Can Dilavar head studs be safely re-used?
 

Just pulled off the heads on a 3.2 engine and have them out for a re-do. 70k on the odo. All the head studs were unbroken. When re-assembling can the Dilavar lower studs be safely re-used. My gut feel is probably not a good idea. The engine is basically stock. The car is used for pleasure, touring, not a track beast. My first order choice would be to replace the lower Dilavars with the same steel studs used on the upper row. Lots higher cost options available including the 993TT full threaded all Divalar (top and bottom rows) but these seem to me like over kill for a cooking engine.
If I recall correctly, John Walker recommends re-torquing at 1500 miles if going to all steel. Any other recommendations?

$282.00 (through Pelican Parts) is not much to spend for a new set of studs when you're right there. Not worth the risk NOT to do them.

My .02

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Hoss4659:

First, I am no expert.

However, when I rebuilt my first 3.0L engine from a 911SC I noticed the bottom studs looked good. But, when I tried to torque the heads to them I could not get a reliable torque value.

Read Wayne's book as well as Bruce Anderson's book and decided to pull them out and replace them with steel studs which has worked great for the steet car. It was a better alternative, as far as my application and much more economical.

Like I said, I am no expert but the steel studs have worked for me since that is what was talked about in both books I read.

The newer fully-threaded 993 dilivar studs are very good to use, so you may have two (2) different alternatives here.

Good luck and hope this helps.

Once you torque them don't put on the valve covers.
Come back in the morning and see how many pieces are on the floor
Bruce

During my top end rebuild, I started to put everything back together with the stock head stud bolts. After rethinking that approach, I took the cylinder off and promptly ordered a new set. I decided that the cost of new head stud bolts was not worth all the extra work I would have had to go through if one (or more) had broken... and I do track my car. You have it apart now, this is the best opportunity for you to put it back together for it to be its best.

The only reason that a stud could fail in this manner would be if it suffered from Hydrogen Embrittlement.

I have seen this suggestion from time to time but it does seem unlikely with a genuine Dilavar Stud.

The basic material used for Dilavar wouldn't tend to suffer from this type of problem.

It is generally accepted that Austenitic Stainless Steels are quite resistant to hydrogen embrittlement and in the rare cases when the formation of strain induced martensite increases sensitivity Molybdenum is used as a stabiliser.

Dilavar has a relatively high Moybdenum content so it would be very surprising if it did behave in this manner.

There are studs being sold on the Aftermarket which claim to be Dilavar and also claim to be magnetic.

There is no way that Dilavar could, under any circumstances be magnetic.

If it is Dilavar it is non-magnetic and it if is magnetic it isn't Dilavar.

IMHO there is no way that an alloy with the composition of Dilavar could produce strain induced martensite.

The environment also needs to be considered.

It is very, very unlikely that any stud used in an engine will be able to pick up 'atomic' hydrogen. So even when wet or hot hydrogen won't diffuse into the stud.

This would mean that the hydrogen would have to have been diffused into the material during manufacture - possibly during an acidic pickling to clean the material after hot rolling.

I am reasonably confident that some 'non-genuine studs' not made from Dilavar found their way into the supply chain a few years ago and they that had been badly made and caused the reported failures.

The main cause of failure in Dilavar studs is due to the material's sensitivity to intergranular corrosion in the presence of chlorides. This process is know as Stress Corrosion Cracking (SCC).

As crack length increases due to continued corrosion the remaining cross section eventually becomes too small to support the preload in the stud and a brittle failure occurs.

This would tend to happen when a cold engine warms up and the cylinders expand.

It wouldn't affect a new, dry stud that had never previously run.

It is, however, a good reason not to use old Dilavar Studs.

The mechanism that causes failure due to Hydrogen embrittlement is quite different and it is the presence of a preload and the migration of hydrogen atoms through a material that causes the problem.

These atoms tend to congregate at crystal defects within a metal and then combine to form hydrogen molecules.

This creates an internal pressure in the crystal resulting in a catastrophic brittle failure.

It doesn't tend to be thermally activated although if small non-propagating defects are present then expansion could mean that the cracks become unstable.

I did some work for British Gas in around 1980 involving Hydrogen cracking in X-70 and X-100 Line Pipe steels and at levels of dissolved hydrogen as low as 2ppm there is a significant influence on mechanical properties.

No

The Dilavar studs tend to break...the steel studs tend not to break. Makes it easy to decide.

Actually saw this happen on a 3.2 rebuild. Everything torqued down fine. Engine sat over night. The next morning, heard a ping and a stud shot up 2 feet towards the ceiling and then fell to the floor. Took off the heads and replaced all the studs. Studs looked perfect prior to reuse.

I replaced the Dilavar with steel and kept the upper steels on my SC. Had some advice to that effect 10 yrs ago. Works for me. I don't think you have to go super expensive. Steel seems to work. But I would throw the Dilavars away. While in theory they may be perfect, they do seem to be the ones that fatigue.
Alan

How long ago and where were the studs bought from as it would be useful to try to track this type of failure down and try to isolate the supply.

I am not sure that studs will ever fail due to fatigue the stress even at its greatest when Nikasil cylinders have fully expanded just isn't high enough unless there is already a defect present.

The ONLY Dilavar ones I would re-use are the late 993TT full-threaded ones.

The rest go into the metal recycling barrel.

Origional Porsche studs on a 3.2

How come most if not all the broken studs in 911 engines are the Dilavars, when present? Just curious - been thru this with two SCs. Both same - broken Dilavars, OK steel studs.
Alan

Three 3.0s in the shop, as we speak, and the two I've torn down so far had one broken dilavar stud ea. All of the steel studs are ok.

Want to put money on number three?

regards,
al

The fatigue failure of any component is fairly straightforward to explain and understand.

Any metal when subjected to an alternating stress does have a tendency to fail and the mechanisms which cause these failures are very well reported and explained.

It should also be clear that in general many steels exhibit an ‘Endurance Limit’ which is an alternating stress below which failure should not occur.

It would be fair to say that Austenitic Steels don’t traditionally exhibit this behaviour and in theory, at least, may eventually fail.

This type of behaviour would suggest that materials of this type shouldn’t be used for critical applications but in practice this is far from true.

By using very basic design tools such as either a Goodman Diagram or Gerber Parabola and basic fatigue test results failures of this type should be avoidable.

It is also fair to say that it is difficult to predict individual fatigue failures but Dilavar studs break on such a routine basis that either Porsche made a complete miscalculation of the alternating stress present in the stud or there are further influences that need to be considered.

The fatigue life of the majority of components needs to be considered as occurring in three basic stages.

1. Stage 1- Initiation.

Initiation is the most complex stage of fatigue/fracture and is the stage most rigorously studied. The most significant factor about the initiation stage of fatigue fracture is that the irreversible changes in a metal are caused by the application of repetitive shear stresses. The accumulation of microscopic amounts of surface damage develop over a large number of load applications and create a small local defect.

The initiation site of a given fatigue fracture is normally located in an area of a severe stress concentration which depends on the geometry of the part as well as on environmental conditions and a few other factors with regard to a material's basic properties.

For very high strength materials fatigue crack initiation may be 95% or more of a components total life.

2. Stage 2 - Propagation.

The propagation stage of fatigue causes the initiated microcrack to change direction and grow perpendicular to the tensile stress. The second, or propagation, stage of fatigue is usually the most readily identifiable area of a fatigue fracture.

3. Stage 3 - Final Rupture.

As the propagation of the fatigue crack continues, gradually reducing the cross-sectional area of the part or test specimen, it eventually weakens the part so greatly that final, complete fracture can occur with only one more load application. The fracture mode may be either ductile (with a dimpled fracture surface) or brittle (with a cleavage, or perhaps even intergranular, fracture surface) or any combination depending upon the metal concerned, the stress level, the environment, etc.

Brittle fractures commonly result in surfaces along the axis the maximum shear stress - Typically around 55 degrees to the tensile axis - but is based on some fundamental material properties.

Stage 3 represents the "last straw" that broke the camel's back.

Precipitation Hardened Austenitic Stainless Steels, which is a group of material with very similar properties to Dilavar do exhibit sensitivity to corrosion pitting and intergranular corrosion in the presence of Chlorides.

There is virtually no published data available for Dilavar but A286 is an alloy with very similar composition and is commonly used for the manufacture of fasteners.

If this material is in a petrochemical environment then its hardness and hence tensile strength must be strictly limited or it will exhibit the behaviour known as stress corrosion cracking.

Basically the alloy develops either pitting on the surface or the grain boundaries of the material are attacked and form crevices.

These pits/crevices act as initiation sites for subsequent fatigue crack propagation and subsequent failure. They significantly affect the Number of Cycles required to make the transition to Stage 2 of the fatigue process.

This process could be described as an Environmentally Assisted Fatigue Failure but I believe it is prudent to differentiate this from the more conventional fatigue failures we see from time to time and generally occur in air and at ambient temperature.

IMHO the sensitivity to SCC is the major weakness of Dilavar and the reason for the coatings on later studs.

= ditch the Dilivars?
Alan

I would never re-use Dilavar.

We use standard steel studs on Aluminium Engines and Titanium Studs of our own manufacture on Magnesium Cases

To try to inspect used studs to ensure they are defect free is a PITA.:)

We never build engines with Dilivar so my suggestion is to throw away any of these studs you have.....new or used....
Just my 2 cents worth.


BTW: Chris, I have seen band new factory Dilivar studs break after installing them on a rebuilt engine before the engine ever runs.
The last time it happened, I had to call a customer to tell him his 356 show car had a dented drivers side quarter panel.

I've been reading this thread with interest, my engine a 2.7 mag case currently undergoing rebuild with ARP bolts and I wonder what the experts think and should I be considering alternative bolts or will the ARPs work fine.