FH Dry build - primary case

One of the problems you always come across when tinkering with unloved old bikes is broken screws. The Huntmaster hasn't had many, but while I am working on the dry build it is time to fix these kind of problems as we come across them.

This is a typical example - a broken 1/4" BSW screw in the primary chain case. Of course, it is too short to get hold of and Plus Gas has no effect on loosening it enough so that the little bit I can get hold of can help it come free.

The only sensible option is to drill it out and and re-tap the thread. I set the inner case up on the pillar drill and used a centre drill to start the hole, opening it progressively (0.5 mm increments) to tapping size, hoping to release the remains of the screw.

This didn't work out, and after attempting to chase the rest of the bolt out with a tap, for the first time in my life I managed to break a tap in the hole. I spent the rest of the evening laboriously grind it out with a diamond coated tool in the Dremel.

Once I had removed the tap, the next step was to drill out the mess for a Helicoil:

Then tap for the Helicoil:

Then screw in the insert. You break off the tang used to put the insert in when you are done:

Charlie's Shed - Let's fix those duff threads

Updated: First published January 2014

Well, I knew I shouldn't have looked too closely at those calipers. I replaced one nipple successfully:

The other one is too far gone and I am going to need to Helicoil it. Time to resurrect an old post that I published a long while ago:

So you've all seen how the trial fit up of the primary cases revealed some duff threads that I really should have seen earlier - well now we have to fix them.

I bought this nice little set from Chronos:

It  comes in a nice little steel box with all the tools you need and 20 or so helicoils, for less than £15 delivered. These are 1/4" BSW.

Here are the drill, tap & a helicoil insert:

Here's the insert tool, which has a depth stop collar supplied with a hex key to adjust it, and the punch for knocking out the tang:

The first step is to drill out the hole to the correct tapping size for the outside diameter of the insert. It is vital to use the correct drill bit size, as the outer thread of the helicoil will match the newly tapped hole, and will dictate the size of the internal thread such that it matches your fastener.

You need the screw to ultimately go into the work at the proper angle, so if you can, set the work up in a drill press.

Next you have to cut the thread for the outside of the insert using the tap that is provided with the kit. The tap has a short taper (called a 'second' tap) to help get the thread started; use a tap wrench (90 degrees forward, 120 degrees back to clear the swarf) with some lubrication to cut the new thread. Cut the thread until you no longer feel any resistance as this means the hole is now completely tapped with the new thread. Gently wind out the tap with the wrench. Make sure you don't try to bend it at any time - you really don't want to have to deal with a broken tap.

Next take one of the helicoil inserts and put it over the end of the installation tool, with the tang of the inset in the slot and in what will become the bottom of the hole - so the tang goes in the hole first. Set the depth stop so the insert will end up 1/2 a pitch below the surface.

Wind it in. You shouldn’t feel any resistance as you do this but if you do you may just need to clean the thread up a little. Note that as you get near the bottom of the hole and the friction increases you will feel the insert start to wind up, spring like, against the tool.

One the coil is in place you use the little punch that should come with the kit to knock the little tang off the end of the helicoil that assisted with winding it in. If you are rethreading something relatively thin, as is the situation with this primary chaincase, you can grind off the spare length of insert with the Dremel:

Job done: