Repairing a threaded hole in aluminium (27/08/14)

The part shown below is from a laser tube mount. One of the threaded holes had been drilled at an angle and subsequently cross-threaded (the first photo shows the angle which a screw enters at - clearly very misaligned). The screw is M4 (4mm diameter).

To fix this, I first drilled out the hole to 6mm and turned an aluminium plug. The plug is a loose fit, maybe 0.1mm undersize. I then glued the plug in using Loctite 638, curing it on a hotplate at maybe 40° for an hour. After cutting off the plug and sanding it down, the hole could then be re-drilled and tapped.

I used to use Loctite 290 for this sort of job - it's an extremely low-viscosity threadlocker and runs readily into cracks, but isn't really intended for adhesive applications. Admittedly, in this application, there's no great force exerted on the screw, so 290 would have worked fine. Loctite 638 is described as being suitable for locking cylindrical parts like shafts, pins etc. so is much better. However, it is quite thick and is harder to spread around evenly.