Spark plug tourque

[chrisfell]

"Following your logic then I assume that you don't torque cylinder head bolts either?"

If the studs were made of aluminium or the domed nuts were made of aluminium, I'd be just as careful.
However, on my car the studs are steel, as are the domed nuts and the studs screw into an iron block. Steel and iron doesn't deteriorate in the same way the aluminium does. So I use normal torque settings.

As already mentioned, you are free to follow any advice you choose.

[cactusman]

Cylinder head removal is not routine maintenance and for sound engineering reasons needs to be torqued up.....how often does one remove the head in comparison to the plugs? Cylinder head gaskets are intended for single use with an appropriate clamp force over their entire surface whereas plug washers are small and suitable for 're use so long as they have not been over crushed...as you say the head studs screw into steel and have steel nuts on top so the aluminium head is merely clamped to the block ensuring a tight seal and ensuring stresses do not distort the head. The spark plugs screw into aluminium alloy. One usually cuts far coarser threads in softer metals but as plugs come with a standard thread that is finer than is probably ideal in alloy the thread in an alloy head needs looking after....just my opinion so feel free to torque if you like.

[Mark Gordon]

Other than, as the OP mentioned, the OCD of his friend, I don't understand all of the concern about "proper" torquing of spark plugs. I agree that it's important to avoid over-torquing and stripping the threads in the head, but what is the concern about under-torquing? The worst that could happen would be a compression leak that would be very obvious when the engine is running in which case you would just snug the offending plug down a bit more. It's not as if anything catastrophic would happen from under-torquing as would be the case if, for example, the big end bearing nuts weren't torqued to specification. The key is to get the plugs tight but not God-awful tight and let it go at that. I would think that most anyone who has put a wrench (spanner?) to a car more than a few times has developed the feel for "tight enough" and lets it go at that.

[Gfhug]

Well Darren as OP, what have you decide to do having 'sparked' quite a difference of opinions for people to torque about?

Geoff

[cactusman]

My thoughts precisely Mark. If a plug gets loose it would be a minor inconvenience and very noticeable. If a big end got loose it would be a bind to fix. If a brake caliper fell off you might wind up in a hedge...

[Series1 Stu]

"If the studs were made of aluminium or the domed nuts were made of aluminium, I'd be just as careful.
However, on my car the studs are steel, as are the domed nuts and the studs screw into an iron block.

Yes, and the cylinder head that they're all clamping down is aluminium. Saying that steel doesn't degrade like aluminium makes no sense at all. It rusts, and drastic reductions in strength can result. How many snapped cylinder head studs have resulted from steel corroding? We read about them all the time on this and other forums.

As I said before, keeping to the properly calculated torque figures is a sound way of ensuring that all of the materials are maintained within their elastic limits. You only usually get problems when somebody hasn't observed the correct procedures.

Saying there are no potentially serious problems resulting from an under-tightened spark plug is erroneous too. I have seen spark plugs fired out of cylinder heads at great force and fires in engine bays resulting from loose plugs. That's not something I'd want to face.

The spark plug crush washer is there to provide a seal. Regardless of that, you still need the correct amount of tension in the plug threads to ensure that it stays put and doesn't rattle loose with the heat cycling and vibration it finds itself subject to.

I'm not sure how aluminium degrades with age. It age hardens, yes, but that happens within weeks of being commissioned due to the heat cycles it sees. It doesn't really change after that, provided it is looked after.

Years of designing and developing safety critical mechanical devices tells me that failing to get your torque settings right will give you problems further down the line. Torque figures are specified on all assembly drawings and service procedures as matter of course.

Carry on and do whatever it is that you do and you'll probably not have a problem. But you might. Or you might just be creating problems for the person that comes after you.

Regards

[abowie]

Hi all anyone know of the top of their head the tourque setting s for spark plugs in jaguar xk engine . A mate asked me today and I said I'll find out ..... mine are done up to the click in my elbow ... but this guy is pedantic about these sort of thing . Cheers for any ideas

https://www.ngksparkplugs.com/about-ngk ... stallation

[dal2.0litrefrogeye]

Well Darren as OP, what have you decide to do having 'sparked' quite a difference of opinions for people to torque about?

Geoff

Well I'm with the gang that hand tighten and then tourque by feel /experience . But as I originally said my chum iis pedantic about this sort of thing (read, less experienced so needs to go by the book ) so I gave him the tourque figure . I personally have never tourqued a plug and never had one come loose or stripped and/or seized ...... yep I'm lucky

[Mark Gordon]

"Saying there are no potentially serious problems resulting from an under-tightened spark plug is erroneous too. I have seen spark plugs fired out of cylinder heads at great force and fires in engine bays resulting from loose plugs. That's not something I'd want to face."

I suppose that almost anything is possible, Stuart, but it is hard to imagine how a loose plug would cause serious damage or an engine fire unless you totally ignore what is going on with your car. I did have a spark plug come loose from the engine once, but the sound was very obvious as soon as it happened. I pulled to the side of the road and found one plug hanging by its HT wire but without any damage to the car or engine. I re-inserted it and tightened it down and had no more problems. From that I learned to start the car after R/R'ing the plugs (or anything else for that matter) and checking for leaks and unusual sounds.

[Rustyred]

To say torquing plugs is necessary is incorrect, it just isn't required. The screwing of the plug till it seats then tighten another 1/4 to 1/2 a turn is more than adequate.
The torquing of any fastener is needed for correct clamping of any component to give it proper squash and to prevent failure of those components or fatigue of the fastener during service.

The spark plug is just a plug, it doesn't hold or clamp anything down, and as with most fasteners the thread has to be clean and free of carbon build up, so you can get the proper feel of plug seating down for good sealing against the washer. I have had many vehicles where the plugs have been loose or not installed snug enough and I have needed a breaker bar to get the little suckers out, cause the carbon leakage up the thread just about welds itself to the head.
Point is if the plug threads are dirty or slightly crossed and the average home mechanic has over-tightened, by stretching the threads you will have issues, regardless if the manual say to torque them to 100f/lbs, that plug will not squish against that sealing washer.

[Series1 Stu]

Well Andrew, it seems that even NGK are wrong on this one.......

[malcolm]

Not seen this much excitement since my Granny broke her arm rushing to get a glass of wine!

[mark10337]

It is interesting (as an observation) that normally we say on the forum, Jaguar did things for a good reason and that we should do it by their recommendations... except of course where we can take an age old practice and do it by mechanic feel of that's good enough, plus some say 1/4 turn some say 1/2. Finger tight to one person is not the same as finger tight to another. Using the numbers given by the torque settings gives a standard to which it should be done. Whether or not that standard is too high is another matter. What would be the torque setting of finger tight + 1/2 turn?

[Series1 Stu]

That's my point exactly. Working by feel is not consistent from person to person or car to car. Using properly calculated torque figures ensures it's all consistent. Most people would be surprised at how little effort is needed to correctly torque their spark plugs, which suggests that they would naturally overtighten them.

Buy a good torque wrench, look after it and it will look after you.

Regards

[Hugo]

There is one other factor which so far we have not considered, and that is temperature. In my youth, we were told never to remove or replace plugs in an al2 head when the engine was hot. Presumably because, as the head cools, whatever torque has been applied effectively is increased as the thread pitch contracts as the head cools

I have a Bristol 2 litre with an aluminum head and weird head bolt arrangement which means you can't get a torque wrench anywhere near them. And they are (very fine) cycle thread of all things. Anyway, I overhauled the head on mine & tightened the head down to what felt about right. When I went to re-torque the nuts, I was amazed to find that I could spin one of the nuts with your fingers. At first I thought I was cracking up & had forgotten to tighten it, but in fact it had completely stripped the threads - as the head expanded with the heat it had just pushed the nut off! And of course it couldn't just strip the nut, could it - it had to be the stud, which meant the head had to come right off again!
Going back to the original post, if your friend needs a torque wrench to tell if his plugs are tight enough, maybe he should be getting somebody else to change the plugs on his engine? I saw a figure of 27 lb/ft in an earlier post - that sounds alarmingly high for a plug in an aluminum head.