, it is not perfect.

 

?But lets fix it,? said I. ?Do it,? he said. I like ?do-it customers?. They have a sense for aesthetics and function and put it ahead of a bit of expense.

 

 

 

The little nut in the bottom recesses of  the inside of the brake calliper down there at ankle level is clearly visible to the discerning bike owner. It sticks out like a borrowed Marie Biscuit tyre on a Porsche.

 

 

The side that?s still OK doesn?t offend at all?it was made like that and should be like that.

 

 

I decided to go the Helicoil* route since there isn?t enough meat on the sides of the tab for fitting a much thicker Vasbyt thread insert. In fact, none of the threads of a bicycle that usually strip can be repaired with a Vasbyt. It?s a pity, since this product is much cheaper and requires no special tools.

 

 

The Vasbyt is the one in the top right corner. It is much thicker than a helicoil and requires lots of space around the hole.

 

 

I removed the calliper and saw that the standard 6mm capscrew was replaced with a 5mm screw and nut on the other side.

 

 

 

The thread was indeed stripped and there was no chance of recovering some of it by chasing it with a 6mm tap.

 

 

 

The thread should look like this. This is the good side.

 

I first drilled the hole out to 6.3mm (you read correctly) and then tapped it using the special tap supplied with a helicoil kit.

 

 

Tapping is straight-forward and quick?if you have the right size drill and a drop of tapping fluid.

 

 

 

This is what the newly tapped hole should look like

 

I cleaned out the hole and applied some thread lock as an extra measure to keep the helicoil in place. This is not necessary with full-depth inserts but the calliper?s mounting tab is a good 2mm thinner than the length of the coil and often they tend to come out if the mechanic has forgotten to use copper compound on the bolt before mounting the calliper. Note that this is not a standard tap but a special one that matches the diamond profile of the coil?s wire. The size is also odd and unfortunately there is no short-cut. You have to buy the special (read expensive) tool for the job.

 

 

Ideally I should have used some permanent lock but medium strength blue is all I had.

 

 

Next step is to insert the helicoil. For this you need a special tool that grabs it by the tang and compresses it as you screw it in. The trick is to judge when it is flush with the mounting surface so that you don?t have an end sticking out and compromising the flush fit of the calliper against the fork.

 

 

This is the tool. It is supplied with the kit. The slot fits over the tang and compresses it through screw-in friction. Very clever.

 

 

The coil is inserted through a clockwise rotation of the tool The tang should go in first and its rounded elbow prevents the hard steel coil from biting into the aluminium and getting stuck. Screw it in until the mounting side (the blind side here) is flush with the surface. Don?t worry about the extra coil on the far end, we?ll cut that off with a good side cutter and the job is done. Mount the calliper and please notice the newly-returned elegance down there.

 

For those who are interested, here?s a close-up of the coil itself.

 

 

And a very close up, clearly showing the rhomboid-shaped wire of the coil. The notch in the tang is where it is supposed to break off when inserted into deep threads. We don?t require that much thread on most of our applications so we cut the coil to length after mounting.

 

 

I have successfully used this method of thread repair on:

 

Stem front-loader clamp mounting bolts.

Derailer cable anchor bolts.

Derailer frame mounting bolts. (This method requires a very rare 10 x 1mm helicoil that costs a bomb?but a lot less than a new frame).

 

 

*Helicoil is a brand name for coiled thread inserts. I?ve used it as a generic term throughout the article. Another brand is Re-coil, from Australia. 

Johan Bornman2009-09-09 11:19:08

[ichnusa]

Awesome! i want one...

[Jaco Steyn]

I cannot see the second-last picture.

[Johan Bornman]

I cannot see the second-last picture.

 

I fixed it.

[DawieO]

People who care are rare. Even more rare are those who care as much about YOUR stuff as you do.

Being a denizen of TheHub, I have always been impressed by Johan Bornman's depth of knowledge and experience.  So much so that, (and those that know me will find this a shock), I entrusted my beautiful bike to him for a number of much needed ministrations.

Johan, you have restored my faith in professional bike-gurus.  My baby is magnificent, as good as new, and every bit of work was done the way I would have aspired to doing it.

Thank you,

The Fastidious Customer.