nylon lock case nuts

[farman]

Hi, I got nylon lock M8 nuts (from our host) for rebuilding my 1978 911sc engine. On their bag it says valve cover nuts. Is that the same as case outer perimeter nuts? The nuts that came off the car were a little (about 1 mm) thiker that the ones I got. Does that make a difference? Thanks, farad

[304065]

Yes, the threads are both M8x1.25 on the valve cover studs and the case perimeter studs.

[Henry Schmidt]

We never use nylock nuts on aluminum cases.
We use the prevailing torque and steel washers on all cases after 1978.
They are expensive and harder to find but far superior for this application.

[Walt Fricke]

Henry

What is a "prevailing torque" nut?

Why won't a nylock nut work? I thought the nylocks were intended to be used 1) where you can't torque the stud to the point it stretches enough to keep the nut from loosening on its own, or 2) where oil can creep up the threads of the stud -- both of which conditions occur with the valve covers.

Otherwise you don't need a nylock - a regular nut will stay put.

But why can't a nylock hold the 18 or so lbs/ft of a case perimiter fastener? I've used them (with steel washers - haven't seen any electrolysis, but these are race motors that don't get out in the rain and salt and mag chloride) on a couple of my motors, and haven't noticed any ill results.

Walt Fricke

[304065]

I am also confused by the "prevailing torque" idea. To me, a "prevailing torque" lock nut is one with a distorted top thread, or with a "triangular" thread shape-- the distorted thread creates friction on the stud which prevents the nut from backing off.

The trouble with that seems to be that for nuts that are torqued to a specific value to approximate the correct preload or stretch in the stud, you're relying on a torque spec based on an old DIN 933 steel nut, or in the case of the later cars, a DIN 985 nylock. If you torque a distorted-thread nut to the same torque value as a nylock, you won't have as much preload in the stud, because the amount of force it takes to overcome the higher friction will before the same level of stretch is achieved.

Also, prevailing torque lock nuts can damage the threads and aren't really used when an assembly is intended to be repeatedly disassembled and reassembled (not that an engine case particularly is, except a race engine).

I would be worried about the right preload particularly on the case perimeter studs, insofar as I managed to snap two of them when torquing a DIN 933 plain nut with steel DIN 137B wave washer to the factory spec. Probably some PO had overtightened those fasteners (they were at the case bottom) in an effort to stop an oil leak-- the resulting weaker studs went "POP."

But I'm sure Henry will chime in and I bet he has a good reason for using them and NOT preferring nylocks, maybe has to do with heat?

[Henry Schmidt]

It has been our experience that they hold better.
That's it. No mechanical engineering degree required.
Of course, if you have experience with other products that work, you are welcome to use them.

Generally when I offer a suggestion it is based in years of trial and error. I offer the suggestions so others don't have to make the mistakes I made.
I generally find asking the question "why" to be counter productive when success is achieved. Perhaps I'm just not smart enough.
Thank god for the engineers who have time to ponder the generally unanswerable question "why".

[lindy 911]

Henry,

You are getting old, like me, and are finding it difficult to tolerate people who question your experience. From the looks of the products you manufacture (I just bought you head studs for my SC) you know what you are talking about.

I design and manufacture lighting systems for stadiums (MLB, NFL, MLS etc,) and have been doing so for over 25 years. There is no college degree for what I do. It's all seat of the pants, in the field, trial and error. Only if there is an error I get to pay to fix it. In my business I have to tolerate the degree'd engineer who asks why I do things the way I do. I find that if you start to explain the real answer to the engineer, he / she soon becomes lost with any concept you might be trying to convey. My answer these days is "because it works".

Lindy

[Walt Fricke]

I guess I could have looked this up. The first Google reference said this:

PREVAILING TORQUE NUT
A type of lock nut which has a prevailing torque to assist in preventing self loosening. There are two main categories of prevailing torque nuts, all metal and nylon insert. All metal torque prevailing nuts generally gain a prevailing torque by distorting the threads at the top of the nut by some means. Nylon insert torque prevailing nuts ultilise a nylon (or other polymer) insert to achieve a prevailing torque.

The factory used ordinary nuts on the case perimeter, did it not?

Fact is, I have never had one of these nuts get loose, but Henry no doubt has seen a whole lot more engines than I have.

Edit: Googling farther, I see that one should measure the "prevailing" torque on a locknut, and add it to the torque required for the fastener. But where a manufacturer has specified a locknut and has specified a torque, you should use the manufacturer's specification without adding anything. Unless you are reusing the nut, in which case you should make an adjustment after measuring what the new, lesser torque might be just to turn the nut before it starts clamping anything.

Perhaps this is behind recommendations to use new nylocks for the valve covers. In any event, it allows one to reach the required clamping force.

That said, I don't think I am going to go this extra mile for non-critical fasteners. The engine I might save is only my own. This is not up there with aircraft requirements, where you die when things fail.

Walt

[Henry Schmidt]

The factory used all steel prevailing torque nuts on all late model 911 based engines. 84 and later ? Maybe 82 1/2 and later when the non sump plate case came out.

[Porschekid962]

Nylocs are not to be used in areas of high heat for starters. Secondly Jet nuts or a K nut is that much better. Use a properly calibrated torque wrench. For peace of mind you can add Loctite high temp red or possibly blue. Dont have my reference material in front of me at the moment.

For Henry,

Some people will always ask why, one would be foolish not too.

[Henry Schmidt]

Originally Posted by Porschekid962

....edit........
For Henry,

Some people will always ask why, one would be foolish not too.

Why ?

[lucittm]

Until someone earns a PhD (Porsche hardware Design) in "Post 1978 Porsche engine repair and rebuilding", I am listening to Henry. Since I cannot easily converse with the engineers back in Germany who designed and spec'd these engine parts (my German is useless), this forum is the next best thing.

I do know, however, from experience, that when the plastic in the nylock nuts melt, it is a mess.

Mark

[88-diamondblue]

What is the average temperature of the engine case? Is it that much higher than that of the valve covers since most of the fasteners (case nuts) have air flow over them. Anybody use these Unbrako FLEXLOC Locknuts would seem to be the best of both nyloc and prevailing torque fasteners.

[Walt Fricke]

I have never had a nylock melt. Unless for some reason I put a torch on it or something very near to it. I could see one used on an exhaust stud melting, though I have not used one there. Isn't going to happen on the case.

Caroll Smith stressed that it is proper torque (more precisely, proper fastener stretch which gives proper clamping force) which keeps fasteners fastened. Despite all the Bellville, wavy, split, and other fancy washers in use, I am inclined to keep his dictum in mind. Which suggests that special precautions should really only be needed where, for some reason, the required torque cannot be obtained: because sufficient torque will crush something, for instance. Like a gasket, seal, crush washer, and so on. Which isn't to say that he ignored lock wiring.

Note that nylocks are a prevailing torque fastener, at least per some definitions, though of course they can't hold up to heat as well as other prevailing torque fasteners which don't rely on a plastic for their extra friction.

But it seems to me the proper question is which fasteners, when properly tightened, tend to come loose if some additional means of preventing rotation is not applied? Porsche uses a mechanical sheet metal lock system for the 911 oil pump. Also for the ring gear bolts to the differential. The nylocks for valve covers and chain housing covers. Not much else?

The flywheel bolts can loosen on the 6 bolt cranks in high RPM race motors, so for that application Loctite red, very carefully applied, can help. I like to use Loctite blue on the grub screw which holds the shift coupler to the transmission. Hard to get all that much torque on an Allen of that size, and the piece it threads into is aluminum. CV bolts can be an issue, but retightening and proper torque make them stay put. It can be tempting to use something on valve cover and othe studs where the stud tends to come out when the nut is turned. What else? Using any kind of prevailing torque nut on a stud is going to increase the chances of the stud backing out during disassembly, is it not?

Walt Fricke