Torque Values - Wet or Dry?

[Tremelune]

Sooo...How do you guys feel about Loctite? My understanding is that it does not effect torque values.

Can I assume that if no angle value is given, torque is pretty much what you have to work with?

[Quicksilver]

A whole bunch of misses on the "replace with stainless" direction. The choice of fasteners wasn't made by accident by Porsche. (even though there is room for improvement...)

A Porsche can be expected to last for years and part of that comes from the material selection the engineers used.

- Stainless fasteners mixed with steel fasteners: Not good
- Stainless fasteners in aluminum: BAD
- Stainless fasteners in magnesium: REALLY BAD

The choice of what dissimilar metals are allowed to be used with each other is really important. The reason has a number of names: Anodic Index or Galvanic Index which all comes under the heading of Galvanic Corrosion

The cathode (more noble) will "eat" the anode (least noble). In the case of stainless with aluminum or magnesium that means the fasteners will eat away at what they are fastened to.

As a rule of thumb you want to keep the different metals within 0.15V of each other if at all possible. If the fastener is more anodic then the metal it is attracted to that is good because it protects the metal of your parts. If you look at the anodic index you will see that steel is very close to aluminum which is good and you will see that zinc galvanizing is very anodic which protects the parts. All of this is done for a good engineering reason.

If you are worried about exhaust components corroding look at copper plated fasteners instead of stainless. It may seem counter intuitive with copper's placement on the anodic index but it isn't in contact with the actual engine parts. Plus copper is very, very resistant to heat and when used as a plating on a steel nut with a steel fastener you end up with a very seize resistant combination.

[reddogmotrsprts]

The best way to look at it is to realise that if you consistently lubricate 10 fasteners and torque tighten them as carefully as possible and measure preload against an accurate loadcell you will see a variation of around 30%.

This would be a good result and assumes calibrated, high quality torque wrenches and skilled technicians.

If you changed to torque and angle this may drop to 20% - a big improvement.

If you now start to mess with plating and lubrication methods you just have a huge potential for error.

This is awesome, thanks. Those are surprisingly big variances and a nice bump for the angle technique.

If you are worried about exhaust components corroding look at copper plated fasteners instead of stainless.

If you don't have the copper-plated stuff handy, even adding a bronze washer would be an improvement. I have a small cache of copper-plated fasteners and it seems I have to keep reminding knuckleheads to stop plundering it for other uses.

I did swap to stainless from the low-grade non-metric aftermarket fasteners on my 987 exhaust hangers. However, if you look to where it matters, the car has stainless secondaries fastened with copper-plated nuts and bronze washers. It's possible that these may even be Porsche parts; the stock nuts are priced at a $1 each, so it seems unlikely that they are zinc-plated steel.

More evidence, perhaps, that Porsche engineers do know what they are doing?

Outside of Porsche, I'm frequently surprised by what I find and there are many more cases where an upgrade (usually in steel grade) seems appropriate. Maybe there are just a lot of people out there using Home Depot hardware?

[chrismorse]

Originally Posted by Quicksilver

Which ignores the fact that the only measurement they gave you was torque. That torque was based on specific conditions which specifically included the resistance of turning the nut and to ignore them and say, "always wet" is inviting damage.

The correct answer is has been given: If the factory torque spec doesn't mention lubrication then install it dry. If it mentions a lube then use it.

To get the best working knowledge of fasteners it should almost be a requirement to get Carroll Smith's books:
- Prepare to Win
- Tune to Win
- Engineer to Win

These are car prep books done in language where you can understand and absorb information.

(Favorite tongue in cheek fastener quote from a friend just back from A&P training in the Air Guard: "I used 'military torque specs'. I tightened it until I felt it get a bit easier then backed it off an eighth of a turn.")

It is rare to find an accomplished, articulate and occasionally humerous man like Carroll Smith.

A little tongue in cheek comment that would have driven him over the edge:

"Cross threading is almost as good as lock tight".

or, "If a hammer won't fix it, the problem is probably electrical".

I just sent my son a copy of Prepare and Tune to Win - and am interested in his "education", (he has a WRX :-(

chris

[sc_rufctr]

FWIW... The differential bolts in my SC failed. I had an experienced Porsche dealership mechanic look at them and he said they were probably torqued wet.

The results... This is one of the better ones. You can just see the crack at the base of the threads.



So I would torque them dry unless otherwise stated.

[reddogmotrsprts]

Carroll Smith would probably reuse that bolt, if it passed his "flotation test". Simply throw it in the ocean. If it floats, it's safe to put back on the car.

[chris_seven]

Originally Posted by Tremelune

Sooo...How do you guys feel about Loctite? My understanding is that it does not effect torque values.

Can I assume that if no angle value is given, torque is pretty much what you have to work with?

Loctite does and will always affect torque to preload relationships - there is nothing you can do to a thread which will not have some effect.

Many articles about Loctite claim it has no effect but IMHO you should decrease the specified dry torque values by around 20% when using the Loctite Purple, Blue, Red or Green thread lockers. We carried out trials many years ago when looking at the bolting of high speed flexible couplings used on helicopter engine test rigs.

The corrosion issue is interesting particularly with regard to Magnesium.

We are using a thread primer on all of the head studs we use in magnesium engines.

Many aftermarket studs are a stainless steel or similar material that is more difficult to threadlock than conventional steels.

We have been making a 170 ksi Titanium Stud for these engines and the corrosion index is even less favourable than stainless steel and needs to be dealt with.

The primer has two effects - it is an activator that forms a bond with the stud lock and it electrically isolates the stud from the thread.

Cost of fasteners in a production environment is also a major concern.

[safe]

In most reference I've seen specifying general torque, for example 24 Nm for an M8 8.8 grade, it has called for clean, untreated and lightly oiled threads.