Making a spark plug mandrel

Okay so this is more of a machining entry then a minimoto post, but its related so here goes.

To machine a head on a minimoto you need to be able to mount it up on the lathe. Thats quite difficult because they're quite intricate pieces with lots of wierd angles, and quite thin as well, so mounting them directly to the chuck or faceplate is generally out of the question.

Thus the only way to mount it up properly while keeping everything concentric is to make a threaded mandrel that will clamp in the chuck and the head threads onto it through the spark plug hole.

For the squish dome i made for the banshee i had used a turned down old spark plug, which while it did work and work quite well, was excedingly dodgy.

So out to the lathe and a piece of rusty old 16mm steel shaft was thrown in the 3 jaw and turned down. I just skimmed enough off the outside diameter to get rid of the rust and turned the end down to 13.80mm which is the major thread diameter.

I also had to make a slot at the end  because my lathe doesnt have a thread chasing dial. If you want to know the reason why google it ;p

Below is a pic of the part ready to be threaded. you can see the threading tool which is specially ground to have the same angle as the thread itself. You could technically grind one yourself but they're really cheap to buy premade anyway..

Heres a brief video detailing the threading process without a threading dial, its much easier to understand with a video i think :)

The thread after the first cut. you can see it taking shape. The cut is about .25mm deep per pass. It seems any deeper and the finish is pretty terrible and you blunt the tool pretty quick.

And after a couple of passes heres the final part! Well technically not true, i accidentally put a gear on the lathe that had 6 teeth too many and the thread was marginally too big in pitch. ARGHHG. so i had to do it all over again. Having done it once it took about 10 minutes instead of an hour :P so all was well.

Now it can be mounted in the lathe and some heads shaved down. This is done to increase compression or correct port timing / squish. The run out is about .08mm when measured on the head itself which is pretty decent. although i think its actually perfect and the .08mm comes from the orginal machining.

The finish on the threads is pretty rough to be honest, even though it works just perfect and after threading it on and off a few times it cleaned right up. So i may have to have a bit more of a play around with threading.