Restoration of Oil Union and other threads with Colsibro Bushings

In restoring vintage machinery we are frequently faced with strlpped or damaged threads. In recent years the Helicoil approach has been most popular, owing to its relative ease of application, often without removal of the damaged part. These solutions have their place, however, I believe Silicon Bronze inserts, commercially available in bar form as Colsibro, offer a better, stronger and more permanent solution. 

Silicon Bronze has a coefficient of thermal expansion only just below typical cast alloy and is about twice as strong, so it is less likely to loosen in use, especially when thread-locked in place. Many will be familiar with its use as valve guide material from the club, where the above, plus its self-lubricating qualities, make it the material of choice. It is a particularly attractive option when the 1/8” BSP oil pump union threads need repair, which often suffer from over-zealous tightening, in a misguided attempt to cure a leak. This is a fine thread which should normally be tightened to only 8-10 lbs.ft. in alloy. In some cases the thread will be so damaged as to be oversize for a Helicoil insert so bushing of some sort is the only solution. I recently repaired such a housing where alternative, non-native, fittings had been inserted and even the original position had been compromised. Although I haven’t had the opportunity to try it yet, I believe this would also be a good solution to the common problem of the head stud adapter pulling out of the crankcase.

Clearly such repairs are beyond the scope of a hand held drill and a milling machine is the ideal tool. 

I would normally expect ½” BSCY to be adequate for the outer size and leave a good margin at the bottom to nip up against, avoiding disturbance to the port area. Having established the outer dia and depth required, it is a simple matter to turn up and thread a suitable bush on the lathe. Colsibro can be difficult to machine in some circumstances, but I find sharp, ground edge CCG type tips, as used for alloy, work well. I made up a threaded tool in 1/8” BSP with locknut to facilitate insertion. Having degreased the components I applied an (over generous) amount of permanent locking compound and nipped it up tight in the crankcase. Once the locking compound had gone off, I spot-faced the outer surface to ensure it was square to the bushing.

I believe this is a high quality repair, which is much stronger than the original, will not wind out, and would probably shear the oil bolt before