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Exhaust Manifold Studs



Exhaust manifold studs on 911 engines can be a real PITA.

They commonly fail when exhausts are removed and the work involved in replacing broken studs can be costly and irritating.

What do we do when we have gone to all this trouble ?

We normally fit another 50 cent standard stud which may as well be made of green cheese.

Standard 911 Studs are made from a Grade 4 material which has a very poor shear strength.

The standard barrel nut corrodes to the stud quite quickly and can cause problems if exhausts have to be removed even after relatively short periods time.

The current trend is also to replace the barrel nut with a cheap and nasty Self-Locking nut with a flash of copper.

These nuts are slightly more corrosion resistant but only marginally and not much use.

Add to that the prevailing torque feature and the risk of another failure increases.

A correctly tightened nut shouldn't need this feature and I just don't think they are a good idea.

We have made a batch of Grade 5 Titanium studs and some matching nuts.

These have about the same strength as a Grade 10.9 bolt and they will be difficult to break.

They have a 'dog point' on one end so they can be gently tightened into the head and then held in place with Loctite 266 which is a High Temperature thread locker.(this will be supplied with stud kits)

The opposite end of the stud is prepared with a 4mm AF socket head to allow tightening.

The threadlocker will also electrically insulate the threads and prevent bi-metallic corrosion.

We have also made Grade 5 Titanium Flange nuts with a reduced AF size that will pass though the heat exchanger.

We have also found a Thin Wall Socket that also passes through the heat exchangers to allow these nuts to be correctly tightened. (Also supplied)

By using a suitable Ceramic Anti-Seize ( manufactured by Molyslip and also supplied) we believe that we can eliminate any further broken studs.

Titanium is extremely corrosion resistant and is used in marine environments so it should be good even on salty winter roads.

We are just organising exhaust manifold gaskets as currently only 42mm diameter parts seem to be available other than from Porsche.
Old 09-12-2016, 10:07 AM
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Maybe not as nice as you have and haven't used these on any air cooled yet but have on a lot of VW Jettas for hard to reach places.




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Old 09-12-2016, 10:39 AM
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Old 09-14-2016, 10:11 AM
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Those sound great.
What's your trick for removing the old studs?
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Old 09-14-2016, 01:06 PM
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VERY nice, Chris.

Are you going to sell these?
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Old 09-14-2016, 01:38 PM
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Chris loves Ti
Old 09-14-2016, 05:44 PM
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Metallurgist......

Quote:
Originally Posted by 3literpwr View Post
Chris loves Ti
I believe Chris is a metallurgist and loves these special materials.

Tony
Old 09-14-2016, 08:14 PM
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We decided to use Ti as the cost to manufacture isn't much different to Stainless Steel and it is even more corrosion resistant. We could have used 17-4PH with a HH1150 Temper but the Titanium is just as good in terms of strength and even more corrosion resistant.

We are just sourcing gaskets and then we will offer complete kits with, High Temp Stud, Lock, release agents and suitable sockets.

For heads we refurbish we don't try to take out the studs we just replace them as a routine.

We mill them off flush and have made a custom cutter to remove the old stud completely.

We then fit a solid threaded insert similar to a Timesert. We have tested the pull out force of the insert from an aluminium block and it is around 10 times higher than the preload in the stud allowing for flange expansion.

We are making a jig with a hardened drill guide but these are already available in the USA.

Last edited by chris_seven; 09-15-2016 at 05:52 AM..
Old 09-15-2016, 04:47 AM
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Any idea of the age limit or number of thermal cycles these will withstand before cracking?
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Old 09-15-2016, 11:47 AM
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I don't believe that there are any significant issues with using 6AL4V for this application.

This alloy should be quite stable at operating temperatures up to about 400deg and I am sure that the studs on a 911 engine will not reach this sort of temperature as otherwise the Aluminium Alloy would suffer from quite serious stress relaxation.

I believe that the Beta Phase should be quite stable at this temperature.

I would think that a standard steel stud would have a maximum operating temperature of around 250 degC as a comparison

Thermomechanical Fatigue shouldn't really be an issue at the operating temperatures of these studs as the expansion of the flange will only add around 10% to the stress in the stud.

6ALV4 doesn't embrittle with age and I am not sure why this should be an issue.

NASA Document TM 53445 - The Heat Treatment of Titanium and Titanium alloys says:-



This statement was made following testing at around 480 degC and the material we are using has been 'beta' annealed to stabilise the material.

There are report of 996 Ti studs having been used for 15 years and while I can't say that I have any real proof I have no reason to dispute the claim.

I don't know the guy making these studs but he seems to be based in Belgium and is using 6AL4V.



If you have any data that shows there is a problem I would be very grateful for a copy.

Last edited by chris_seven; 09-15-2016 at 05:56 PM..
Old 09-15-2016, 05:53 PM
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no data was just asking since titanium rods have a fixed lifetime. And titanium in other applications does also.
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88 turbo Guards red Targa slant nose, and yes I am a horsepower junkie, 3.4liter,7.5 to 1 JE pistons, Adjustable WUR, Imagine fuel head, 1 bar waste gate headers,allthe cis toys. Now apart to become the next EFI monster. fabbing my own intake, headers Individual throttle bodies, MS-3, pauter rods, Xtreme twin plugged heads, gt-2 evo cams cop's.
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Old 09-17-2016, 09:09 PM
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Those are just like the ones I put in my engine when I redid my engine's top end. Perfect fit in place, extremely nice & ridiculously expensive. Zero issues and the multi point nuts were a lot easier to torque than the 6point nuts. I bought mine from EBS Racing. That was a couple of years ago so maybe the prices have dropped since then.
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Old 09-18-2016, 01:42 AM
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I am not sure why 12 point nuts are easier to torque than a 6 point nut ?

A 12 point nut is usually used on very high strength nuts used for Aero applications and do have a much higher contact area to allow a high torque to be applied.

Most 12 Fairchild, Alcoa and Simmonds 12 point nuts also have a prevailing torque capability which also tends to increase the 'drive' torque when fastening these nuts.

An exhaust nut is only torqued to around 18lbs ft and the reduced AF nuts we have made as quite capable of being torqued to this level with only 6 points. 6 point nuts are much less expensive to manufacture than 12 point.

The Thin Wall Socket we will supply with each set of studs is a 'wall drive' design and will not caise damage to the nuts. They are sufficiently small enough to pass through the restricted access holes in the heat exchanger.

I would agree that 12 point nuts are easier to use than the standard barrel nut and it is also difficult to fond a socket small enough to allow the use of a standard 13mm AF Hex Nut.

The Fatigue Life of a con rod is a very different issue to the the life of a bolt.

When it was initially developed 6AL4V was not considered to develop the pronounced 'knee' in the SN fatigue curves that were typical of ferritic steels and due to variability in production control individual batches of this material were fatigue rated on the basis of a batch by batch inspection.

Back in the mid Seventies we used to carry out these tests on behalf of a Underwater Weapons development lab and the variation was substantial.

During the intervening 40 years the production control of Alpha-Beta Titanium Alloys has improved to the extent that most manufacturers will now specify an endurance limit for thiese materials.

Although the knee in the SN Curve isn't as distinct as that pf steel the curve will flatten out at around 10^7 cycles,

If we add to this that Ti also shows less sensitivity to notches than steel of a similar strength that in many cases the KThreshold stresses give better results than steel.

The reality is that Exhaust studs really don't experience any significant degree of fatigue loading and that the number of cycles of Thermo-mechanically induced stress is not likely to result in a failure within the life of a car.

The number of Ti Springs now being manufactured should also give confidence that fatigue of this type of Ti alloy is less a concern now than in the past.

The subject of Ti rods is much more interesting as is their 'life'. It doesn't seem to worry Honda with the NSX but rods on 911s seem to be rated for only 40 hours.

There are statements made about scratches and nicks affecting Titanium but this is more likely to be concerned with Alpha alloys which will be sheet materials that will be stretched or formed during manufacture.

I have seen issues of this type when manufacturing Hydrazine fuel tanks and great care is needed.

I am quite confident that these studs will be durable and reliable in service.
Old 09-18-2016, 03:41 AM
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Hey Chris, any idea when these Ti stud kits will be available? Christmas season is just around the corner... :-)
Old 11-01-2016, 02:10 PM
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Ti exhaust studs

Very interested... as soon as they are available in a kit.

Thanks for solving this problem.
Harry
Old 11-08-2016, 03:45 PM
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