Spark plug hole tapping

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Model T Ford Forum: Forum 2017: Spark plug hole tapping
Top of pagePrevious messageNext messageBottom of page Link to this message  By Nevada Bob Middleton on Saturday, July 22, 2017 - 08:27 pm:

Ok stock T is 1/2 pipe thread.
The Aluminum head i bought off the forum has some odd thread in the plug holes. Thought it was 14mm but that doesnt seem to be the case.
Since i have a set of adapters think it be good idea to install them
I have the tap 1/2 pipe but unsure how or what
size or is it just a drilled hole or do i need taper reamer as well
Dont want screw it up
Also drill size to use
Tap Chart - Pipe Threads


Drill size w/ taper pipe reamer Drill size w/out taper reamer or straight pipe


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mark Gregush Portland Oregon on Saturday, July 22, 2017 - 08:41 pm:

Check first to see if they are 18MM. I hope the holes drilled are centered in the casting, if not you could be going into the water jacket. Ok that is worse case.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kenneth W DeLong on Saturday, July 22, 2017 - 08:55 pm:

Years ago i had a rare head with bad 1/2 pipe threads so i took it to the local wizard.He used 3/4 to 1/2 bushings.He turned the 3/4 threads off and threaded a fine thread on the od.He then drilled and taped the head to match the bushing.Turned the hex off the bushing,cut,and installed.Bud.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Richard Gould, Folsom, CA on Saturday, July 22, 2017 - 09:11 pm:

Bob, hi there. Its been a while. I had a Z head that was manf'd to accept the smaller metric threads. I purchased a 1/2" NPT tap and tried to tap it. It was damn near impossible to do by simply drilling the holes recommended in the Enco catalogue for tapping. I held the head on a mill and the head of the tap guide in a collet in in the quill. Even though I used a large crescent wrench to turn the tap, it took all my strength just to cut enough threads to hold the spark plug. I'd strongly recommend the use of a tapered reamer. Even that might not be enough.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Gene Carrothers Huntington Beach on Saturday, July 22, 2017 - 10:14 pm:

Bob, Your 1/2 pipe tap drill would be a 23/32". You might get by with a 3/4. To go with that I would try it out first on a piece of scrap. I have not ever tried a taper reamer since common practice is simply drill the hole and then tap it with the pipe tap. Since the pipe threads are tapered you can adjust the depth but be careful not to go too deep.
I've done several threads in mild steel and would certainly think alum would be much easier. Some Rapid Tap will be good so you don't gall the threads. A mill or good drill press is important to get a straight hole and the tap started true.

Let us know what you do.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Nevada Bob Middleton on Sunday, July 23, 2017 - 12:59 am:

Well thanks guys take its going to a machine shoo tap the adapters and head let someone do it
I have get bit
14mm plug wont thread in and i think 18 mm is bigger dia


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kevin Matthiesen on Sunday, July 23, 2017 - 02:03 am:

Bob, I know that you say that you think a 18 mm spark plug is to big, but if it turns out that 18 mm is to small, may be the head uses the 7/8" Model A spark plug.


Top of pagePrevious messageNext messageBottom of page Link to this message  By David Stroud on Sunday, July 23, 2017 - 04:57 am:

One thing to remember when tapping threads by hand. Don't just try to screw the tap in and keep going until you are done. You have turn it in a bit, then backup a bit. Then do it again, maybe add some lube each time, something like the Rapid Tap that Gene mentioned. I think there are other brands that work well also. Take your time and make sure the tap is square with the hole. Dave


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