Head stud broken, now what?

[OldTee]

I am interested in the advisability of several "while you are in there" decsions I must make. I realize they are highly personal but there must be some common $ense logic about the way to go.

Background. The car is a 87 Targa with 97K miles of uncertian history. It was running fine except the clutch was shot. We are refurbising the whole car with new bushings shocks etc. and planned to refresh the engine with new hoses, some new sensors, gaskets, etc. When the valve cover came off to adjust the valves the head stud for #2 fell out. Leak down on all cyls was good as was the compression. (Ie) no symptoms. The engine is out of the car and on the engine stand. When finsihed the car will be a 'keeper' used for semi daily transportation and occasional DEs. I Don't plan to break the case.

Now the questions.

With this many miles is it advisable to just replace the stud?

Valve job now?

If I remove the heads to do a valve job must I also remove the barrels to replace the lower gaskets?

What is the going engineering cost to do the valves. Labor and parts?

Valve springs?

This is a lot to decide, but some of the experts here have faced this delimma. A road map would be appreciated.

[zbph10]

I was in about the same boat except I had 4 broken head studs. I decided to completely go through the engine so I knew what I had.

I sent my heads and rods to Henry at Supertech, new ARP rod bolts, balanced everything and am replacing basically everything Wayne suggests in the rebuilding book (gaskets/sensors/bolts) and cleaning and painting everything.

I'll have $3500-$4000K in the rebuild doing it myself and that's reusing my P&C's. FYI Having the heads done were only $350. About $400 of that was specialty tools

Motor is going back together this month, wish me luck.

Brian

[zbph10]

Forgot to mention I replaced all of the head studs with new steel ones, couldn't justify the extra expense for ARP or Supertech. I was told unless I'm racing the car it wasn't needed. I'd replace all of the head studs IIWY

[88-diamondblue]

I did my rebuild due to 2 broken exhaust studs. Top end cost (valve job) was about $400.00 from one of our well known shops here on PP. I did a complete rebuild. Definitely replace all the lower dilivar studs and should consider ARP rod bolts since this is a Carrera. You can replace the rod bolts without splitting the case. I would at the very least do the studs and heads. If everything else is ok, you are good for another 100,000+ before a rebuild. Do a search as there is lots a of good reading.

[OldTee]

Thanks. Looks like step one is to figure out how the thing comes apart without butchering it. Then I'll pull the head on the offending cylinder and see what I have. Then I'll make the hard decisions to top or not to top. My goal is to retrun the engine to a condition that I can get 50Kmiles out of it with some reliability. I recognize the risk. The budget has been warned. We've cancelled the newspaper, sold the dog, will let the cat catch mice to eat and just get on with it.

[boxermania]

OldTee

Since the engine is out and the car will be a keeper I would sugest the following:

1) Replace all of the lower studs with the steel version
2) Refurbish the heads (valve job) the whole job typically runs around $600 with out parts. There were some issues with the 3.2 valve guides early on but I would sumize that by '87 the issues where corrected
3)If the car had basic maintenance, you should still see the crosshatch (honing) on the cylinders with hardly any wear (ridge) on the cylinders. No need to go there, they are good for 200K+ miles.
4) replace the front and rear crank seals if there is evidence of leaks.

Of course reseal everything on the outside and you should have a sound engine for a long while.

Good luck

[OldTee]

Well stated. The makings of a good plan. Thank you.

[casperlabs]

If you don't plan on splitting the case, we recommend using the exact same p/n OEM stud. Mixing stud materials is asking for trouble, as the difference in thermal growth characteristic will result in unequal load sharing, and failure someplace else.

Broken studs are the result of Low Cycle Fatigue (LCF). The rapid thermal expansion of all aluminum cylinders puts a tensile load on the stud that far exceeds that generated by the installation torque. A heavy foot on a cold engine is the worst thing you can do for your stud hardware.

It is a misconception that high tensile/fatigue strength studs are only needed by racing engines. The race engine thermal cycle is actually much less severe than what is typically seen in a daily driver. Reusing old studs that have been subjected to decades of LCF cycles should never be done w/o a full Magnaflux inspection. For coated studs this means a strip and recoat.

Casper Labs considers the two shortfalls of the 911 engine to be the cam chain tensioners and the cylinder head attachments. Right now the tensioner problem is widely understood, but the stud issue is surrounded by lots of mystery and conjecture for most owners. Porsche has compounded confusion over the years with different many different p/n's, and little real technical data on why they are different.

The message we try to get through to our customers is that if you are rebuilding an engine (that you intend to keep) and it has linerless aluminum barrels, then the $500 cost of our kit is insignificant if you have to pull an engine apart to replace an old stud that has seen one too many cycles.

Bill Ryan
Casper Labs, Inc.

[john walker's workshop]

the factory mixed stud materials. steel top/ dilivar bottom. people have been replacing the dilivars with steel for years with no problems. so you would reuse an old dilivar stud?

[Eagledriver]

I used to ask if anyone had seen a factory steel stud break. One person said they did so I don't ask anymore but I still think it's extremly rare.

I would replace the lower row with steel studs and do a valve job. Your engine will run for another 100k miles or more if you do. It's a good idea to check the pistons/rings/cylinders while you have the engine apart but normally they will measure up just fine. I agree with you on not doing the lower end without cause.

-Andy

[casperlabs]

After reading jwworkshop's reply to our posting, we felt further clarification of our earlier message was needed. Casper Labs has done its homework on stud failures, and we will share our data with anyone who is interested. The failure modes are not complex. Really understanding these modes from the engineering perspective is all that is needed to give engine builders the information they need to make parts selection. If they understand why the failure occurs, they have a heads-up in determining how much risk they are willing to accept on each engine they build.

Using the word "steel" in conection with these studs is terribly misleading because there are so many different p/n's out there, and technically they are ALL made of some form of steel. Porsche's reluctance to put identifying marks on the studs, can make this a real grab bag of hardware.

The desk drawer in front of me has at least a dozen pieces of broken studs. These have been sectioned and mounted in resin for metalurgical analysis. We have broken studs made of Dilavar, stainless steel, and various alloy steels. Fortunately for us, there are some of the country's top metalurgical engineers in this area, because Pratt & Whitney's rocket engine business is just down the street. Across the road from P&W is Moroso Speedway. Its a great combination when you put together talent, resources, and experience.

While I do not doubt jwworkshops comments, please consider the following on the Dilavar application. While Porsche may have mixed Divalar studs on engines as an OEM, let me remind you that they have had several failed attempts at fixing this problem, and we view this one no differently. The lower studs run hotter than the top ones because the cooling air has picked up heat as it passes over the cylinder fins. When you combine the greater thermal expansion of Dilavar with the additional temperature, you have caused a big difference in tensile load top to bottom on a hot engine. Anybody who does this on a mag case engine is headed for trouble.

Stud failure (on all aluminum cylinder engines) is a random process. The random nature is driven by the condition of the stud as it goes into the engine. If it has a crack or scratch in the wrong place from machining, thread rolling, or installation, (that the others do not have) then it will be the first to fail. That is why we suggest ANY reused or new stud be magnetic particle inspected. A stud that has been heated for removal should be considered suspect, its heat treating may have been compromised. It is cheaper to replace it than have a lab do a workup on its condition. As time passes there are going to be fewer and fewer of these engines driving around. However, the ones that are out there are going to have a lot of LCF cycles on old studs. This means the probability of a stud breaking will be higher on any given engine.

It all comes down to the cost/risk question. The solution exists. The question for decision makers is if it is worth the expense. If the engine builder does not snap a stud when torquing, he knows that it is structurally sound to last a few miles. Is 20K, 30K, or ??? miles?? That is going to be determined by how many cracks have started and what the engine duty cycle will be. Our business is to guarantee our studs WILL NOT FAIL, for those customers who do not want to deal with the problem. If you solve the stud problem, the next weakness is the case. That is why we design our studs specifically for the grip length of a threaded insert. You get additional threads and more load carrying area simultaneously.

Bill Ryan
Casper Labs, Inc.

[JBO]

...after all that, I'm still not sure of the answer - is it to use steel 993 studs, which match the OEM steel top row??

[casperlabs]

JBO.....don't recognize your member ID, as this chain started with Old Tee, but.... we will try to answer your question anyway.

We will assume the engine's lifetime maintenance history is unknown. The first thing we would do to an assembled engine (that you don't want to breakdown) is check each stud visually to see if there are any apparent differences in stud appearance in the remaining 23 (color/shape). If there are not differences, we would order a brand new fully traceable stud for the model year engine, and compare it with the others. If identical, we would mag particle (takes only a few minutes) check it and install with low temp. threadlocker in a super clean case hole. We would be careful to look for unexplained oil leaks at the cylinder base which could be an indication of case distortion resulting from the "3 studded cylinder". All of the cylinder stud nuts on each side should be loosened in the same sequence as used for tightening. Record the breakaway torque on each for future reference. Then retorque all twelve on each side. If you try to use threadlocker on the nuts, it will be impossible to get accurate torques, as the stuff sets up so fast you cannot progressively torque up 12 nuts without it starting to set. Post application of penetrating locker does not seem to work, as we have not found one that survives the CHT very long. (This is why we use all metal lock nuts in our kit). This approach is not terribly scientific, but it is reasonable and practical under the circumstances.

You should also remember that this engine has demonstrated stud failure, and other studs might/could be near their limit. I would check them with a wrench as frequently as practicable. Record the starting torque on each every time you check. If you create a table or graph of these torque values, over time you will likely be able to predict a pending failure. Watch your acceleration rate carefully until the oil temp. has stabilized on each cold start.

Bill Ryan
Casper Labs, Inc.

[88911coupe]

Something I'm still not clear on is in regard to the rod bolts and main crank bearings. Other than taking them apart is there any way to determine if they are still good. I realize it's pretty east for the rod bolts but do you risk anything by just putting new bearings in...or are they just replacable assuming there is no damage to the surfaces?
TIA

[Eagledriver]

When I have an engine apart and want to check the lower end I remove one of the center rods and inspect the bearing. If the bearing condition looks good then I re install with new bolts. I make the assumption that the other bearings are in similar condition. The center rods are the first to show wear because they have the lowest oil pressure to them.

All the bearings are replaceable assuming the journals aren't worn or damaged.

If the engine has good oil pressure then the bearings are likely to be in good condition.

-Andy

[john walker's workshop]

seems like a lot of smoke and mirrors just to replace a lower row of dilivars with steel. that's OE factory steel for the record. ya want to hotrod, ya might want to go supertech or ARP, all 24 in that case. that method has been working fine for at least 25 years now.

[Eagledriver]

Imagine the staggering amount of effort that would go into building one of these engines if every aspect that was as reliable as the factory steel headstuds was given this kind of attention. Sounds a little like rocket science!

-Andy

[kuehl]

Originally posted by john walker's workshop
seems like a lot of smoke and mirrors just to replace a lower row of dilivars with steel. that's OE factory steel for the record. ya want to hotrod, ya might want to go supertech or ARP, all 24 in that case. that method has been working fine for at least 25 years now.

My 87 3.2 had a broken stud at 90k. Appeared to be corrosion started after the coating was chipped. Whatever.
A new set of 993 studs did the job easily. Of course while we were in there she got rings, valve job, clutch, flywheel, alternator, starter, yada. All for the sake of a $9.00 stud. In for a penny in for a pound.

[OldTee]

Originally posted by kuehl
My 87 3.2 had a broken stud at 90k. Appeared to be corrosion started after the coating was chipped. Whatever.
A new set of 993 studs did the job easily. Of course while we were in there she got rings, valve job, clutch, flywheel, alternator, starter, yada. All for the sake of a $9.00 stud. In for a penny in for a pound.

Whoa .... Just got a quote of $30 each for OEM steel and $11+ for stainless steel. (plus shipping) That is $360 vs your $108. Big diff!

[304065]

Uncertain history + Carrera + possible over-rev = replace rod bolts with ARP. This can be done without splitting the case.