Head Stud Nuts- Why Not Locking?

[KTL]

It occurred to me that the head studs are always fastened with "regular" nuts, regardless of whether the studs and/or nuts are original Porsche hardware or aftermarket improved hardware like Supertec, ARP, Raceware. Why is that?

I assume the stretch induced on the loooong stud does a fine job of maintaining the nut's position and it's not expected to come loose? Typically it seems that head nuts are found loose (as in, hopping around inside the valve cover) only when a stud breaks.

Thanks,
Kevin

[AlfonsoR]

I hear one reason was not being able to properly/ accurately torque the nuts. The friction of the locking mechanism would throw off the reading of course.

You could probably do it by installing a regular nut, tightening to spec torque, measuring with a depth mic from the top of the stud to the nut face, then removing the regular nut and installing the lock nut and installing to same depth while accounting for any difference in nut height. You would need to have a good, flat surface on the end of the stud.

I have also seen where some people, after torquing the regular nut down, apply red loctite to the remaining exposed threads. They think that the loctite won't allow the nut to back out. Not sure that works though.

[pharlap71]

The Supertec studs have a serrated flange for this very purpose. It locks the nut to the washer.

[KTL]

Well I found some nice M10 flange nuts that are the deformed thread/toplock type that I thought about using. They have a 15mm hex so they'll give more clearance for the lower spark plugs (12mm plugs on exhaust side). I think I may give them a try.

[chris_seven]

I have recently seen a couple of SC engines with loose head studs on the upper side of the head rather than the Dilavar Studs. The studs were not broken. I think, but can't be sure, that this is to do with stress relaxation in the case causing a loss of preload but it could be an issue with the nuts as both had standard parts.


If you measure preload and compare to torque then you can typically find a +/- 25% variation even with free running nuts. Prevailing torque nuts may be no worse in terms of variation but the mean preload level will almost certainly change.

There are several types of 'Prevailing Torque' nut available but they all need a nut factor to be determined in order to allow torque and preload to be correlated and all types will have a different factor.

The 12 point K Nuts manufactured by Simmonds are very consistent but there is no published data concerning nut factors so you would need to carry out some trials to determine the correct torque.

This means you would need to take an average of a reasonable number of studs to gain confidence in obtaining the correct preload.

I spoke with Simmonds Technical department a few weeks ago and asked if they could provide any figures but they expect the end user to develop the information to suit individual applications.

I think the studs supplied by Casper Labs (sold by Pelican) are a Binx style nut which have a lower prevailing torque than a K Nut and possibly a good solution as they may not need much more torque than a standard nut.

I have heard of people using a dial gauge on the end of the stud and a Crow’s Foot Wrench so that they can measure the stretch of a stud but I don’t believe that this is reliable as it assumes that the stud is correctly seated in the case and that there is no relative movement at the case end of the stud.

Trying to correlate torque to preload using dead reckoning from nut position seems a bit tricky.

I am not sure how a depth gauge between the nut and the end of the stud would be accurate or very repeatable and you would have to make this check on every stud/nut but the pitch accuracy of the thread cutting could be a significant variable and I wouldn’t be confident in this method.

There are several ways to determine preload in studs and to obtain reliable information but they are all a little costly.

SPS Technology manufacture preload indicating washers which are very useful and I have seen used in some critical nuclear installations but I have no idea of cost, but I can imagine they would be expensive.

http://www.spstech.com/aero/prod_lit/pli_brochure.pdf


If these were used under every nut confidence in the correct preload would be quite high.

Another good way is to use an ultrasonic preload measurement. This instrumentation is becoming more readily available and allows every stud to be checked. The typical cost of a suitable instrument is around $3000 which is why they are currently used in very high end race shops.

We have just ordered a strain gauged load washer and plan to make some measurements with a Standard Simmonds 12 Point K Nut and to try to determine the torque to preload relationship and typical variation.

We plan to use the last published tightening method used by Porsche which is torque to 15ftlbs and then tighten 90degrees.

It is probably not as good as direct measurement but will most likely give a good balance between cost and repeatability.

Once we have the correct torque established I will publish the figures and the part number of the nuts. I think this will be a reasonable compromise in terms of cost/ease of use.

I am not sure that it is fair to describe a serrated nut as having a locking feature when used in conjunction with a ground washer although the serration will provide additional resistance to vibration.

[pharlap71]

I am not sure that it is fair to describe a serrated nut as having a locking feature when used in conjunction with a ground washer although the serration will provide additional resistance to vibration

excuse me, "lock or "locking" is probably not accurate, but serrated would tend to have more friction with ratcheting to help from backing off. I guess the only sure way of "locking" in the literal sense would be to spot weld the nut to stud.

[KTL]

Thanks for the detailed explanation of studs and locking nuts/torque application.

I agree that the serrated nut surface is of questionable benefit when used atop the thick factory Porsche washer. The smooth surface of the washer against the head surface will be the area where loosening/slippage will first occur. Having the serrated nut against the big washer is similar to it being a giant flange nut with a smooth bearing surface.

There's no need to weld the nut to the stud to lock it. That's crazy talk! All sorts of thread locking compounds will lock it. Red Loctite 262 or 271 is some strong stuff. It needs considerable heat to break the bond and allow the fastener to be removed. It's a common use on the 911 for flywheel bolts, stock-type rod bolt nuts and head studs into the case.

[pharlap71]

the reason I used the "crazy talk" was addressing the quote "the serrated nut not having a locking feature" above. Even with locktite, using this standard is not totally "locking" because you can remove it, although a pain. I was being facetious to make the point other than absolute union everything else is just aids in preventing rotation, some better than others.

[pharlap71]

PS- I agree welding is "the crazy talk", although Porsche has been known to WELD the heads to the cylinders! Which I also think is crazy.

[KTL]

Originally Posted by pharlap71

PS- I agree welding is "the crazy talk", although Porsche has been known to WELD the heads to the cylinders! Which I also think is crazy.

Yeah I remember reading that in Anderson's book. Electron beam welding? WTF...... The extremes Porsche went to way back when, to make their engines survive. Amazing.

[Flieger]

I believe Electron Beam Welding is fundamentally arc welding (like TIG) using a vacuum instead of a shielding gas like Argon.

[KTL]

Yeah, but it's more precise than typical TIG. Aerospace and DOD usage means it's precise and certainly not cheap. Especially back in the days when Porsche was doing it.

Electron Beam Welding LLC - electron beam welding - design

[Flieger]

Yes the vacuum should mean a more stable "arc" and a thinner one than the plasma arc which TIG uses.

[chris_seven]

It is custom and practice to describe any 'prevailing torque' nut as a lock nut and if you look in any cataloge from SPS, Simmonds, Farichild and any other high end fastener manufacturer you will find this description.

There are many NAS, AN and MIL specifications covering the torque needed to loosen this type of nut.

Serrated flange nuts don't have any prevailing torque feature and the 'wedge action' only works when this type of nut is used in connection with a soft and deformable contact surface.

The simple point I was trying to make was that it isn't usual to describe this as a locking feature.

[ix0ifan]

Originally Posted by pharlap71

I was being facetious to make the point other than absolute union everything else is just aids in preventing rotation, some better than others.

fa·ce·tious/fəˈsēSHəs/Adjective: Treating serious issues with deliberately inappropriate humor; flippant

[chris_seven]

Electron Beam Welding is just what it says on the tin.

A beam of high energy electrons is accelerated in a column which is similar to an electron micropscope.

The kinetic energy of the electrons is converted to heat when they hit the components being welded and cause fusion.

The vacuum, typically 10^-5 Torr is needed to obtain a decent electron efficiency.

The development of high capacity Turbomolecular pumps has enabled these high level vacuums to be developed even in relatively large chambers and I think some of the more modern plant can work quite well even at relatively soft vacuum levels such as 10 ^-3 Torr.

The benefit of EBW is that the beam is very narrow, highly focussed and very high levels of fit and alignment can be obtained with careful fixturing of components and very little distortion occurs.

[AlfonsoR]

Would love to see the rig and jigs Porsche used to weld the cylinders to the heads. I read the stuff on wiki Electron beam welding - Wikipedia, the free encyclopedia It looks fairly elaborate. Probably was done by an outside shop.