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Discussion Starter · #1 ·
Many manufacturers make and spec their plug to our engines. I have heard lots of weird plug thread stories with modular engines.
I just noticed again that the products are not similarly configured at least in form, and I fail to have found a solid answer as to why or the obvious other question which is what plug is best?

Here are two specified for the DOHC engine:




WTF?
With different thread lengths, where does the ground strap actually live?

Does the one with more threads just only screw in part way or does it put the electrode/strap deeper in the chamber?

And if it is the former, how would I presume to index such a thing properly without access to several gross of plugs?
The indexing washers I use set against the tapered thread extreme inside shoulder, not someplace halfway in the threads.

If I squint really hard it looks like the longer threaded plug on screws in part way to me....

Thanks
 

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There are three threads in your heads. Thus the short threaded part.
The seat is the cone shaped part just after the threads.
There are heads that are fully threaded, so they needfully threaded plugs.
 

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Discussion Starter · #3 ·
I know I am no ticket scientist.

What does "There are three threads in your heads" mean?
 

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I know I am no ticket scientist.

What does "There are three threads in your heads" mean?
It means that there are three turns, or threads, in your head.

It also means you need the one with the short threads, not the one with the long threads, for your spark plug.

Unless you want the spark plug to become intimiately related to the piston head. And not in the good way, but in the "Oops, I dropped my bar of soap" way.

RwP
 
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You may be over thinking this. The collar above the threads is the seat as mentioned above. As long as the seat to electrodes is the same district they will both work. I'd you look in the hole you will see only 3 threads down towards the combustion chamber. The number of threads above that do not matter.
 

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The Parts Guy
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Either one will fit your "B" heads. The fully threaded one is necessary on the later heads (which have more threads).
 

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Discussion Starter · #7 ·
Thank you all.
Three threads per plug location into the head.
Therefore the shortest.
 

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Where are you going to index the plugs to? On a DOHC 4v setup, the spark plug goes in straight up and down, and is dead center of the combustion chamber, so with equal amounts of combustion chamber all around, there is no angle that is preferable to any other, so no benefit to indexing the plugs. If you really want to open up the spark to the combustion, you could side gap the plugs, but indexing won’t have any benefit on a DOHC motor.
 

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But with a 4V, which intake do you face it to?

RwP
 
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