I have just looked carefully at a Nelson plug and 049 adapter.

If one used a lathe, it looks like the diameter of the threaded portion of a Nelson plug could be turned down. The tapered seat face could be reduced by about 1/2. This would then allow you to re-thread the outside of the formerly threaded portion, down to a size to fit a newly designed 020 head adapter or even a used 020 plug itself. The question is how much meat is there inside the Nelson plug to for allow this reduction in diameter? Is there a large cavity for the stem sealing material such that there would be no material after turning down the body?

What about the turbo plugs being similarly turned down to a suitable smaller diameter for 020 or 010 use? I don't have any Turbo plugs to examine. Would a conventional glow plug be small enough for the 010? I am looking at one and it appears small enough in the threaded portion. Has any one used a conventional short plug and made up an 010 adapter?

Just some thoughts on a quiet night.

Jim H