Spark plug threads, a definitive summary?

[It's Bill]

Someone is currently advertising a number of spark plugs on the forum and is listing their thread forms and diameters. There are differences in these plugs. How do you determine what is correct for your head? Is there .a chart that specs what thread fits what year? My searching reveals only piecemeal information, but I am an amateur at search. Cheers, Bill

[Peter, Memphis TN]

The standard Model T cylinder head was outfitted with 1/2 inch female pipe threads for the spark plugs.

Male pipe threads, as found on standard Model T spark plugs, are tapered, so that the joint becomes better seated and therefore better sealed, as it is tightened.

Because there are scads of spark plugs made that use other thread configurations, most of which are not tapered but use a crushable metal 'gasket' that bottoms out against the head and forms a seal that way, lots of folks have used an adaptor to reduce the hole in the head from half-inch pipe threads to whatever threads are on the plug they want to use. These adaptors are available from the various parts sellers of Model T parts.

It is possible, or even likely, that a cylinder head other than a Model T head, could have any of a number of threads in the spark plug holes, but I wouldn't be able to give you any help on that.

Suffice it to say, if you have an original head on your engine, you're looking at 1/2" pipe threads, and there are many spark plugs available in that configuration. In fact, the Champion Type X plug, which was the norm in Model T times, is still available from your neighborhood parts store, although they may nmot have them in stock - but can order them.

[Terry_007]

Good response. 1/2" pipe thread is the Model T standard, and the Champion X is my own recommendation, and probably a majority of other T owners. That said, there are a lot of other brands of spark plugs that were produced with that thread, and still are (Motorcraft for example). However-some are better than others (some, a lot worse!). Thousands of different cars were produced that used that thread so it's understandable a lot of plug manufacturers would offer it over the years. As a collector of spark plugs for nearly 50 years, I can tell you that nearly 5000 different brand names have been produced world-wide over the years. That number continues to climb. I'd guess that a large majority of them were available in 1/2 Pipe thread.
Terry

[Steve Jelf]

...the Champion Type X plug, which was the norm in Model T times, is still available from your neighborhood parts store...

...and will give you sticker shock.

Mister Thrifty would use Autolite 3095 (about $12 a set) or some other modern plug while shopping for some original plugs at swap meets or online. Champion X and other old Model T plugs sometimes turn up on eBay for $10 or less, and I have bought them at meets for $2-$5.

http://dauntlessgeezer.com/DG95.html

[TXGOAT2]

If you have a cylinder head with worn NPT threads, you can have a situation where the plug will screw in to the last thread and still have a slight leak. Further tightening won't help. I've had good luck using teflon pipe thread tape. I didn't expect it to hold, but it did. I believe someone here mentioned using aluminum foil in a similar way. That would probably be best, since it would not have any tendency to insulate the plug, either thermally or electrically.

[Mark Gregush]

Generally speaking in the automotive field the plugs are; 1/2 NPT, 7/8", 18mm, 14mm. There are others but most cars/trucks used those sizes. Each of those thread sizes come in different styles at the electrode end depending on application.

[TXGOAT2]

Packard used a tiny plug in some engines, perhaps 10 MM. I think some older Chevrolets did also.