I've been dealing with a bolt problem at work for a week. To make a long story short...we've had some bolts that were being torqued to 1000 ft-lbs and some were snapping. In bolts...tension is the goal...not torque. Torque is just a method to acheive tension. High tech bolting procedures call for measuring bolt elongation using ultrasonic waves. Torque is not important because of the many variables in torque readings.

Some of the bolts were supposed to get lube on the threads to acheive the right stretch at the given torque. We were getting the right torque...but since the threads were dry the elongation was way off. Proof that lube makes a big difference in what applied torque does to the bolt.

Thus...on a spark plug its a little different. The torque applied holds the plug steady and were not concerned with "stretch". I was wondering about Ford using dry threads and those who apply antiseeze. Leaving the theads dry...one would get the torque way sooner than those who use antiseeze. Something like 80-90% of torque is lost to friction so lube makes a huge difference at the thread level. I'd like to use antiseeze for other reasons besides torquing. I just don't want to stress the threads more than Ford had designed them for. Any thoughts on this?

 

I happen to think that anti-sieze on the Triton application is the only way to achieve the proper torque. The reason I believe that is because in todays production processes there is no way in knowing that the plug manufacturer and the Triton engine's head manufacturer are holding the same tolerances on their threads. That's not to say that the threads are junk by any means, but that the tolerances are a little loose and could be tight here and loose there. The anti-sieze will not replace exact thread-fit, but can compensate for those tolerance differences by giving (or allowing) the proper torque.
Heat and expansion of alum alloy and carbon steel are different and sometimes the only thing that will prevent a bolt or plug from working loose is the proper torque.

 

Most things I work on that require a specific torque value, we are given two values, one dry and one lubricated. The lubricated fastener always takes less torque to achieve the correct stretch on the fastener. If fasteners are breaking they may be the wrong grade. In the corrosive environment of a pulp mill, we use anti-seize on almost everything we do just so we can get it apart. We use a lot of SS fasteners and anti-seize helps prevent the stainless from galling or picking up. Two days ago we were taking off the hydraulic cylinders of a refiner. These socket head bolts are torqued to 2000 ft pounds with anti-seize on the threads. We use a 4x multiplier wrench with a 40" long torque wrench set at 600 ft pounds. We figure on a certain loss for friction so that is why we set at 600. Anyway comming apart with the same equipement can take way more effort on certain fasteners. Some of them one man can get by himself, some take two of us pulling our guts out. Just an example of how some things stay tighter than others even if they are installed exactly the same. I guess machining is never 100 % identical on any two parts. I guess this is why some motors get loose plugs and some don't. Wrench.

 

Do not forget the physics of fasteners vs. plugs. Also be aware that most of the PLUG type book information is usually refering to NPT. National Pipe Thread is a deliberate quench fit due to angular differences of the male to femal taper.

In straigh bore threaded holes the object of the thread pitch, and count, is to HOLD preassure on the MATEING surface.

In this case the MATING surface top most wide flat or tapered surface on the base of the plug that contacts to top most portion of the aluminum head.

The un oiled, dry mating surface is sized in diameter and total surface area to resist vibration, hot + cold expansion differences, and retain the plug in the bore by friction tension. This is also the electrical bonding point. It is also the surface interface between the steel and aluminum that has the highest probability to gall. Galling is always influenced by localized heat. High turning speed in meters per second generats heat on a sliding surface at a rate that is proportional to the surface area and speed and duration.

I never apply anti sieze to this interface contact area. I always use anit sieze to the threaded portion only. My two stage torque sequence is done to A) relax the threads during the 10 foot pound twist and B) To Let the plug to head interface cool. The final torque to the final 14 foot pounds is always less than 25% further clockwise rotation and with the two metals back to room temperature any galling is reduced or eliminated.

Of course we are in no way concerned with any stretch of the plug threaded area. But must be cautious with the probability that over torque may shear the aluminum threads. That is why I am always harping about using good calibrated torque wrenches (preferably "click" type) and I try to always promote the idea that more torque is NOT good.

I have decades of wrenching on aluminum and magniesium motor cycle engines.. anti sieze (properly applied) is your friend. Calibrated torque wrenches are manditory. This is not your daddys iron head 460!

As to the implyed question from 3speed about what effect anit sieze has to the final applied torque of 14 ftlbs... is it higher or lower due to the slipperiness of the anit sieze compound?

The answer is no the torque wrence will record the exact point where 14 foot lbs of rotating force is felt. but the total turns in degrees of the dry torque will be less.

Assuming a test of identical threaded bores and materials and tempurature and torquing technique... two plugs one dry and totaly oil free the other dry and oil free but the plug threads lightly coated with anti sieze.

If you mark the plug and a corresponding mark on each hole the dry hole will indicate 14ftlbs "x" fractions of an inch less total rotation or turns than the anti sieze plug. Measured as a fraction of total turns and the implied added stress towards shear this further rotation "wet" is very small at this relativly low 14 ftlb torque setting.

That is not to say it is insignificant. If you assume the original aluminum threads have never been over torqued then we are well within the design tolerences of the materials diameters and alloys in question.

BUT if a threaded bore in aluminum alloy has ever been over torqued then the shear stress distortions to the base of each threads' lands has been compromised and will no longer A) have the same elastic strength and B) a greater tendency to fail under the stresses of hot/cold high pressure excursions up and down the scale.

If you ever suspect plugs have been significantly over torqued you should consider using the dry instalation or "wet" but calculated with a 10% reduction in total torque.

I believe Ford engineers specified 12 to 14 footlbs for the modular motor plugs. As a function of percent this is a very broad range.

 

To explain that thread land a little for others, Fred (if I may): Aluminum Alloy is unforgiving in elasticity. As soon as you overtorque you are always in danger of cracking (not stretching) the threads.

I'm through now, Fred.

 

It's threads like this that make me wish the engine was made of wood. Then I could work on it with complete confidence. Until that happens I'll have to keep reading to see what you fellows have to say. LOL