ARP vs Stock studs

[Mark Henry]

Originally Posted by manbridge 74

Other ARP studs were likely counterfeit. Becoming more and more common in today's world.

The ARP studs I used would be early-mid 2000 from here IIRC.

[Ronnie's.930]

Originally Posted by Lapkritis

Ever get stuck on an elevator ride with someone who farts loudly? It's not enough to just have heard it once. No, no way. Then you smell it...inescapable agony. If they lack decorum they next describe to their captives what they had for dinner the night before to produce such a noxious cloud. Not taco night for sure... maybe broccoli or brussel sprouts you guess to yourself. This is every headstud conversation on Pelican. Whoever smelt it dealt it, whoever denied it supplied it... but we all heard and know the first offending tooter polluter. Please toot your horn once per thread/elevator ride if you must... please also refrain from wafting it about.

Lap, you are a sick, sick man - perhaps a long lost brother of mine?!?!

[racing97]

"I co-edited a book in 1996 entitled 'Computational Methods and Testing for Engineering Integrity' which sheds some light on the methods that I believe we should try to use whenever possible."
Chris,
I have not read your book and someday look forward to doing so. Peterson's Stress Concentration Factors has been my go to resource. Many clever people such as Henry
Ford have not had a formal education and yet have a understanding of what they want to accomplish and need a Mentor such as yourself, in Henry Fords case he was friends with Edison, Alexis Carrel etc.
To say you are condescending is ludicrous you are a gentleman of first order and your posts are packed with information that should insprire even that most autodidactic amongst us.

regards

[Lapkritis]

Quite thankful for the contributions Chris makes. Always insightful.

[Robert Espeseth]

I really enjoy reading posts from Chris...as a mechanical engineer I like the approach of understanding why things work...not only knowing that it works :-)

[Henry Schmidt]

Originally Posted by chris_seven

I found your comment in regards to the Ti Head studs we manufacture to be interesting.

Your comments were roughly as follows:

'When you have 2-300 hundreds of sets in hundreds of engines with 100 000+ miles of success we will re-visit the question'

In all fairness Chris, you took my comment out of context.
The complete statement was:

Originally Posted by Henry Schmidt

Good morning Chris
There is no issue at all. Like I said, "it's flattering".
There are many companies attempting to make a replacement for 911 head studs and competition breeds innovation...If you design a better head stud that is also cost effective, I may even use them. After you have two or three hundred sets in hundreds of engines with 100,000+ miles of success, let's revisit the question...........

The comment clearly states that if you make a better stud we may use them....The caveat is "better" and the only way we will know is to test them in the real world.
Engineering is essential in the design stage of any new product, that is why we hired a mechanical engineer to design our stud.
But after the design we had to test them because we also understand that engineers make mistakes. Testing gave us 7 different design iterations.
The top engineers at Porsche gave us the dreaded Dilivar stud in the first place. They also go us thermal reactors, five bladed fans and shorter rods every time they increased the stroke.
All designed by the best engineers available and all ridiculous mistakes that needed a remedy.
There is a place for engineering and an equal place for real world experience.
We designed a product 15+ years ago that helps reduce friction to enhance bearing life in air-cooled engines and because we don't have a lab we put sample into real world testing.
The engineers will ask "what's in it?" so they can analyze it and my response to that is, "try it."
We have and now we're bringing it to market. Design, testing and marketing.
All three are important if the consumer is to benefit.
If you guys are tired of my head stud "hype" place me on ignore because my lubricants make me even prouder.

[chris_seven]

Henry,

Please explain what you mean by 'test them in the real world'?

[AlfonsoR]

Henry,

Would you share more details on the testing of your studs? What about details on the design goals and what led you and your design group to each of the 6 revisions?

Henry and Chris, both of you are great contributors to this forum, so I hope that neither of you ever gets discouraged from posting here, it would be a loss. The practical end vs the analytical. You guys are coming from different worlds, so is it really surprising that you don't see eye to eye? Please take it easy & have patience, focus on the technical, not the personal.

[chris_seven]

There is always great value in Henry's comments and his depth of knowledge regarding 911 engines is invaluable and provides an excellent platform to try to analyse some of the issues of not only keeping old engines running but in continuing to try to make small improvement.

Debate regarding details doesn't always need to be right or wrong, it is valuable in itself as it makes us question our own assumptions and hopefully improve.

[jimbeaux911]

The use of a fine pitch in stead of a coarse pitch is interesting as an exercise but I am not sure that the torque/preload levels used in 911 head studs it has much practical significance.



Seems to me that the finer pitch is quite significant, and is why I bought them! to replace the BROKEN originals. Its a matter of fine tuning, and accuracy. With the finer thread, the correct torque is much more likely to be achieved, and not under or overshot.

[chris_seven]

Originally Posted by jimbeaux911

Seems to me that the finer pitch is quite significant, and is why I bought them! to replace the BROKEN originals. Its a matter of fine tuning, and accuracy. With the finer thread, the correct torque is much more likely to be achieved, and not under or overshot.

I don't really agree - but an interesting issue.

It is commonly stated that fine pitch fasteners do resist loosening more effectively than coarse pitch fasteners and that they are better at resisting vibrations and from a theoretical stand point these statements are clearly correct BUT they completely neglect the influence of thread friction.

If we look at some basic numbers and calculate the increase in preload for a nominal torque for a pitch change of one step - that is M10 x 1.5 to M10 x 1.25 we will find and increase in preload of around 5%.

The typical scatter in preload for a specific fastener which has been torque tightened would be typically +/- 25% and this will be the dominant effect.

We should also take into consideration that the pitch line on a fine thread is longer for a given diameter so 'K' or nut factors are likely to increase and this will tend to reduce the preload for a given torque.

It is also well established that fine pitch threads have a greater tendency to 'gall' during tightening and this will also have a negative impact on the consistency of the 'nut factor'

My conclusion is that in practical terms there is little or no real difference in preload with either coarse or fine pitch at a torque of only 24 lbsft.

I would point out that the accuracy of torque setting is not really the issue it the relationship between torque and preload that is the major concern and this will always be relatively poor for applications of this type.

It would also be fair to say that the difference between coarse and fine pitches would be better exploited if alternative tightening methods such as 'torque and angle' were employed instead of a simple torque measurement.

There are two main reasons why 'fine pitch' fasteners are less likely to loosen due to vibration.

The first is simply due to the increase in preload - which I have tried to explain is not always assured.

The second is that as the incline of the thread is lower and so the 'off torque' tends to be lower but there are many influences that impact on this phenomenon with 'burnished' threads tending to be more resistant and even bolt length having an influence.

The 'off torque' is the reduction in preload caused the relaxation of the torsional elastic stress stored in the shank of the stud to twist back and reduce preload.

Again these are theoretical considerations and are much more likely to be influenced by the finish of the individual thread in terms of surface roughness, the material used and its tendency to gall and the plating, if any, that has been applied to the threads.

The ultimate questions is how many head nuts loosen and how well have they been preloaded?

I would suspect that the real issue is preload variability and this will be due to these issue I have tried to explain.

There has been a significant amount of work on nut loosening carried out by Simmonds using 'Junkers' Test set ups which introduce transverse vibrations into the thread. It is quite well accepted that transverse vibrations have the most influence on nut loosening and using this test method is a good guide to the 'best' solution.

The results do show that fine pitch threads are better than coarse pitch threads but that this influence is greatest at high levels of preload.

Simmonds results also show that 'prevailing torque' was also significant and that a Kaynar style nut was helpful.

We are currently testing 12 point A286 Prevailing torque nuts which are manufactured with a repeatability of less than 1 lbft variation in prevailing torque.

Interestingly Simmonds also found that low friction coatings and lubricants which had a positive impact on preload variability had a negative effect on vibration loosening.

Simmonds also learnt that vibration loosening was also affected significantly influenced by the clearance in the threads.

Threads with the 'highest' class of fit providing the best results.

It has been my experience that when checking Porsche supplied studs the class of fit of the threads is better than the majority of aftermarket studs and we are trying to make improvements in this area.

I would be interested to know how your studs failed - I have never seen a broken steel stud only broken Dilavar studs and they always fail in the shank. I have never seen a Dilavar or steel stud fail in the threads.

This would tend to suggest that the strength improvement due to stress area and increase in minor diameter for the fine pitch threads is not very relevant.

As I said in my post we use a fine pitch thread to reduce the cost of the 12 point nuts rather than to improve preload variability.

We do tighten the nuts we using to a torque and angle method rather than just torque which I believe helps.

If we switch to 12 point prevailing torque nuts we will also carry on using fine pitch but this because the flange diameter of the nut is smaller rather than any other reason.

I am still unconvinced that a fine pitch really contributes much in this area but I am happy to agree that it can only help.