Sunday, 6 September 2015

Retrieving broken studs & screws

I've been meaning to take a few hours to write up the removal of a couple of broken screws.

These two examples are both parts of my Huntmaster - One is a fork clamp bolt, in which the screw retaining the headlamp casquette has become seized in the past and broken by someone; the other is a engine mount spacer, which in these bikes is provided with a small 2 BA lug to retain a pressed metal cover, which enhances the appearance of the engine mount and prevents it getting full of road dirt. Of course, in this exposed position these little screws seize solid. I managed to get one out, aided by some prehistoric oil leak, but the other was dry as a bone and broke as I worked it.

So the process for fixing this kind of problem is this:

Put the part solidly in the vice, and file the broken bolt flat; then centre punch the broken fastener right in the centre.

File the broken screw flat

Using a small drill, make a pilot hole right through the broken fastener. As you start it, make sure it's still on-centre

Pilot hole in the old 2 BA engine mount screw

Using a larger drill, open the hole out until you are approaching the minor diameter of the thread
Small pilot drill hole in the old screw

Continue drilling out the old screw
Try an Easy-Out in the hole, selecting the largest one possible that will bite in the hole. Turn it gently with a tap wrench. Don't force it - if you break it in the hole you are in real trouble.

Small Easy-Out
If you don't have any success with an easy-out (and I can't remember when one worked for me), go to the next drill. You will be able to see if you are approaching the major diameter of the thread - if you have already cut into the thread, stop.

Using a small punch or a chisel, tap out as much of the old fastener as you can - you can often shift a lot of it from the top of the hole, tapping toward the centre. Removing a couple of threads from the top of the hole is sufficient to go to the next step:

Finished drilling - only the old thread remains

Peeling the top threads away

Using a taper tap of the appropriate size, start the tap in the hole and attempt to recut the thread. Give it a few turns, and then disassemble everything and knock all the debris out of the bottom of the hole.
Starting the thread with the taper tap
Clean it out will so there is room for the second and plug taps.

All that remains of a 5/16" BSC screw
Move on to the second or intermediate tap and then the bottoming or plug tap, and complete the recutting of the hole. There will be a lot more debris in the hole and you will have to clean it all out again.

5/16" BSC Plug tap

Ariel Huntmaster engine mount repaired
Test it with a new fastener and you are done.

Ariel fork bolt repaired

No comments:

Post a comment