Do I have a broken head stud?

[Otter74]

I'm doing an oil change and valve adjustment and other maintenance on my SC, and while I have the valve covers off I checked the studs. All were good except one - on the stud to the right of the exhaust valve rocker for cylinder #3, the nut turns. Everything is in place, and the stud stays in place when I turn the nut either way. If It were broken, I'd expect it to be just rattling around loose; if the nut were merely loose I'd expect to be able to tighten it and back it off. But it just turns in place, as if the thread in the stud were stripped. Th car displays no functional symptoms of a broken stud - noise, leaks, etc. I'm a little puzzled at what I'm seeing, mostly because it's my first time.

My car got an engine rebuild in 2005 at 55k miles and the heads were done again in 2013, about 17k miles ago. Current mileage is 95k. I've looked at the invoices for the rebuild and the head job and I'm not sure studs were replaced either time. Nonetheless, 23/24 are fine and this one is odd - but evidence not what I'd expect if it were broken. Any input welcome!

[pampadori]

Is it magnetic?
How many turns did you rotate it? its either broken or stripped if you couldn't get it to tighten after a handful of turns. It matters not, which.

Drive it thru the summer and fall and then handle it when winter comes.

[targa80]

If the stud was broken it would have fallen out when you removed the valve cover. If the stud is not rotating with the nut then you are looking at either stripped threads on the nut or on the stud or both. I would guess that there are a few cars being driven with broken head studs. Losing one head stud on a cylinder is a concern, two head studs on a cylinder is asking for trouble. I would do a compression check of the cylinder and compare with specs and other cylinders. If the compression is in spec then you have a concern that you will need to address over time not immediately. As I mentioned earlier there are cars out there with the issue. In the mid 90's my car had three broken head studs on three cylinders with no major compression issues. It was a concern but I did not address until I had a major oil leak in 1998. The car had 98K on the odometer when I did the engine rebuild. Replaced all the head studs with what was the recommended replacement at the time. The studs were aircraft grade if I remember and they were costly.

[Otter74]

Yes, it does seem consistent with a stripped thread; that just seems so...unexpected. Since the nut doesn't go anywhere on the stud,I suppose there is still at least some clamping force in the joint. My feeling was that one stud was not an urgent matter - glad that seems to be correct. I've rotated the nut probably a full turn. Hard to eyeball the threads, but I'm guessing a 1.5mm pitch.

[T77911S]

it depends on where it breaks as far as it falling out.
try to pull it out.

either way, sounds like you will be tearing it down.

[jlex]

Maybe its not the head stud at all, but the threads in the engine itself are deteriorated. On my VW, when the engine was rebuilt, time certs (or maybe helicoils) were machined into the block to give more support to the studs. Haven't seen that written about much on Porsche overhauls though... maybe Porsche engine materials are more robust so it's an unusual situation?

[john walker's workshop]

Aluminum SC/Carrera cases really don't pull studs. The lower ones break either between cyl and head or at the cyl base 1/2" out of the case.

[Jack Stands]

I believe Dilivar was being used for the head studs back in the 80s, so hopefully it’s just a failed head stud due to corrosion.

[walt]

Yes Dilivar on the exhaust side but I would think when your engine was rebuilt that would have been replaced without question. You can check with a magnet, I think Dilivar is non-magnetic. At any rate give it a pull and see if it comes out. Aluminum on the stud threads indicates the thread in the case is stripped out.

[Otter74]

It seems very unlikely that when the engine was rebuilt in 2005 that they weren't replaced, but I have an invoice for the rebuild and I can't find it. Then again it's a long parts list, so perhaps I'm just missing it. I'll ask the PO, who used to be on this forum.

I couldn't find a magnet yesterday to check whether the stud was magnetic or not, and since then I've put the VCs back on and refilled the case with oil. Perhaps if I find a magnet I'll jack up just that corner and pull the VC off and check. In terms of appearance, the stud IIRC had a matte silver finish. I pulled on it and nothing was moving. The nut just spins, with some resistance, in place on the stud. I suppose if I'm lucky it's the nut threads that are stripped, but I'm betting the stud is softer.

I changed plugs (NGK, '8' heat range, same as what was in there) and all plugs looked good, clean and light grey - nothing different on #3. No sign of oil leaking either other than a bit of weeping from the old gasket.

[walt]

I suppose its possible the rebuilder didn't set the height correct on the stud and therefore had insufficient thread engagement in the case. As John said studs don't normally pull.

[pmax]

Do you really have a 3.0 ? Just to confirm.

[Bill Douglas]

Why don't you pull the stud out and look at it.

It's either broken or will have bit of aluminium sticking to the treads at the other end - good luck.

[Otter74]

Originally Posted by Bill Douglas

Why don't you pull the stud out and look at it.

It's either broken or will have bit of aluminium sticking to the treads at the other end - good luck.

Because I'm doing maintenance on the car so I can take it on a road trip this weekend If I have to, I can do it at the end of the season this year.

Originally Posted by walt

I suppose its possible the rebuilder didn't set the height correct on the stud and therefore had insufficient thread engagement in the case. As John said studs don't normally pull.

Not likely - there were about two threads exposed above the top of the nut, same as the other studs.

And yes, I have a 3.0

[Walt Fricke]

1) If the stud wasn't screwed deep enough into the case, you'd have trouble with the barrel nut - the stud would push your Allen out, perhaps before you could get torqued to spec. You don't report this, and you can visually inspect the functional depth of the Allen (though with some difficulty some times).

2) I've not heard of stripped threads on the barrel nut or that end of the stud. I think coarse threads are harder to cross thread than finer ones (I'm thinking of the transmission to torque tube bolts, which are all too easy to cross thread). And unlikely the stud would strip the threads in the aluminum. So that's the conundrum here.

3) How do you know that the barrel nut and the stud are turning together? With the car jacked up, you can see the stud (since it is the exhaust) where it emerges from the case and before it goes into the cylinder. And you can see it off and on as it passes through the fins as well. If heat exchangers and plumbing obscure the view, well for $6 or so you can buy a borescope (USB end cable with a camera on the other end) to use with a laptop or tablet which will allow you to look without using dental mirrors and the like. With an assistant turning up top, you can watch the stud and see if it is turning.

If the stud doesn't turn, it's got to be the top connection, and vice versa. You can, however, measure if you can back anything off turning counterclockwise, because you can measure down to the top of the barrel nut and see if that distance changes. You could do the same for clockwise turning.

Because I have air tools, I'd try an impact wrench to loosen the barrel nut.

Just for grins, you could measure the torque it takes to move the barrel nut both directions.

4) How far have you turned it? Both clockwise and counter? But I gather you are confident that if the barrel nut was only a bit loose, you would recognize the feel of a nut getting tighter as you turned it, and when it was just the same amount of torque feel throughout however many degrees of rotation?

5) Buy from a FLAPS a sort of magnet on a ball point pen like stick. Or a longer one on a long malleable aluminum rod. You can't check through the valve cover opening, because the magnet will stick to the barrel nut. But you can check from under the car, where you can see the stud.

As I recall, Dilivar is much less magnetic, but easy to check the difference with some nearby steel piece.

6) I concur with the advice that you do a compression check (Leak downs are best for checking valves and ring seal) to see if #3 is about what the other five are, and if so, just drive it and keep observing for what look like oil leaks around the head to cylinder seal which differ from what you see on the other five. And, since there really is nothing you can do if a threaded part is stripped, especially if you can't get the barrel nut off, all this diagnosis is sort of moot (but a challenge for us in the peanut gallery).

7) When ultimately the engine comes out you will be faced with how to get that cylinder off. At that point, knowing what is what might help with a strategy. With the right small tools you can cut the stud after lots of other stuff has been removed, but that's not easy. With the other three nuts removed, you might be able to exert upward force on the head or cylinder (without breaking fins or other stuff) while trying to back the nut out.

[Walt Fricke]

Hold the press - you say there are two threads above the top of the nut? This means you don't have the stock barrel nut, but an aftermarket flanged nut? Those are a better mousetrap because you can't have the engagement problem with the stud end blocking the Allen engagement. That means it is likely that this is an aftermarket stud. Those are not dilivar, and are of higher strength steel. It isn't an ARP, because those have a bull nose end which makes getting the nut started ever so much easier. But there are at least two others with regular threaded ends. One of them uses fine thread on the nut end, so my guess would be this is Raceware with the regular 1.5 or whatever thread. Or they might just be the stock studs with different nuts.

Have a worry free weekend, though. I found a broken stud on my SC race motor. Who knows how long it had been broken, but there were no signs of problems.

[Otter74]

Walt, I would like to thank you for your post in particular, because it made me realize that I do not have the problem that I thought I had! This is rather embarrassing on my part, what with this being a relatively-public forum and all, but what's learning without occasional embarrassment? And embarrassment is a lot cheaper than engine work on a 911

When I was down under the car and thought to check this, I looked inside the head and drew a conclusion based on location of where the head studs were, but without following them up visually from the outside of the case or, y'know, looking in my Bentley manual. So this nut that I thought was attached to a head stud is, in fact, not. It was the mention of the allen socket and barrel nuts that gave me the 'light dawns over Marblehead' moment. The nut in question is seen immediately to the right of the #4 rocker arm in this photo, but on #3 instead:
https://www.google.com/imgres?imgurl=http%3A%2F%2Fforums.pelicanparts.com %2Fuploads%2FbrokenStud4.jpg&imgrefurl=http%3A%2F%2Fforums.pelicanparts.com%2Fp orsche-911-technical-forum%2F109238-arrrr-i-have-broken-head-stud.html&docid=DbjBPTaGWtSh1M&tbnid=NkrOhYEKFaRtYM%3A&vet=10ahUKEwi8m-O69OTiAhULi6wKHYkwDxoQMwipASgHMAc..i&w=500&h=375&client=firefox-b-1-d&bih=888&biw=1776&q=911%20SC%20head%20studs&ved=0ahUKEwi8m-O69OTiAhULi6wKHYkwDxoQMwipASgHMAc&iact=mrc&uact=8

this is, I now realize, not one of the head studs and that's obviously not a barrel nut.

Thank you for illuminating my cluelessness. I've been doing my own maintenance for over 20 years and have gotten to know my car pretty well, but even I am prone to dumb-bunny mistakes eveyr so often

[Bob Kontak]

Originally Posted by Otter74

this is, I now realize, not one of the head studs and that's obviously not a barrel nut.

Thank you for illuminating my cluelessness.

You are not clueless, friend. Sleep well and God bless Walt Fricke.

Now, how to pull that cam tower stud and put this to bed? I say winter project.

[Walt Fricke]

The photo won't open on my screen. I'm guessing this was an 8mm stud with a 13mm wrench nut? Holds the cam carrier to the head?

[pmax]

Hah, I was close !

Congrats.

Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth. Arthur Conan Doyle