V12 plug gaps

[Jaguar_Enthusiast]

Right, a real head scratcher for me in what I thought might have been a well-trodden question!

As background, my car is a UK mid-1977 V12.

I’m trying to get to the bottom of what’s the right spark plug gap. These are the three paper-based places I’ve looked:

(1) Owner’s manual tells me 0.025 in (0.64mm)

(2) Workshop manual tells me: “

Gap - Except cars to federal specification 0.025in )
)Early Cars
Gap - Cars to federal specification 0.035in )

Gap - All cars 0.035in. )Later Cars”


(3) Haynes tells me:

“All models (excluding North America):

Up to 1977 0.025in
1977 on 0.035in”

From the workshop manual I’ve no idea what are “early” or “later“ cars and in the change from one to the other what changed elsewhere to the engine which needed a change to the plug gap. Compression maybe?

Going by Haynes, as my car was built mid-77 I ought to be going for the 35 thou gap and not 25.

This all seems strange to me - my car was one of the latter V12 Coupes, so can’t work out why there would be such specification changes just before the plug was pulled on the car (no pun intended!)

Really interested in what others do or think. I saw with interest Mike’s post from a few years ago about decals for earlier models!

[mike020150]

You are quite right about the confusion. A long time ago, I tried running my 1977 V12 coupe with 35 thou gaps and I wasn't impressed. That appeared to introduce occasional misfires. Ignoring the emission spec engines for N America, what I suspect is that the 35 thou clearances were introduced for the high compression P Digital equipped flathead V12 engines, none of which were fitted to series 2 cars. Hence, my advice is to stick with the Jaguar handbook recommendation of 25 thou plug gaps. Mike.