Hello Aero Nut,


You ask me about how to make an adjustable glow plug holder / contra-piston adapter for insertion into a standard sparkplug hole in a personnel email.

I spent a about 10 minutes on the lathe and make a simple mandrel to hold a glow plug and then turned down the outside of the glow plug. The glow plug hole is a .250 x 32 threads per inch. I turned the plug down to .260 of an inch. If I went further it would have cut too deep into the plug and head of the glow plug will pop off.

Okay you can see in the photos the plug and you ask now what.

The rest of the device (Not shown since I have not made it yet) is just an old spark plug with the electrode remove and a 3/8 fine thread cut into the jacket with a tap.

Next a 3/8 fine threaded bolt in mounted into the lathe. A #3 drill is run down the center and tapped for ¼ x32 threads. Next it is counter drill from the other end with a #2 drill almost to the bottom leaving just the amount of threads to hold the glow plug (about 0.250). A wire is attached to the center electrode and the threads on the glow plug are coated with Stud Mounting LockTite.

The glow plug is inserted down into the 3/8 bolts and turned into the hole with the extended electrical wire.

This 3/8 bolt then is inserted into the old sparkplug. Now you can run the engine and adjust the timing while screwing the 3/8 bolt into or out of the cylinder. Of course you need to attach a leaver to the bolt, such a cross pin to be able to turn the bolt.