Spark Plugs, Half Thread or Full Thread???

[JKLedbetter]

I was looking on rockauto for some sparkplugs for the SVO and noticed there are some listed as "Half-Thread Original Equipment Design", and others listed as "Full-Thread Original Equipment Design Change", and yet others which do not specify.

I've never noticed this before, how do I know which to use? I would just look at the ones that are in it now, but how do I know they're the right ones?

-Kennon

[MikeFleming]

Yes, either will work.

The iron heads are not fully threaded. Most tapered seat iron heads are like that.

[JKLedbetter]

I find this confusing.

If the heads are not fully threaded, then wouldn't full thread plugs not go in all the way?

If either will work, then wouldn't it be better to use the full thread plugs so they'll seal better and be more secure?

Does the electrode sit at the same depth on both?

-Kennon

[in2fords]

I believe that behind the threaded part is the same diameter as the outside of the threads and not the inside that way the threads will pass thru it and it doesn't matter what type you use, the full thread ones are still only using the same amount and placement of the half thread ones, if that makes sense :)

[MikeFleming]

The spark plug hole in iron heads is counterbored to a larger diameter than the threaded section below. So the "upper" threaded section of a full-thread plug won't touch anything.

Spark plugs don't (can't??) seal on the threads. They seal on the taper (or washer seat in other heads).

Taper seat plugs get tightened to 10 Ft-Lbs, btw.