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Today I got hold of a copy of a Porsche Technical Bulletin dated 12/85 (Dutch language) regarding 2 changes regarding repair instructions on the 928.

1) camshaft adjustment 928S USA 4-valve engines
2) torquevalues for cilinderheads 2 and 4 valve engines with studs (not bolts)

1. Camshaft adjustment for cilinderbank 1-4
New value: 1,6 mm +/- 0,1 mm (old value: 2,0 mm +/-0,1 mm) valvelift height at 20 after TDC
Cilinderbank 5-8 remains 2,0 mm +/- 0,1 mm valvelift height at 20 degr after TDC
An altered procedure is given how to perform this adjustment (including pictures)

2. Torquevalues for cilinderhead
New torquevalues in 4 stages:
stage 1: 20 Nm
stage 2: 90 degr turn
stage 3: 90 degr turn
stage 4: 90 degr turn
The sequence give in the picture has to be followed (not reproduced here).

INSTRUCTIONS:
Thread of stud and nut MUST be lightly oiled using engine oil or Optimoly HT.
No lubricants between ring and cilinderhead.
While turning the nut, the ring should not turn. If needed: mark ring & surface of the head to check if the ring is turning.
If needed, the surface of the ring may be roughened on the cilinderhead side using abrasive paper.

IMPORTANT:
The new torquevalues using the turningangle method is only applicable on engines with new studs (gold/yellow) and with new improved strengthened type of cilinderheadgaskets with silicon circulating seal.

USING BOTH NEW AND OLD STUDS ON ONE CILINDERHEAD IS NOT ACCEPTABLE

Only new type of studs are available as replacement parts. Also, only new type of cilinderheadgaskets are available as replacementpart.

The use of inferior material for the studs of the 1984 model, the old type of studs (black) have been used for a while.
In case of repairs, engines with the old type of stud (black), the torquevalues for the old type of studs should be used:
Stage 1: 20 Nm
Stage 2: 50 Nm
Stage 3: 90 Nm

--END OF QUOTE--

Reading all of this there remains only one conclusion:

If you use new studs, use the new torque values
If you use the old studs, use the old torque values
If you install new studs, you can only buy the new type.

I still have one question:
Is it necessary to change to new studs? It appears not from the info-sheet but at the same time, it isn't very clear on that point.

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I tried to highlight an important section but instead it is unreadible. Here it comes again:

IMPORTANT:
The new torquevalues using the turningangle method is only applicable on engines with new studs (gold/yellow) and with new improved strengthened type of cilinderheadgaskets with silicon circulating seal.

a4sfed928

I think a little tention should be put on the studs before the loctite cures. This will ensure that there is good metal to metal contact between the pitch diameter flank on the stud and block. If you just screw them down to the proper height and the loctite cures most of it will be cured on top of the flank due to the weight of the stud. Once you torque the nut all the clamping force has to transfer through the cured loctite which may soften under heat or crush out initally. I'm not sure if this would cause altered torque readings but it seems logical to me that it could.
just my .02.

69gaugeman

In all my experience I have never seen 3 - 90 degree turns. I just rebuilt an Audi A6 engine and it had bolts and used the low torque then 2 - 90's.

I even used the old head bolts

WAAAAy back in mechanical engineering materials class we spent a long time pulling material apart and analyzing it.

I cannot fathom taking a bolt or stud to into plastic deformation. Everything should be in the elastic range as strength is reduced once the plastic deformation has occurred.

Calculation of the required ratio should be easy if we know what type of material the bolts or studs are made from. I don't have a stud to measure, but if someone wants to measure (accurately with a micrometer) I can look into some common materials and calculate what percentage of elongation there would be at the elastic and plastic deformation stages, then see if we are there with 3 - 90's or 2 - 90's.

GregBBRD

Yes, very familiar with this bulletin.

The new studs (which is all you can buy), combined with the new nuts (which is all you can buy), with the new head gaskets (which is all you can buy) are what I've been using and writing about. Buy some new studs and try the above sequence. They will be ruined....just like the ones I tried it on. Then buy another set and do something different.

GregBBRD

Exactly.

Porsche doesn't either...except for this one vehicle.

blown 87

I think that is closer to the truth.

GregBBRD

Me too.

Fabio421

So Greg, can you clear this up for us. When should we be tightening to 2-90 degree turns and when should we be tightening to lb/ft ratings only?

Used studs = lb/ft setting ?????
New Studs = 2-90 degrees ????

69gaugeman

Always use 2 - 90's reguardless of old or new.

The reason that this method is superior is for the following reason:

The idea is that you take the stud or bolt and stretch it to some point before the stretching becomes permanent. This is called the elastic stage. When you remove the torque (loosen the fastener) It reverts to its original length. If you take it and stretch it past that point, it retains that length and doesn't go back, That is the plastic stage. Due to the dissimilar materials (head aluminum, bolt steel) this allows for the engine to heat up and grow at different rates (aluminum expands more than steel at any given temperature rise). Once you are at the plastic stage any further stretching also stays that way. Now if you torqued to the plastic stage, then heated the engine, it would further stretch the stud or bolt. It would stay there, then when the engine cooled you would have less force holding the head on. Whereas if it was at the elastic stage the stud would return to its original length and maintain the pressure on the head.

The idea is you need to stretch the bolt a given DISTANCE so that it has the maximum amount of holding force without stretching the bolt / stud.

When you use torque only the following become an issue:

When you apply torque to a fastener, as the torque increases, the friction on the fastener (the threads and under the head) become an increasing factor in how much torque it takes to turn it. Any contamination changes how much torque it take to rotate it.

So, if you add ,let say, grease to the threads, it is easier to turn and for the same amount of torque, you stretch the bolt farther, potentially getting into the plastic stage. If you add grit, you would have the opposite effect, and it wouldn't have the same amount of hold down force as the screw that had oil or grease on it.

The range of variables makes for HUGE swings in the amount of stretch on a bolt.

Now, at low torques, this is not nearly as much of a factor. This effect is logarithmic and not linear so the difference very quickly climbs with the torque applied.

So now if we torque is some calculated torque that does not vary in the amount of stretch for clean or dirty threads, we can now turn 180 degrees and stretch ALL the fasteners the same amount.

Clear as mud?

69gaugeman

The reason that it is recommended to replace all these fasteners, is there is no way to check if the stud had reached the plastic stage and stretched (say as in if the engine overheated and the thermal expansion was greater than accounted for in their calculations). Or any stress fractures (that could be internal) have occurred.

Like has been stated before, unless you are going to boost, probably not an issue. But If you are adding extra cylinder pressure, you best change them all.

I personally reuse the studs bolts on a factory engine.

GlenL

The problem with the angle loading method is that the initial preload doesn't necessarily put the same force on all stud and isn't repeatable. It is the same argument as why the torque sequence method isn't good.

I do like multiple torques as it provides and easy way to re-torque the nuts: Back off 1/8th turn and then go forward to the specified torque. In my experience, this reduces water leaks.

I put a new fully-threaded stud in a block and it seemed to stretch forever. Had that disturbing feel when being torqued that I was pulling the threads at the bottom or yielding a bolt. I pulled it and put in an ugly original stud and that torqued-up just fine. (Phew!)

69gaugeman

Unfortunately you are very wrong about this. Low torques are repeatable. High torques vary the amount of stretch GREATLY.

EVERY engine builder in the world that I know of (and I have been to a lot of engine plants from many makers) uses low torque and then angle.

Of course ALL those guys could be wrong.

Kiln_Red

Very compelling thread as I am in the middle of replacing original head gaskets on my '84. Upon removing the first head, I mistakenly pulled one of the studs out rather than just the nut. I guess that the nut was more corroded than I realized. Anyway, that got me thinking about replacing the studs BUT NOT ANYMORE.. In fact, I plan on re-installing the original since the new one may or MAY NOT match the other 9. Loctite 270 for the stud, right?

Very, very interesting reading guys. So would you guys say it is safe to say that the procedure on cars using studs with M12x1.5 nuts should read as follows:

Step 1: 20nm or 15ft/lb
Step 2: 90 degree turn
Step 3: 90 degree turn
DONE

A thread with this much dialogue by so many knowledgeable enthusiasts must surely reach some sort of clear conclusion on the CORRECT torque procedure.

GlenL

"Very wrong?" At least I didn't get "very, very."

Of course anyone who knows me will expect that I stand by my position and will proceed to explain why I'm am not wrong let alone "very wrong." Thusly:

First off, consider everything people say about why torquing to spec is inaccurate. The washers move. The surfaces aren't perfect. Same for low torque. Add in variances in gasket material, perhaps?

Then there's the torque sequence. The first bolt in going to snug the head down and the following ones more. Once that tenth bolt is tight, the first will be looser than the torque spec. If I was doing angles, I'd go back around, loosening and then tightening to spec until all returned to the same position after the loose/tight cycle.

Then there's re-torquing. I am going to repeat myself because I enjoy that at least as much as "MK." Since the process, however you do it, is fraught with error sources and preambles, I like to re-torque the heads. Harder to "re-angle" as you've got to loosen the nut up a lot. (Or maybe that's OK?) It seems that periods of 1 hour, 8 hours and 24 hours are fine. Then the nuts go back to the same angle after being loosened. Helps avoid leakage. I know ordinary "modern" gaskets say you don't need this, but it helps. I've had problem heads that stopped being problems when I've done this so I do it all the time.

No, not getting to auto manufacturers, so ya got me there.

A. Embry: "good luck with that."