Recommended spark plug

What is the latest recommended spark plug for the 5.3L HE? I checked archives and Kirby’s book and I came up with more than 8.

I went with a BP6ES and that turned out to be big for the hole in the head.

Um… huh? That completely confuses me.

The hex at the outer circumference on the plug is big and interferes with the hole for the plug and with the plug hex socket, the socket will not fit in the hole at all.


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From the manual:

We discussed spark plugs last week on the XJS side:

I think non-projected means the gap itself is even with the end of the threaded body of the plug, while projected means it sticks out a bit farther into the combustion chamber. I don’t actually know if it makes any difference on the H.E. engine. Supposedly there’s a swirl going on as a result of compression, and the swirl should be bringing a combustible charge past the spark plug gap.

There is some possibility that this charge will be richer or leaner depending on centrifugal effects. If fuel vapor is higher density than air (which may or may not be true), then perhaps the little swirl causes a localized rich area around the circumference of the swirl chamber. Richness would be good for spark ignition, so you’d want the plug gap to be right there at the rich area. Perhaps that means non-projected would work better than projected, I dunno.

Another possibility is that the swirl causes some of the fuel to settle out on the surface of the chamber, again resulting in a localized rich area, and again making the non-projected the better plug.

OTOH, perhaps both of these ideas are completely backwards and the swirl causes a localized lean area instead, meaning the projected is a better plug.

This is where I have suggested indexing plugs might help. If that swirl is truly bringing the charge past the plug gap, the one thing you don’t want is the ground electrode being directly upstream in this flow path because it would shroud the gap. The plug rotated to any other position would be better, with the ground electrode being to either side or behind the gap as the flow approaches.

Thanks Steve, I appreciate that.


I remain unconvinced, that on a relatively low-performance, naturally-aspirated engine, that plug indexing buys you bupkus. **

I did it on my Datsun 1200 race engine, but that was because of the 13:1 Cosworth pop top pistons: I HAD to have the ground electrode facing away from the piston crown, not to mention the 0.050" valve-to-piston clearances.

** Except cars-n-coffee bragging rights…:wink:

Ah… looks like they are 5/8ths plugs.

On any normal engine I would agree. The only reason this might be an exception is due to the swirl chamber in the H.E. head. And even if it did make a difference, I’m not sure we’d be able to verify it. It’d be fun if someone with a rough idle indexed the plugs and made no other changes and ended up with a smoother idle, but we’d still really only have a seat-of-the-pants opinion that it made a difference.

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True, and fairly-easily done, with a stack of indexing washers, aligned plugs, and a good, known car/engine.

DO NOT, I repeat, DO NOT use “indexing washers” on the Jaguar V12! The H.E. uses taper seal plugs and the spark plug gap is positioned at a carefully controlled depth. If you choose to index, simply mark the insulators to indicate where the ground electrodes are and swap plugs around as necessary.

Learn something new, every day! Tapers, like older Fords did?

The older Fords were great big tapers, I think. These are smaller, same size threads as washer-seal plugs, and the hex is smaller yet, 5/8" instead of the conventional 13/16" IIRC.