cylinder head

L

Lacrown

Junior Member
I just had a cylinder head planed for my Model A ,put it on torqued it down and went to put the spark plugs in and the holes are all over sized. Help, what have I got?
 
Oversized as in worn out or just way to big? Are you sure you have the correct plugs? What is the make and number on the plugs and where did you get them? Any pictures of the head and plugs?

Charlie Stephens
 
I actually thought the head looked NOS. They are plugs that should work C4 Champion. I never even thought to check the spark plug holes, they looked good to me (not worn out).
 
Have you tried another plug in one of the holes? Quality control is very poor these days. Maybe they are misidentified or incorrectly machined. Have you tried the plugs in another head? Are you saying they are just a loose fit in the holes? Pictures?

Charlie Stephens
 
These plugs come out of a Model A, I have had Model A's for years and had a few parts around including heads and I picked this one because it looked NOS . I am not very good on this thing to send a pix but they look like a normal threaded hole but the plug will slip right through, not even close. (The whole plug won't slip through just the threaded part).
 
Take a photo, make sure it's a jpeg less than 500k, go to Advanced on the post option, manage the photo as an attachment, upload it, and Bob's your uncle.
 
Lacrown said:
These plugs come out of a Model A, I have had Model A's for years and had a few parts around including heads and I picked this one because it looked NOS . I am not very good on this thing to send a pix but they look like a normal threaded hole but the plug will slip right through, not even close. (The whole plug won't slip through just the threaded part).
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If the thread looks good then it's obvious that the holes have been tapped for a larger plug. The original holes are 7/8-18.

Maybe the holes have been tapped for Heli-coils and the Heli-coils are not there.
 
I found a chart that calls out a 1 1/8" thread for a spark but I can't find any reference for a plug in that size.

As ednondclinton said maybe tapped for inserts.
 
This is a Champion RS79N plug. 1-1/8" thread. Only available in heat range 3. $93.99 EACH.
598_p04_top.jpg
Several industrial engine heads used them through the early 50's.
Even if you spend the $$ for a set your connectors will not reach the towers.
Get a different head.
 
This problem is not holding me up I already have another head on and that one on the shelf with a note. I thought about them being tapped out for heli-coils but why would do all four.
 
Lacrown said:
I thought about them being tapped out for heli-coils but why would do all four.
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That's easy to answer. Many of the original spark plug holes have had the plugs removed and replaced and often strong-armed so many times in the last 80 plus years that the threads are practically nothing compared to what they were when new and can leak compression and combustion.

Many Model Aers would be in for a surprise if they would spray a solution of soapy water around the base of their spark plugs while the engine is running and watch the bubbles pop up.

It would not surprise me if someone did all 4. One thinks of using a Heli-Coil only when a threaded hole has been stripped out but it's a different concept when considering having to hold high pressure like combustion.
 
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