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|
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|
|All that remains of a 5/16" BSC screw|
|5/16" BSC Plug tap|
|Ariel Huntmaster engine mount repaired|
|Ariel fork bolt repaired|