Spark plug tightening torque

Hi,
As per my search result of just 3 threads after I input 'spark plug tightening torque'

http://ferrarichat.com/forum/search.php?searchid=3728825

I would like a little more information if you all don't mind helping me with my confusion:

1. The service manual for the car (88 TR) indicates a spark plug tightening torque of 1.6 daN-m, which is = 16 N-m = 11.8 lb-ft = 141 in-ft of force.
One of the above 3 threads mentions a force of 15 ft-lb. Is this meant to be 15 lb-ft?

2. The same thread also mentions first bringing the force to 17 ft-lb, then backing off a bit, then re-tightening to the desired 15 ft-lb. Would someone explain why this is done.

Thanks ahead of time.

 

NOT very tight, IMO....stripped aluminum heads in cramp locations is a expensive %^#$&!

I took a set out not long ago that were so tight, I didn't belive it!
I was using breaker bars to get them OUT!!

You are sealing a crush washer. I don't know why you'd overtighten, the redo it....

I take it down until I feel the threads tighten up and call it good, if it blows out I guess I'll put it back in!!

 

Someone more knowledgable will have to chime in.

 

Routine I use with new plugs is to torque them to about 18 ft lbs to crush the washer, then back them out & re-torque to 12-14 ft lbs.

Used plugs just go in at 12-14 ft lbs

 

This is part of what I would like to understand the need for. Thanks.

 

I have never used a torque wrench on spark plugs and I find it impossible to relate in words how tight to make them. I have no doubt that if you went to Champion Spark plug or NGK web site the information is readily available.

I look up torque info all the time on the net. I just type in "Torque for XXX" and get more info than I could ever use.


Go to the horse's mouth for info. It is almost always more reliable than you will find in some forum anyway.

 

Brian is right, many years of experience is is your torque. In 25+ years in automotive I never had to use torque wrench on spark plugs and never strip a thread. You should be able to feel it but that goes with experience.

 

I'd swear that that method (torque higher, loosen, then retorque to a lower value) used to be given in the instructions on the NGK boxes, but now they just say "torque to X ft-lbs" (although the single torque value given seemed to be the higher one for a cast iron head, not an aluminum head -- I just put fresh NGKs in my Honda Civic, and the Honda "spec" was even lower than the NGK aluminum head "spec").

The instruction given the the 308GT4 WSM (torque to 1.5 kgm, loosen, then retorque to 2 kgm) seems more logical to me -- i.e., torque most of the way, loosen to relieve any frictional weirdness that occurred during the majority of the sealing washer deformation, then retorque to the final, slightly higher value.

PS The link in your first post isn't working, but 15 ft-lb = 15 lb-ft (although can't comment about using 15 instead of the TR WSM spec of ~12).

 

Thanks for all your coments. Since I don't have the experience needed to do this by feel, I will rely on good equipment and the Workshop Manual, woven in with your inputs.
Sincerely,
-j-

 

Thanks!!

I thought I might have been f**cking up.......

Good 'n tight is just that, in expensive aluminum, especially!

 

Seems to me the purpose of a crush washer is to maintain some "spring" or local pressure in the washer around the plug to help seal against air escape. The idea of torqueing a plug, backing it out, and retorqueing to a lower value makes no sense at all.

I have read that if you remove a plug you should use a new crush washer when you reinstall it. Torqueing to a lower value then a higher one would be consistent with what you do to headbolts etc, but is probably not necessary.

 

+1

IMHO, "feel" is one of those things that can't be taught in schools - You either "get it", or you don't.....

Torquing down a head, yep, use a torque wrench. Tightening lug-nuts - Yep, same deal. But tightening plugs is just one of those things where you "feel" it..... (IMHO)

My 02c,
Cheers,
Ian

 

In almost 30 years of working on cars and motors, I don't think I have ever used a torque wrench on spark plugs. I use a torque wrench on most everything else, but rarely spark plugs. Maybe last time I did the plugs on my friend's truck I used my digital torque wrench now that I think about it. But even then, I went more by feeling and just used the torque wrench as a guide.

A couple of important things about installing spark plugs. First, make sure the motor is cold, since you are typically going into aluminum threads. Second, I use copper anti-seize on the threads and also the crush washer surface. This will prevent galling and also help ensure an accurate torque. You do need to be careful that you don't get too much anti-seize on the threads, because it can end up fouling the plug if you aren't careful.

As far as installing the plugs, it's not so much a matter of torque, but more a matter of "seating". New plugs have a crush washer, so when you torque them down, you should be able to feel this washer crush down. Then it's more a matter of snugging the plug into place, without putting too much force on it. While you can use a torque wrench, I would go more by feel than by a reading on a torque wrench. Nothing is worse than busting the ceramic insulator on a spark plug, or worse, popping the threads in the head.

The other thing to be careful of is cross threading. Many times, spark plugs are in difficult to reach location and/or require strange angles for installation. Sometimes, you are working blind. The important thing to keep in mind is that the initial threading of the plug should be effortless. You should be able to make several turns by hand without any resistance. For this reason, I never use a wrench to start plugs. I always start them by hand (either using an extension or a length of hose slipped over the plug). Again, if there is resistance right away, something is wrong. It should take almost no effort to start the plug and run it nearly all the way down. I've seen people start right out installing plugs using a 3/8 or 1/2 ratchet and this is a great method for cross threading because it doesn't allow you to really feel what's going on. So always start the plugs by hand and turn very gently. If the plug doesn't turn freely and go right into place, then you are cross threading something most likely.

Also, keep in mind that it's always possible someone before you did, in fact, cross thread something. If that's the case, it's easy to accidentally catch their threads, instead of the correct threads. If you can see the plug as it goes into the hole, it's important to make sure it's square to the head. Also, they sell a tool which will chase the threads in the cylinder head and clean them out. I also generally blow compressed air down into the thread area to clean it. Start by spraying some cleaner down there (with the old plug still in place) then use compressed air to blow the dirt and stuff out. It pays to make sure everything is nice and clean in that area when it comes to installing new plugs.

As far as spark plug torque, my advice would be to use a 3/8 ratchet and just give it a little force once it crushes the washer and seats up. Not very much. Plugs will tend to tighten up on their own, so as long as you crush the washer and snug them up a bit, then you should be okay. You aren't torquing down lug nuts - a little force goes a long way on a spark plug. It doesn't take nearly as much as you think. It's generally best to err on the side of too little torque than too much when it comes to spark plugs.

Go by feel mostly - at least that's how I do it. I've never had any problems. "Tight" but not super tight. If I had to put it into specific terms, then I would say that once the plug seats up, then give it maybe 1/16th to 1/8th additional turn - but without applying too much force on the wrench.

I think if you go only by a figure on a torque wrench, then you are setting yourself up for possible problems. Again, you are not dealing with lug nuts here.

Ray

 

+1

Lug bolts, rod bolts, main caps, head bolts, brake caliper mounting bolts, etc. For anything like that, I break out the digital torque wrenches. For spark plugs, I go by feel.

Also, I rarely use a torque wrench for anything under 10 mm in size.

Like you say, it's all about feel on some stuff. Either you have it or you don't. And usually, developing the proper feel comes from having respect for how easily you can strip the threads on something when you have the leverage of a ratchet at your disposal.

Ray

 

TR WSM spec of ~12ft-lb

No doubt many folks on the board can attest to removing plugs that were merely finger tight (or worse).

Speculative logic:
The crush washer may not fully compress before the 12 ft-lb is reached.

Result:
If the crush washer does not fully compress, it will leave the plug seated against a partially crushed 'spring'. The partially crushed, steel washer by definition is already deformed past its elastic limit. Subsequent thermal and pressure cycle(s) may further deform the washer even further passed its elastic limit. When steel deforms past its elastic limit - it will not 'spring' back. After thermal and pressure cycles, the plug may be left with no 'spring' to hold it in place.

Fix:
Ensure the washer is fully compressed by going to 15 ft-lb (25% adder - don't push it any further - 17 is too much!). Back it out - and retorque to 12 ft-lb thereby allowing the threads to carry the load rather than the 'spring'.

Reminder - this was 'speculative' logic.

PS - Use the torque wrench!

Hope this helps,
Vincenzo

 

I agree, thus was confused to see this technique mentioned.

Thanks.

Thanks Vincenzo,
My take on this, from having a simple mind, is that if Ferrari wanted plugs to be finger-tightened, it would not have bothered to print a value for the specified torque in a WSM.
To me the numbers in a WSM mean something, even as guidelines; unless WSMs are published as mere reading materials for owners and wanna-bes while service technicians are issued guarded and secretive volumes with 'actual' specifications.

Of course all becomes moot if I now run out to the nearby Harbor Freight, bring back a $10 torque wrench and end up breaking the plug into the engine because 12 lb-ft on it is actually 50 and I had to keep applying force in wait for that 'click'.

I think a chat about torque wrenches would be more interesting and informative.

 

The reason you would tighten-loosen-tighten is to coin the threads which improves surface finish and thus improves the axial force-to torque ratio by reducing friction. But I agree it also deforms the crush washer needlessly. With good quality plugs and assuming the plugs have been changed a couple of times on our cars, and with proper lubrication, coining is not needed.

By the way the axial force to torque ratio can vary easily by 50% or more from dry threads to lubed, and the type of lube also plays a role.

Here are the Bosch instructions for plug installation
View attachment Bosch spark plug torque.pdf

 

With sincere thanks. I saved the pdf to my reference files.

 

Remember to subtract at least 25% from the torque spec if you lubricate the threads.

 

If you torque to crush the washer and then loosen it and then retorque, why don't sparkplugs just come with pre-crushed washers?

 

Copper crush washers have NO SPRING back. If you could back off a springy crush washer you could reuse them.

Examine a machined surface under a microscope and find very slight variations. The copper crush washer's purpose is to deform, CONFORMING to microscopically differences in both machined mating surfaces.

They should not be rotated after crushing as the microscopic seal can then be lost! Unfortunately you cannot visually observe it with spark plugs. In high pressure hydraulic environment, the 'Don't Rotate After Crushing' rule becomes obvious with leaks under pressure and the lesson quickly learned.

Just because Bosch or Champion put it in print doesn't make it so. Most of large companies printed crap comes from the marketing department, not from the engineers.

 

Question is: should the threads be lighted lubed or should the the threads in the block be thoroughly and carefully cleaned? I believe some plugs, like those from Bosch, have some lubricant property formed into the threads already.

Paul_308 below answers this good question for us both. So crushing creates a surface molecular seal and thus has to be specific to that immediate area.

...with blessings from the legal department

Thank you, Paul_308

 

No one has mentioned using anti-seize on the plugs. If you look at you manual you can see that Ferrari recommends using anti-seize made by Champion. Champion denys ever making the stuff, and tells you not to use any, but if you look on Aircraft Spruces website, you can find it.
I use the copper stuff from Wurth, less is better.

Brian Brown
Patrick Ottis Co.

 

That seems to be whole other can of worms! Seems about 50% of people swear by it, and the other 50% swear against it.......

I don't know if Brian uses it on my car, but I'm sure he'll let us know......

FWIW, I *always* used a little "copaslip" on the racing engines years back.

Cheers,
Ian

 

I wonder if just cleaning the threads thoroughly beforehand is sufficient.