Spark plug torque and cam cap torque specs

[2cams70]

Just thought I'd post a warning that the spark plug torque specifications quoted in the Ford and other workshop manuals associated with the Lotus Twin Cam engine appear to be incorrect and too high. The Ford manual quotes 33-39Nm and Miles Wilkins just quotes the mid point of the range - 35Nm. Every other engine with an aluminium cylinder head using flat seat spark plugs and also the NGK data for engines with aluminium cylinder heads quote lower figures in the range 25-30Nm. Personally I use the lower end of the range - 25Nm for aluminium heads and never ever have I stripped a thread or had problems with spark plugs not sealing, coming loose or being difficult to remove. I also never use anti-seize on the threads and just install them dry.
I suspect the original the source of the error is that Ford mistakenly quoted the figure for their standard cast iron headed engines in the manuals and didn't make a distinction for aluminium and so the error just carried along in all other printed literature into perpetuity.
Another one to be aware of are the cam bearing cap bolts. The figure, correctly quoted in the manuals, is only 12Nm. The standard figure for a 5/16" thread is normally much higher at 21Nm. This is why the nuts here are lock nuts and not standard ones. At 12Nm there's insufficient stretch in the bolts to keep them tight through the friction acting between the nut and camp cap interface surfaces and so the extra friction provided by a lock nut is required.
So the moral of the story is - do yourself some big favours when tightening these bolts. Use an accurate torque wrench, torque to only 12Nm and use lock and not standard nuts. There's a big risk of stripping the threads in the aluminium cylinder head otherwise.

[reb53]

I've never believed the spark plug torque numbers quoted in books.

The first time I decided , "I should really do this properly", and used a torque wrench on some new plugs I very quickly thought, "you've gotta be kidding", as I felt it getting up to where I'd normally stop, and the torque wrench was telling me to keep going as I was miles away.

There's a good adage by which to live,
"Rules are for the guidance of the wise,
and the obedience of fools"....

[2cams70]

Yes I suspect the original figures are based on old spark plugs sealed with copper washers installed in iron heads. These days all flat seat spark plugs use crush type sealing washers and the clamping pressures required to make them seal properly is much less.

[mikealdren]

My Lotus +2 manual says 24-28 ft lbs.

[2cams70]

Metric now in countries except USA!
24-28 ft/lb is 33-38Nm (i.e too high - same as Ford manual)

[rgh0]

Why would you not follow the instructions on the side of the NGK sparkplug box ? This tells you how much to tighten to ensure correct crush on the washer ?

How hard can it be ?

cheers
Rohan

[2cams70]

Why would you not follow the instructions on the side of the NGK sparkplug box ? This tells you how much to tighten to ensure correct crush on the washer ?

How hard can it be ?

cheers
Rohan

The reason for me raising this as a topic is that I hear on this forum that many people have stripped their spark plug hole threads and was therefore trying to offer possible reasons and a helpful suggestion in that regard.

[alan.barker]

I've never ever torqued spark plugs. Always screw in untill contact then a light nip, no problem.
Never stripped a spark plug thread.
Alan

[2cams70]

You'll find with the NGK spark plugs when you install them fresh out of the box to the torque specifications NGK themselves recommend for aluminium heads (i.e 25-30Nm) the angle they turn after the crush washer makes contact is within the same range as if you were to use their alternative angle of turn method (i.e 1/2-2/3rd turn).

The problem arises if you remove and reinstall them in service after the crush washer has already been crushed once. If you use the1/2 -2/3rd angle of turn method again under these conditions the plugs would definitely be overtightened. Therefore best to always use a torque wrench although doing it by feel is also ok if you are experienced. I prefer to use a low range torque wrench for the most consistent result and positive confirmation that it's been tightened to spec.

[rgh0]

You'll find with the NGK spark plugs when you install them fresh out of the box to the torque specifications NGK themselves recommend for aluminium heads (i.e 25-30Nm) the angle they turn after the crush washer makes contact is within the same range as if you were to use their alternative angle of turn method (i.e 1/2-2/3rd turn).

The problem arises if you remove and reinstall them in service after the crush washer has already been crushed once. If you use the1/2 -2/3rd angle of turn method again under these conditions the plugs would definitely be overtightened. Therefore best to always use a torque wrench although doing it by feel is also ok if you are experienced. I prefer to use a low range torque wrench for the most consistent result and positive confirmation that it's been tightened to spec.

Yes I measure the torque as I tighten them to the specified amount of turn when new and tighten to the same torque if removing and replacing used plugs

cheers
Rohan

[h20hamelan]

Some people put wrenches on, before getting 3 turns by hand

[2cams70]

Some people put wrenches on, before getting 3 turns by hand

I assume by that you mean that some DPO tighten it with a wrench until the thread strips and then turn it by hand 3 turns afterwards just to be sure the job is done properly

[USA64]

I believe the instructions actually say always use a new crush washer! I have only seen them come with a spark plug in them. Can they be purchased separately? My Honda had a one-use-only washer on the oil plug. When I asked the part-man for a new one he had them right at hand. He explained he didn't give them out when he sold a filter because they cost $0.25 and people though he was cheating them!

[2cams70]

I believe the instructions actually say always use a new crush washer! I have only seen them come with a spark plug in them. Can they be purchased separately? My Honda had a one-use-only washer on the oil plug. When I asked the part-man for a new one he had them right at hand. He explained he didn't give them out when he sold a filter because they cost $0.25 and people though he was cheating them!

It depends on the type and construction of the crush washer. The ones on spark plugs are reusable and you can't get them separately anyway. Just remember to use a torque wrench rather than using the angle turn method for tightening them the second time around

[Slowtus]

I've never ever torqued spark plugs. Always screw in untill contact then a light nip, no problem.
Never stripped a spark plug thread.
Alan

Ditto and ditto.