When stripping down the rear hubs, the heads of the two smaller screws securing the brake backing plate to one of the hubs had sheared off. Now it is time to attempt to remove the remaining parts of the screws which are still stuck in the hub.
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| Today's mission - removal of two sheared screws... |
There are many suggested ways, it seems, to remove broken screws and bolts.
The most straightforward would have been to position a suitable sized nut over the top of the sheared-off screw and using a hot metal glue gun, blob some weld into the middle of the nut to join it to the screw. Then it should just be a matter of using a spanner to undo the nut/thread combo. My only issue with this was that the sheared-off thread is just below the surface of the hub and, while the theory is that the steel weld won't stick to the aluminium of the hub, I wasn't prepared to take that chance with my hub.
Next on the list of discarded options was the use of a suitable sized screw/stud extractor. The theory behind these is that a pilot hole is drilled into the broken screw into which the extractor is inserted; the extractor has some reverse cut threads that bite into the metal and undo the screw. Now I have never had much luck with using screw extractors even on screws in wood with chewed up heads. And given that these broken screws had resisted all attempts to remove them initially before breaking, including heat, impact screwdrivers and penetrating oil, I seriously doubted that using an extractor was suddenly going to persuade these screws to give up their grip on my hub.
So the method I decided upon was to slowly try and drill out the broken screws, using drill bits of increasing size until the hole is drilled out to the diameter of the screw shank and then to retap the hole with the required thread size.
Of course, I should have known it was never going to be quite that simple.
The first problem is that actually trying to centre punch the end of a sheared-off screw is quite difficult - the broken surface was uneven which made it hard to locate the punch in the exact centre. In hindsight, I should have used a Dremel or similar to grind the end of the screw flat before using the punch...
The second problem was that smaller diameter drill bits are very short, the chuck on my drill is quite large and the offending holes are very close to the hub bearing housing; the result that I could barely get any depth to my initial pilot holes in the screws using a 2mm bit. Worse to get any depth I had to fix the drill bit with only a very short length of the shank actually in the chuck. This means that the drill bits are very flexible and the end is prone to wander (of course compounded by the difficulty of punching the centre of the screw in the first place).
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| Drilling depth restricted by the clash of the drill chuck and the hub... |
The end result was that the pilot holes on both screws did not really start off on centre so by the time I started opening the holes up with 3mm and 4mm bits it was clear that the holes were going very awry. The other issue is of course that with the hub being aluminium the drill bits found it much easier to slip off the hard steel surface of the screw and chew their way through the softer aluminium.
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| Opening up initial pilot holes... |
By the time I got to the 5mm bit, I realised I needed a Plan B, as the result was that I was drilling out about 3/4 of the area of the screw and the rest of the hole was through the aluminium hub.
Plan B was to use a Helicoil kit (thanks to that well known on-line retailer and their next day delivery service). Helicoil inserts are a threaded metal insert used to repair stripped threads; the stripped hole is drilled oversize and tapped for the metal insert which is then inserted into the tapped hole to reinstate the original thread.
My logic for this approach was that by drilling the hole slightly oversize for the Helicoil that this would allow me to remove all the remaining portion of the screw thread from my misaligned drilling.
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| Helicoil Kit - includes drill, tap, insertion tools and a selection of inserts |
The hole in the hub should be an M6x1.0 thread so the first step was to drill out the hole using the supplied drill bit for the corresponding insert.
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| Prior to drilling - misaligned attempt to drill out screw is clearly visible! |
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| After drilling - operation didn't quite clear out all of the remaining screw |
Next step is to tap the holes for the Helicoil inserts using the tap provided for the M6 inserts.
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| Tapping holes for inserts. During... |
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| ...and after |
Next stage is to insert the Helicoil. The inserts have a small tang on the bottom of them which is inserted into the slot on the appropriate sized insertion tool. This is then used to twist the insert into the threaded hole until the insert is approximately half a turn below the surface. Care needs to be taken to allow the insert to thread itself into the hole without applying too much downward pressure. My first attempt unwound the bottom of the insert... Fortunately, it was not too far into the hole and I managed to extract it without too much excitement.
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| M6 threaded insert in place on the insertion tool |
The final step is to use the punch supplied with the kit and a sharp tap with a hammer to break the tang off the bottom of the insert.
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| One repaired hole! |
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| Replacement screw fits perfectly! |
In the end, it was not as traumatic a job as I was expecting it to be (although equally, it was not a simple as I would have hoped...). Both holes are now repaired albeit both are slightly off from where they were originally. Hopefully, the brake backing plate will still line up - if worst comes to worst I may have to slightly oversize the holes in the backing plate.
Right, time for a cup of tea to celebrate.
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