Spark Plug Tightening Torque

For years I’ve just done what most people do I suppose which is tighten them until they feel “right”. However, in running through the Bentley manual on page B55 I came across the apparently Jaguar required setting of 27lb ft. I tried tightening one plug down to that and stopped short fearing the worst. Anyone talk to why the Bentley manual has such an apparently high torque setting…maybe a misprint…maybe correct???

That seems like tremendous overkill and likely to lead to thread damage. Maybe the crush gaskets they had back in the day were not as good. Every spec I’ve seen says 14-16. On new plugs I’ve had no issues with 14. If they need to come out for inspection and go back in again, then I’ll retorque to 16 so it gets some extra crush.

I use a stubby ratchet which helps prevent over tightening and I swear I can feel when the crush washer has crushed. That’s when I stop.

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Yes I feel the same crush. If the plugs are not leaking why overtighten.

I have always done it by feel. A typical 3/8” drive socket wrench is not so long where you can get a great deal of leverage, and you can feel it as it snugs up. I would be apprehensive about using anything bigger / longer. Remember you’re putting steel into aluminum threads. And if there is one place you don’t want to pull aluminum threads it’s in the head.

From the early days, circa 46 til now, I’ve relied on “snug and a tug” for spark plugs in iron or alloy…

Only once has that failed me. In my 63 Corvair. stripped one!!! lead to an engine overhaul!!! Shipwright disease…


Stand by for some heresy.

Like most here I am scared of a plug thread going soft, as it is likely to do on our 50 plus year old cylinder heads. I have had one go on me, about 14 years ago. Since then I’ve been using iridium spark plugs.


The electrodes of Iridium plugs do not ‘wear’ like normal plugs do. They can stay in the head for tens of thousands of miles. Modern fuels are so clean they never foul up, so the central electrode stays clean. I have just bought my third set to be fitted this winter. I let them go for 15,000 miles or so, about four years of my driving. I also gap them slightly larger than normal, as I’m using a 40,000 volt coil.

Using plugs that work well several times longer than standard means I replace them less often. And that means re-tightening them in the head fewer times as well. And it is the continued stressing of the thread, rather than over tightening, that is likely to make the thread go soft. Aluminium is like that, it looses its maliability when worked, it goes brittle. And that’s when threads get stripped.

So using iridium plugs puts off the inevitable requirement to helicoil the plug threads, or at least the five that are original.

Helpfully, the NGK plugs I use have the same number for their heat range as the standard plug. So where I used to use BPR5ES the iridium alternative is BPR5EIX.

If these plugs are ever likely to cause a problem, it hasn’t happened to me in well over 30,000 miles of driving.

If you are thinking this is heretical enough for critical comment, just wait until I tell you about using synthetic engine oil.

Not in the least. …

The spec for the V12 H.E. is 8 ft-lb. That’s so low that the slightest bit of crud on the threads can have you stopping short before they are tight, and we recently had one report of a guy who had done just that without knowing it. Basically, you need to chase the threads and apply anti-seize compound to make proper use of that spec.

Note there is a difference, though. Your XK engines, as well as the V12 pre-H.E., use washer seals on the spark plugs, but the H.E. uses taper seals – no washers. I dunno why that’d make any difference to the torque spec, since the threads are the same. It will make a difference to the feel, though, as you can generally feel the washers starting to crush, but the taper seal plugs just come to an abrupt halt.

My recommendation: Stop by a gun store and pick up a gun cleaning brush for a shotgun, probably about 20 gauge. These are high-quality wire brushes of various diameters, and by picking the right gun size you can pick a good size for what you’re doing. These brushes attach to the end of a long rod for pushing them down gun barrels, again very high quality tools. Just the thing for twisting them down a spark plug hole and cleaning up the threads.

Most of those gun cleaning brushes have brass bristles, but I found a guy selling them with stainless steel bristles and I bought one of each. Great for cleaning up whatever threads I encounter.

Thanks for all the feedback. There’s no way I was going to wind them down to the printed 27 but was curious of what others thought…has to be a misprint.

Plug info from NGK

So according to the NGK plug torque guide, a 14mm plug should be tightened to about 18.5ft lb (25Nm). Still seems like a lot.

I think it is completely unnecessary honestly

according to the big green official jaguar manual on page B.55 it says "Tighten the sparking plugs to a torque of 27 lb.ft. (3.73kgm)

nevermind, I see you already pointed this out. sorry for the pointless post

personally never have gone beyond 21 myself

I never, ever go beyond half a neck cord.


My notes (from somewhere years ago) say to use 18 - 21.6 ft lbs for an aluminum head and I always go with 18 (actually 216 inch lbs).

I use a torque wrench every time and suspect that those who think they know when it is just right are possibly over-torquing the plugs.

At 18 lbs I have never had one come loose and have never stripped one.

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Some, possibly correct: I’ve done so many-- and, yes, every so often, I check my neck cords with a torque stick-- I get them within 1(ish) ft-lbs, every time.

That said, for those who haven’t changed a gabillion plugs, a torque wrench is a good idea.

If you wanna use a torque wrench on the plugs on the V12 H.E., you’d better set aside the rest of the day, because you’re going to have to move the A/C compressor and the throttle pedestal to get a torque wrench in there.

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Neck cords, it is! :grimacing:

Truth of the matter is, I’ll likely only change a set of plugs two, maybe three more times in my life.

hard to over torque when you use one of these.


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