Alloy Diff Housing Repair – Reclaiming Stripped Housing Thread.

After several decades of use and perhaps abuse, the elan alloy diff housing can become worn in various areas, one particular problem is that of repairing/reclaiming stripped or broken threaded banjo mounting studs.

The threaded holes in most old diff housings show signs of serious wear from over tightening created by numerous diff strip downs and rebuilds over the lifetime of a car.

There is nothing worse than almost completing a rebuild and you get caught out with a striped thread in a particular stud hole, they can be hard to detect, and usually only found when the last amount of torque is applied when tightening the studs up, and one or maybe two studs will let go and strip the thread in the alloy housing.

It is a pain, but the grief and anxiety can be lessened by a close inspection to look at the threaded holes in the diff housing during the strip down and cleaning, each threaded hole should be cleaned & chased out with a plug tap to make sure the hole is clear threaded right to the bottom.  Then screw each respective stud into it’s hole to see if there is actually a good fit, if any studs feel very loose, there is a chance that it may strip during reassembly.

So what to do? Coiled inserts are a popular way to repair damaged threaded holes, most of the time they work fine, but a preference of the writer is to drill oversize, and re-cut the thread with a suitable tap into the parent metal of the housing, this I believe, is a more permanent solution, but if the helli-coil insert seems right for you, then stick with that.

So the first thing to do is to drain the oil out of the diff, a simple way to do that without creating a mess is to remove the rear filler plug and then up-end the diff over a large tin or bucket and let the oil drain for an hour or so. Replace the plug and commence with a thorough cleaning of the exterior of the housing, before any dismantling is carried out.

The special over size thread is machined in two sizes, the over sized part that is inserted into the alloy diff casing and the outer part of the stud remains as a 5/16″ UNC threaded stud. A stud such as this can be easily machined and threaded on the lathe.

The easiest method to insert the screw stud is to use the double lock nut method.

To prevent the stud from unscrewing in the future, place some loktite (C) thread locker fluid on the threaded shank, then bed bed down the stud with a spanner, do not over tighten. The job is now complete, unlock the nuts and remove, and reassembly can commence.
There are ways and means of refurbishing and reclaiming parts that have been damaged, it just takes a bit of thought, to sort out the right approach.