Thread size for a Model A spark plug

midgetracer

midgetracer

Well-known member
I purchased a Snap On spark plug tap that is listed as 7/8 18NS. Spark plug tap
Stock #T2818. I needed to clean up threads on a high compression head. The tap appears to be 7/8 18 tpi, but when I tried to clean up the threads, it cut more than I expected and the spark plug would not thread into the hole. I tried it on an old head I had laying around and it did the same thing. purchased it on EBay some time ago, so return is not possible. Any thoughts on what I have and what I need. I measured the tpi and it is 18.
 
How important is originality to you? If it is high priority, you may need another head. If you'd rather have your car going than looking original, establish what thread you have by measuring the diameter and making sure of the thread count, then consulting a thread chart. A suggestion here is to have 4 adapters made with male thread to match the thread you have cut in the head and internal to take a modern plug. Now drive on.
 
Are you sure that it's T2818NS? You need T2818NF tap
The S tells me that is's a special thread
 
Lay your tap onto the threads on your plugs and see how they match up. Threads have changed in the last 90 years so not all modern taps or dies (or nuts and bolts, for that matter) truly match original A threads.
 
midgetracer said:
I purchased a Snap On spark plug tap that is listed as 7/8 18NS. Spark plug tap
Stock #T2818. I needed to clean up threads on a high compression head. The tap appears to be 7/8 18 tpi, but when I tried to clean up the threads, it cut more than I expected and the spark plug would not thread into the hole. I tried it on an old head I had laying around and it did the same thing. purchased it on EBay some time ago, so return is not possible. Any thoughts on what I have and what I need. I measured the tpi and it is 18.
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delete
Bill
 
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according to my spark plug tap set it is 7/8- 18 that is NF
 

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In the Unified threads, the "S" in the UNS is for special threads. This can be a number of different thread per inch sizes but 7/8" X 18 is the spark plug thread that was common to the model A, B, and some early 30s V8 cars. I would check that tap closely and make certain it's a 7/8 X 18 size using a tpi gauge and a rough measurement of the OD of the thread pitches.

It's also a very good idea to check the internal thread of the head before cutting. A tap that fits properly should not take much effort to turn in cleaning up old threads. Some old aftermarket heads had different spark plugs than original.

Aircraft engines still used Unified fractional type threads to this day.
 
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The tap is labeled Snap ON 7/8 18NS. It measures 18 TPI and so does the C4 spark plugs I am using. It must have a taper of some sort that it will not turn in a stock Model A head. The threads on my HC head were slightly wrecked on one cylinder, so I bought this tap to clean them up. Waste of $45 and now I have a spark plug hole that really needs help. JW is your tap the correct one for a Model A and labeled 7/8 18 NF. I will look for one with that designation. My machine shop could not help me. Thanks for all the comments given.
 
I have an engine that has a spark plug base in the oil drain hole, it is the same size, I left it in there with no leaks. I was surprised they were the same threads.
 
midgetracer said:
My machine shop could not help me. Thanks for all the comments given.
Click to expand...

I think you need a new machine shop. They should be able to tell you what you have, what you need, and the willingness to do the job necessary to get you back on the road. You are dealing with rudimentary shop work.
 
Can you measure/compare the OD of the tap and the C4 plugs you have?
I have a NOS C4 that measures .872" OD.
 
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