Xylan Coating for Head Studs?

[cribbja]

Curious if anyone has experimented with using Xylan coated studs in aluminum engine blocks? Particularly as head studs?

Steel studs set into aluminum blocks are nearly always troublesome to remove after many years of use, and many times will take the aluminum threads with them on extraction.

It would seem that Xylan would help alleviate this problem. Are they perhaps "too" slick to be used? Perhaps even thread lockers aren't effective with Xylan?

TIA,

John

 

[CoryPad]

I can't say for certain that a Xylan brand coating is used for head studs, but similar low friction coatings are used everyday in production car engines. Whitford, the manufacturer of Xylan, has not penetrated the automotive market very quickly, but they are working on it.

Regarding thread adhesives, in general they do adhere to Xylan fastener coatings.

 

[cribbja]

Thanks Cory, I'll be talking to Whitford this morning about the application and will update the thread with any useful information.

 

[FennLane]

The use of coated studs is quite common on air cooled Porsche 911 engines.

The lower studs on this engine are exposed to the road surface and certainly in North America tend to become contaminated by salt.

Early 911 engines used steel studs which seemed to cope reasonably well without coating but in the Seventies when engine cases were changed to magnesium die castings and cylinders were made from aluminium instead of cast iron the stud material was changed.

The new material used had the trade name of Dilavar and seems to be an Austenitic Precipitation Hardening Stainless Steel.

Due to its relatively high CoE it cured the problem of head studs pulling out of engine cases due to cylinder expansion but it does suffer from SCC and broken studs have become a common issue with these emgines.

the latest generation of stud uses a coating to eliminate SCC but this coating is not applied to the threads so that studs can still be installed using an appropriate threadlocker.

It is also possible to buy thread coatings which electrically isolate the stud from the engine case and we apply this type of finish to a Titanium Stud that we use in Magnesium cases so we eliminate the chance of bi-metallic corrosion.

 

[cribbja]

Thanks FennLane, this is good info. Strange that I didn't find much discussion about coated head studs with the usual Googling.

After speaking with Whitford, they advised that several of the automotive OEM's in recent years have used Xylan or similar fluoropolymers to fix certain head warping issues, then revised their head & stud designs to eliminate the warping (and the need to use coatings). Apparently the fluoropolymer coatings on head studs also helped with warping issues for a major diesel manufacturer as well, however they haven't been adopted by this manufacturer for reasons unknown.

One point that I'm a bit unclear on is this: For a given clamping load, assuming the makeup torque for the head studs is greatly reduced due to the lowered coefficient of friction of the Xylan coating, is there a greater possibility of the nuts loosening? Presumably, since the clamping load is the same, the stud stretch is the same too - the torque reduction is due solely to the reduction of friction from the use of the Xylan.

Perhaps in this case, additional friction is a good thing?

John

 

[FennLane]

Stiction/Friction is the major cause of nuts coming loose.

Too much friction means that there is too little preload to hold the assembly together and any vibration allows the nut to rotate.

If the preload is correct the elastic deformation in the threads keeps it all together.

The relationship between torque and clamping force which is what holds the joint together is extremely dependent on thread friction (Google K or Nut Factors and you will find significant information)

If you reduce friction you will certainly have to re-define the torque you apply or you may overtighten the stud with catastrophic results.

ARP for example suggest reducing 'dry' torque figures by up to 50% when using their proprietary thread lube.

If you can do some calibration with an ultrasonic gauge it would help to be sure you had the correct values.

 

[CoryPad]

Nut loosening is almost always caused by lateral loading and not related to friction coefficient.

 

[Screwman1]

I agree with Cory and in addition the amount of stretch on a properly tightened head bolt should prevent loosening. You will need to adjust the assembly strategy if you are using torque as a seating strategy. If you use torque angle you should not need to adjust the final angle, maybe reduce it a few degrees because of reduced torsional windup in the shank if you installing bolts, if studs you won't need to adjust.

 

[FennLane]

Coerypad - I think what you are describing is the Jost Effect which is rarely present in automobile engines and specifically cylinder head studs which was the point of the discussion.

The basic discussion involved cylinder head studs and without using costly ultrasonic equipment it is difficult to use stretch as a tightening strategy although this would be my preferred method for tightening connecting rod bolts.

However, stretch only holds a joint together due to the preload developed in creating elastic deformation. Stretch is clearly a more accurate method of determination than torque where nut factors have a significant influence. This was my point as the statement "that more friction maybe useful" was being made and this is incorrect.

In my experience the most common problem with nuts coming loose on cylinder heads is lack of care in terms of thread quality and cleanliness when using torque as a tightening procedure. A failure to apply the correct preload will alays cause problems.

I could agree that using a lower initial torque and then an angle will reduce the variations in preload that occur when just using torque but many manufacturers do not provide this data and only give simple torque settings.

 

[CoryPad]

I was describing transverse slip, which does occur in automobile engines.

 

[FennLane]

Dissimilar metal joints may see some very small amounts of transverse slip due to differential expansion but this is very unlikely to have sufficient magnitude to overcome friction in a circumferential direction and cause nuts to loosen.

For this to be a problem the coefficient of friction of the thread will probably have to be less than 0.03.

Of course the lower the preload the more likely the forces created by differential expansion are likely to cause loosening.

For similar metal joints transverse slip due to differential expansion can be neglected.

Axial vibrations have very small potential to cause loosening unless once again preload is very low.

Bending can be be an issue but cylinder head and block faces need to be parallel and if not other problems will rapidly cause engine failures so nut loosening is unlikely to be much of an issue.

Torsional vibrations have most influence on nut loosening but I don't think there are torsional vibrations affecting cylinder heads.