In my never ending head gasket job, I think I just learned something about exhaust manifold studs. I was paranoid about wrecking the aluminum head by stripping out the exhaust manifold stud holes. The stud (new from AutohausAZ) wouldn't start to thread in easily. I decided to clean it up with a die. Now it went in just great. The next one same way. Third one, I decided something wasn't right. A web search revealed that these are Tap End Studs, and the thread is called an interference thread, designed to hold tighter and not vibrate out. Makes sense. They look the same and measure the same on a traditional teeth type thread gauge, but they are not the same. This also begs the question if studs should EVER be reused due to the possible deformation of the interference thread and loss of holding capability. A ruling by a pro would be welcome. Is this common knowledge and I'm woefully uninformed?

 

Certainly not an expert, but I think it has something to do with the metal of the head and the metal of the stud having 2 different rates of expansion and contraction during heat cycling, Hub.

I ran into the same stud issue during a bike restoration a few years back. If I recall correctly, that same kind of stud was used on the Suzuki GT750 "Water Buffalo." Mine was a '73 specifically. It was one of the first ever factory produced water-cooled bikes, and even better, it was a 2-stroker. Hot like Hell on the inside, blasted by cool air on the outside. That thing ran great, but the cylinders rattled like golf ***** in a Hills Brothers can when she was at idle. Pulled the top end to go a few mm over bore, and figured at 36 years old (at the time), I might as well go the full Monty. Funny thing is, we did the exact same as you did. I had never heard of an interference thread before, either.

I wouldn't be too worried about it. Oils and coolants and ATF's what they are nowadays, I don't think you're hitting the rapid changes in temps that those studs were designed to tolerate back then, anyways.

Common knowledge? No. Over-engineered? Hell yes.

 

I did the exact same thing too when I did my heads this may. Used a die to run the studs through.

 

I did the exact same thing too when I did my heads this may. Used a die to run the studs through.

Yes, I think so too. make sure you get the OEM stud for the exhaust manifold and tapping the hole on head ( both end has thread 1.25p x 8.00 mm, length? bolt).

Exhaust manifold must be flatten ( u have to use straight gauge for warp) and torquing nuts accordingly otherwise, stud will break again. x19

 

Another good topic and thanks for detailing your head(s) trip Hub! As I have no direct 12V exhaust stud experience either, (but hey Loud I did drive a water-buffalo with a lovely 3 into 1 expansion chamber for many years!) I have some fear and questions on the subject.

To your last question (Re: OEM head to ex. manifold stud re-use; what I have read only) a qualified "no, always replace with strictly OEM", i.e. unless you've just installed them and had to then remove the manifolds shortly thereafter. But the reason stated was always to avoid breakage of stud itself; and what fixing that requires.

- How do you turn them into place, both interference and non?

- When do you stop turning?

- What do you torque your modded studs to, on installation into the heads?

- Do they stop by either bottoming-out against the bottom of the in-hole threads, or against the stud shoulder because your out of stud threads?

- What keeps them from loosening?

- Where your (interference) studs from the dealer?

Where I am going with this is I suspect two things:

a.) perhaps correct OEM is interference type: &

b.) perhaps the stud should not be "bottomed" (which is a standard industry practice) and hence why it is an interference type thread.

The OEM stud is certainly a DIN spec item, my guess would be DIN 939. These can be had either with interference threads, or without; as I have seen listings for both.

- Is the thread pitch 1.25?

- Which type of threads are OEM? Are the OEM subj. studs either:

DIN 939 (nut and tap end with normal thread tolerance 6g)

or

DIN 939 (tap end with interference thread fit Sk 6)?

- Are OEM studs plated (939(zinc)); or plain (939 Fo (ferrous oxide?))?

- Are the "manifold to down-pipe" studs, nuts & washers all the same items as these?

- I doubt ETKA goes into that much detail. Can someone with an ETKA please confirm any of the above, or perhaps the listed dimensions there?

- Has anyone tried using a regular, a bottoming, a re-chamfered broken, or no tap at all to clean the head-holes, and then installed the interference studs, and how did it go?

This pdf (below) explains the calculation of length measurement of the subject studs; and mentions that the interference thread type is "normal":

http://mdmetric.com/pdf/studexpl2.pdf

Also from their site, M8 stud dimensions:

b. = 22 mm
e. = 10
x. = max. 3,2

These look interesting (pages 51 & 78), may I assume they are reuse-able?:

http://video.arp-bolts.com/catalog/ARPCatalog.pdf

Separately, if you where already there (like yourself, heads off), would VAP's suggested simple exhaust gasket mod to avoid some head ex. port to ex. manifold gasket mismatch be worthwhile alone, even without further gasket-matching; to the manifold itself?

https://www.audiworld.com/forums/sho...exhaust+gasket

 

You can see the entire fastener manual here, quite the book:

http://www.fas.org/man/dod-101/sys/ship/nstm/ch075.pdf

Note that they recommend lubing interference fit studs on installation (which makes for Army & Air Force humour!)

S9086-CJ-STM-010/CH-075R2

075-8.6.3 REWORK/REPLACEMENT OF CLASS 5 INTERFERENCE FIT THREADS.

For non-high temperature applications (less than 300°F), Class 5 interference fit studs may be replaced by studs installed using anaerobic-locking compounds. For higher temperature applications, Class 5 (interference fit) studs are required unless specific approval is obtained for use of anaerobic or other thread locking compounds.

075-8.6.3.1 Replacements Using Anaerobic Compounds.

The following procedure applies for using replacement
studs installed with anaerobic compounds.

a. Remove existing studs. Re-tap holes. While Class 3 threads are preferred, Class 2 threads are acceptable.
b. Clean the hole thoroughly by repeated flushing and scrubbing with a general purpose liquid detergent (MILD-
16791 or equivalent) and a soft wire brush. See FSC Class 7930 in Afloat Shopping Guide. Blow out all liquid, and
dry with oil-free compressed air or a clean cloth.
c. If studs are in good condition, use thread die to clean up the studs to a Class 3 (preferred) or Class 2 (acceptable) dimensions.
If studs are damaged, obtain new studs. Scrub the studs with the same detergent used to clean the tapped holes and dry with oil free compressed air.
d. Apply primer (activator) as required. See paragraphs 075-5.7.2.2 and 075-5.7.2.5 for selection of primers. Table 075-5-5 provides
stock numbers for primers. Apply sparingly to both male and female threads in accordance with manufacturers directions.
e. Apply anaerobic locking compound to both male and female threads. (Select anaerobic compound in accordance with the guidance in paragraph 075-5.7.2.) Apply enough compound so that the gap between the male and female threads will be completely filled.
f. Install the studs using the standout specified on the installation drawing. No lubricant is to be used as the anaerobic compound will act
as a lubricant. See paragraph 075-5.7.2.9 for more detailed instructions.
g. After curing is completed, ensure the studs are properly bonded by applying an inspection torque in accordance with paragraph 075-5.7.2.10.7.

075-8.6.3.2 Replacement Interference Fit Studs.

Because of many slight variations in thread forms used by various activities it is not possible to provide detailed guidance
for the replacement of interference fit threads. In many cases the repair or replacement of interference fit threads may be beyond
forces afloat capability. The following general information applies.

a. Re-installation of a removed interference fit stud may sometimes be feasible. Clean tapped hole and stud in accordance with paragraph 075-8.6.3.1, above.
b. Apply appropriate thread lubricant sparingly.
c. Install studs using the standout specified on applicable drawings.
d. Subject each stud to an inspection torque the same as for a stud set with anaerobic compound. It is recommended that the torque in Table 075-5-6 be used. If a torque wrench is not available, any unused (new) self-locking nut with a plastic element may be used.
If a self-locking nut with plastic element is to be installed, it is recommended that the torque in Table 075-5-6 for Grade N Locking Compound be used. If the stud turns when the torque is applied the installation is unsatisfactory.
e. When installations are unsatisfactory, re-installation without use of a lubricant and a repeat of the inspection torque may be attempted. If still unsatisfactory, an oversize custom fit stud will be required. S9086-CJ-STM-010/CH-075R2 75-130
f. Prior to manufacturing or selecting an oversize stud, measure the tapped hole threads at three places: near the bottom, midway, and three threads from the top of the hole. Take thread readings as accurately as possible.
g. Provide the tap hole dimensions taken above to an engineering activity (shipyard, NAVSEA) which has a copy of ASME B1.12, Class 5 Interference-Fit Thread. Ask the engineering activity to determine the major pitch diameter required to maintain the same interference as that which would be obtained using the thread forms recommended in paragraph 075-2.2.2.2.1.
h. Have a stud or studs made to the required dimensions.
i. Lubricate stud with the appropriate lubricant (see Table 075-4-3 for listing of lubricants).
j. Install studs to the specified standout length.
k. Whenever some studs are set with anaerobic compound and others are interference fit, document the installation with a sketch with each location numbered and the set end pitch diameter readings recorded.

 

I did some of the same research and there is obviously an entire science devoted to this. Some of the aviation manuals state that the tap end thread should never be bottomed, the hole (aluminum head) should be chamfered or you will get cracking at the hole when the shoulder of the stud hits it. The audi head stud holes were not chamfered, but I snugged them up with the traditional two nuts locked to each other, making sure not to get too tight and deform aluminum at the site. The manifold to exhaust pipe studs I purchased from Audi. I did chase those holes with a tap and the studs went in fairly easily, but then we are talking a steel stud to steel casting mating, not steel to aluminum, so there could be a difference. If I can't get six new head/exhaust manifold studs locally, I will order them from the dealer and report back on what I discover. Thanks to all for the discussion.

 

I went to the dealership to order the studs. We could not find them on ETKA. A mechanic stated that yes, they are interference threads. He also said that if they came out without breaking, that he would have no hesitation to reuse them. I did reuse them on one side. They did go in just slightly easier than the new studs, but not as easily as the ones that I recut the threads on. I believe the new studs had a coating on them and that may have added to the tighter fit. The old studs were almost shiny like stainless at the tap end threads which were, of course, protected in the aluminum head. I think that ends this story unless one side backs out. Only time will tell.