Plug thread lengths


Founding Member
Feb 6, 2002
San Rafael, Ca
14mm, .708" reach, half thread, 5/8" hex, tapered seat, resistor, power tip, copper core, nickel plated

14mm, .708"(18mm) reach, 5/8" hex, tapered seat, resistor, projected tip, V-power (v-grooved center electrode), .039"gap

I have a question about plugs for my 2002. I want to put some TR6s in, but it seems they have a longer thread than the stock plug. I am assuming this because motorcraft, and autolite ate the same. The stock replacement autolite has a half thread, and the TR6 are has a full thread. They both have the same reach. I know many people use the TR6, and ar103s, and ar94s, both have a full thread. I have not looked at the stock plugs yet, so am guessing from peoples comments on the lack of threads in the head, and the stock plugs having less threads they are a half thread plug. It seems all the replacement plugs, but the autolites are a full thread plug.

Will using the plugs be an issue? Will they protrude into the combustion chamber further, would this not cause hot any spots. They are tapered plugs will they seal correctly.

If someone who has experience with the thread length issue can shed some light on it for me I would appreciate it.

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It shouldnt be an issue what so ever. I've ran autolites, motorcrafts and NGK's all in my cobra and also the GT when I had it. The only plug I had to ever give me issues was a motorcraft plug shooting out of the head. Long story for another time, i am sure you can search and find it.

Make sure you use some anti-seize compound on it and you will be fine.
The threads on some plugs are longer than others, but will only go into the head the same distance. That worried me once too so I put one of my new NGK plugs in, and before I started the engine measured how far down into the head it went, and comparred it to the stockers, and noticed it was the same.