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tubman · 11-17-2019, 10:07 AM

After all the back and forth on this, I decided that there must be something else going on. At Anthony's request, I went to my shop yesterday and spent some time looking more carefully at all aspects of this. It has become apparent that I am responsible for some misinformation here, and I want to correct the situation.

I made a bunch of measurements that Anthony requested and I think I found the problem. My N5C measures just under .750" from the bottom of the main plug body to the bottom of the minor plug body (the part with the threads). I then measured from the bottom of the washer to the bottom of the minor plug body and got .663". I then measured the depths of the holes in the head. I measured a couple of times just to be sure, and got between .540" and .580" for the 4 plug holes (yes, there was that much variation). Given these figures, my plug should have protruded about .100" from the bottom of the head. But it doesn't. With 20 lbs torque on the plug, the bottom is just about even with the bottom surface of the head. I tried up to 40 lbs, but it was still the same. This was quite puzzling to me, so just for the heck of it, I tried to thread the plug up from the inside of the head. I tried all four holes and try as I might, I couldn't get a plug started from the bottom. This leads me to the inescapable conclusion that the heads are not tapped all the way through and the N-series plugs are bottoming out on the threads in the head. I do not have a 14 mm tap, but I do have a quality "K-D Tools" spark plug thread chaser. I could not force the thread chaser all the way through the heads, even though I applied what I considered to be reasonable force. t appears that the N5 plugs are bottoming out on the threads in my never used heads and are sealing that way, and not with the normal gasket surface.

I'll bet that was what was happening back when I was racing as well. Since we never ran the engine for any extended periods of time, I don't think I ever even knew this problem existed. Who knows? Maybe a set of N5's would work well in an engine with my new heads, but it wouldn't be right.

So bottom line, the N5's are not the answer and we are back where we started from. I am very sorry that I have mislead everyone, but I guess this is the kind of the thing that can happen when you get casually involved in a thread and don't fully research the problem.

The bottom line is that I still believe that it would be better to have plugs that fit the heads properly, but it looks like that just isn't the case here. I guess spacers may be necessary.

11-17-2019, 10:07 AM	#118
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 10,304	Re: Spark Plug Protrusion into Head? After all the back and forth on this, I decided that there must be something else going on. At Anthony's request, I went to my shop yesterday and spent some time looking more carefully at all aspects of this. It has become apparent that I am responsible for some misinformation here, and I want to correct the situation. I made a bunch of measurements that Anthony requested and I think I found the problem. My N5C measures just under .750" from the bottom of the main plug body to the bottom of the minor plug body (the part with the threads). I then measured from the bottom of the washer to the bottom of the minor plug body and got .663". I then measured the depths of the holes in the head. I measured a couple of times just to be sure, and got between .540" and .580" for the 4 plug holes (yes, there was that much variation). Given these figures, my plug should have protruded about .100" from the bottom of the head. But it doesn't. With 20 lbs torque on the plug, the bottom is just about even with the bottom surface of the head. I tried up to 40 lbs, but it was still the same. This was quite puzzling to me, so just for the heck of it, I tried to thread the plug up from the inside of the head. I tried all four holes and try as I might, I couldn't get a plug started from the bottom. This leads me to the inescapable conclusion that the heads are not tapped all the way through and the N-series plugs are bottoming out on the threads in the head. I do not have a 14 mm tap, but I do have a quality "K-D Tools" spark plug thread chaser. I could not force the thread chaser all the way through the heads, even though I applied what I considered to be reasonable force. t appears that the N5 plugs are bottoming out on the threads in my never used heads and are sealing that way, and not with the normal gasket surface. I'll bet that was what was happening back when I was racing as well. Since we never ran the engine for any extended periods of time, I don't think I ever even knew this problem existed. Who knows? Maybe a set of N5's would work well in an engine with my new heads, but it wouldn't be right. So bottom line, the N5's are not the answer and we are back where we started from. I am very sorry that I have mislead everyone, but I guess this is the kind of the thing that can happen when you get casually involved in a thread and don't fully research the problem. The bottom line is that I still believe that it would be better to have plugs that fit the heads properly, but it looks like that just isn't the case here. I guess spacers may be necessary.