99% on this forum probably know this, but the compression adapter for a Model A does not work for a Model T. Sigh. My engine runs fairly well idling, but a compression test will really tell the story. I bought a Model A adapter on Ebay and the threads are different than a Model T. After completing a couple of other project I just rewarded myself by doing what I really wanted, checking the TT's compression. Darn, now I have to wait until I get the correct adapter.
I do have a related question though. The plugs are Champion 25. I tighten them to what I feel is right and there are still about two threads showing. Is that correct?
The plugs are 1/2NPT and as snug as you can tighten with spark plug wrench is adequate.
You can go to your favorite plumbing store and use a 1/2" pipe nipple and adapt it to fit your compression tester. I made a Rube Goldberg tester using a 1/2" thread to stab fitting, a short piece of hose, and another stab fitting that my gauge threads into. It works ok for me but it won't win any beauty contests.
As Dennis posted. Got this K-D at auto store, added the brass 1/2" Nat'l Pipe Thread nipple for adapter to Model T spark plug hole in the cyl head.
Used it to check new high compression head
Nice psi bump
Get, (or make), one of these and you can use any compression tester out there, (though you may have trouble finding one that reads low enough to check T engines).
Just got off the phone with lang's. The set of 4 spark plug adapters they sell, they will sell you one only for 1/4 the price of the set of 4. I had an order in house, so I had them add one to my order and now I can check my compression with my auto parts store gauge.
TT, The amount of threads showing on a T plug when installed will vary depending on the wear in the threads in the head AND on the plug. It's a tapered thread, so when it tightens, that's where it is. Do not over-tighten trying to get the same number of threads showing. They just need to be snug enough to not leak, nor loosen themselves, any tighter is likely to cause other problems.
I used a dial tire gage and pipe fitting from the hardware store to make my compression gage.
I was always taught to install and remove plugs with the engine cold. I was told that if you install them in a hot engine, they will be too tight once the engine cools off and you might have to run the engine and get it hot again before the plugs will come out. FWIW.