Why doesn't broken head stud destroy the engine within a few seconds ?

[sugarwood]

This is something I have never understood. I assume the head stud holds the head in place. There are 2 or 4 per head? There appears to be a long rod passing through the cylinder and head.

If the stud also holds on the cylinder, when it breaks, I would expect the cylinder and head to move. The piston is attached to the connecting rod and has a very tight tolerance. If the cylinder moved, I would think the piston would smash into the cylinder wall, and destroy itself instantly. Why does this not happen, and people drive for years and not even know they have a broken head stud.

Or does only the head move and there is no way for it to hit the piston, since it can't get any closer due to mating surface of head being fixed.

I get if only one is broken, maybe the other cylinders keep it from shifting. But, how many can you get away with. With 2 studs broken, I would think the above scenario would be inevitable.

[Travis Neff]

There are 4 headstuds per cylinder. all headstuds on each engine side bolt the cam tower with the heads sandwiched in between. The remaining unbroken studs hold the load keeping everything together.

[sugarwood]

Thanks. I am guessing 2 broken studs could still keep everything still. Any 2 corners of a square are fixed, the square stays fixed. So, it would take 3 broken studs from the same cylinder for a catastrophic shift in the engine causing interference?

Has anyone ever reported 3 or all studs breaking in the same cylinder and resulting in a catastrophic engine failure ?

[john walker's workshop]

[boyt911sc]

[Rawknees'Turbo]

Wood, the cylinder and head assembly doesn't move left or right, or up and down (as in up/sky and down/ground) much at all (as in basically not at all), even if many of the studs on a bank are broken (even just two remaining, intact studs would do a lot to steady that kind of movement, as everything is still held together, left to right & up and down, by the cam housing), but what happens is compression and combustion pressures lift the heads that have broken studs, and that allows combusting fuel and oxygen to escape, which then burns/erodes the surface where the heads meet the cylinders - even with such a slight loss in clamping force that there is no escaping combustion, you can end up with slight-movement-caused damage to those surfaces (called fretting).

You are not going to see catastrophic, 911 engine failures due to broken studs, as they do not break all at once (not even the super shiity, dilivar studs that Porsche used for decades and could not get right until the 993TT, do that), and when a couple of them have broken (even just one, for people with experienced and attentive ears), the tell-tail noise of escaping combustion is loud and clear, so someone who is thinking straight will not operate the engine further until repairs are made.

[LIRS6]

I drove my car for years with no issues, save the dirty block and a drip or two .. or ten



Jason

[MBAtarga]

Looks like 3 broken studs above.

[sugarwood]

The hypothesis was 3 broken studs in the same cylinder. Center cylinder has 2 broken.

I will try to find a photo of cam housing to understand how that keeps all 3 cylinders and heads from moving. I am guessing it is like an inverted egg carton with 3 eggs that "cups" and forces things to stay in line.

[boyt911sc]

Originally Posted by sugarwood

The hypothesis was 3 broken studs in the same cylinder. Center cylinder has 2 broken.

I will try to find a photo of cam housing to understand how that keeps all 3 cylinders and heads from moving. I am guessing it is like an inverted egg carton with 3 eggs that "cups" and forces things to stay in line.

Sugar,

Are you aware why and where the broken head studs always occur? Take a closer look at Jason’s picture and you will find the obvious. The three-in-one (3 in 1) scenario has never been documented or not seen in the last 40 years. I doubt you will find one.

Tony

[KTL]

Originally Posted by sugarwood

This is something I have never understood. I assume the head stud holds the head in place. There are 2 or 4 per head? There appears to be a long rod passing through the cylinder and head.

If the stud also holds on the cylinder, when it breaks, I would expect the cylinder and head to move. The piston is attached to the connecting rod and has a very tight tolerance. If the cylinder moved, I would think the piston would smash into the cylinder wall, and destroy itself instantly. Why does this not happen, and people drive for years and not even know they have a broken head stud.

Or does only the head move and there is no way for it to hit the piston, since it can't get any closer due to mating surface of head being fixed.

I get if only one is broken, maybe the other cylinders keep it from shifting. But, how many can you get away with. With 2 studs broken, I would think the above scenario would be inevitable.

Your original post does not say three broken studs. It says two. Like Tony mentioned, the upper studs don't ever break. They just don't.

I echo the by others. Chicken little the sky is falling, here we go again........

[GH85Carrera]

On my 85 3.2 engine I was doing my typical inspection after another long road trip at 150,000 miles total mileage and noticed a wetness on one head. No major leak, but a wetness ain't good.

I took it up to my engine builder and we dropped it, and started looking. Of course to get the head off, is a lot of digging. Got the suspect cylinder and one stud was loose. Not broken, it had somehow backed out of the case. No thread damage, no stud damage. Everyone says "I had never seen that before" and I guess I was the lucky lottery winner.

Of course the engine was grimy and gross after all those miles. If one side had an issue, we had to check the other side. And "while we are in there" struck big time and down the slippery slope to a full rebuild. New ARP studs for everything and rod bolts and might as well......................

[donbecker1234]

My understanding is that although head studs breaking isn’t a “good thing”, the concern is gap between the cylinder and the case, possibly caused by a broken head stud.

[356911930]

Don,

It is the top of the cylinder and the head that will separate, causing fretting that is the worry of permanent damage. This will require the head mating surface to be "cleaned" up some.

Rahl

[sugarwood]

Originally Posted by boyt911sc

Sugar,

Are you aware why and where the broken head studs always occur? Take a closer look at Jason’s picture and you will find the obvious. The three-in-one (3 in 1) scenario has never been documented or not seen in the last 40 years. I doubt you will find one.

Tony

Why. I read that it's due to moisture. I did see Franny mention something about upper and lower bolts being different materials since different temps? Intake vs. exhaust?

I see that only the lower studs are broken. Looks like top ones don't break? Hence, never getting 3 of the 4? Thanks!

[sugarwood]

Originally Posted by GH85Carrera

. Got the suspect cylinder and one stud was loose. Not broken, it had somehow backed out of the case. No thread damage, no stud damage. Everyone says "I had never seen that before" and I guess I was the lucky lottery winner.
......

https://youtu.be/-0gUImuE5e0?t=409

[sugarwood]

Originally Posted by 356911930

Don,

It is the top of the cylinder and the head that will separate, causing fretting that is the worry of permanent damage. This will require the head mating surface to be "cleaned" up some.

Rahl

What if the fretting damage is beyond repair?
Can one simply buy a new cylinder and head?
What do those 2 parts cost these days?

Cylinder is $700
930-103-990-01
https://www.pelicanparts.com/catalog/SuperCat/911M/POR_911M_ENGPIS_pg1.htm

Cylinder head is $1500
930-104-033-06
https://www.pelicanparts.com/cgi-bin/ksearch/PEL_search_2016.cgi?command=DWsearch&description=Cylinder+Head+

[juanbenae]

Originally Posted by john walker's workshop

Originally Posted by boyt911sc

:d:d:d

[GH85Carrera]

Originally Posted by sugarwood

https://youtu.be/-0gUImuE5e0?t=409

On my engine, just that one stud was loose. All the others were still fine. None were broken. But that one stud allowed combustion gasses to escape, and damage the head a little. So it needed to be surfaced just a bit. Of course you can't do that without removing the head, and that really means all 6 heads have to come off. So a full top end has to be done. It would be really stupid not to do the valve guides, and clean up the valve seats. And since the heads off it is easy to pull the cylinders and check them. And while yer in there the rod bolts can be replaced. But heck, while yer in the re why not split the case and check it all out.

I was real glad we did split the case and the intermediate shaft bearing had a decent amount of wear. One of the rod bearings showed a little bit of a "hot spot" so while we were in there.........

In the end my engine was all better and I love it.



Before


After

[patkeefe]

I think it is only possible to break three head studs in one cylinder if you use the wrong viscosity oil below 20F.