Questions on Cylinder Base Gasket and Assembly

dtxscott · 01-21-2021, 04:18 PM

Doing a valve adjustment on my '89 Carrera 3.2L recently and noticed pitting on Cyl 2&3 exhaust side cam lobes.

I have the cam housings off the engine and am looking at options on the head studs. Thankfully all are intact but I don't want to re-assemble the engine without addressing the exhaust side head studs.

I plan to go with the 993 studs to keep things simple. This is a stock engine in a bone-stock car.

I also plan to address the exhaust-side valve guides as I get black soot on the back of the car and burn a little oil.

Question1: By pulling the heads, will I be disturbing the cylinder base gasket?
Question 2: Will I need to replace the cylinder base gasket?
Question 3: The Bentley manual indicates to snug the head nuts down by hand then torque the cam housing to the heads.... Then come back and torque the head nuts afterward. Is this the correct?

Thanks for any and all help!

Henry Schmidt · Henry Schmidt's Garage

Quote:

Originally Posted by dtxscott

Doing a valve adjustment on my '89 Carrera 3.2L recently and noticed pitting on Cyl 2&3 exhaust side cam lobes.

I have the cam housings off the engine and am looking at options on the head studs. Thankfully all are intact but I don't want to re-assemble the engine without addressing the exhaust side head studs.

I plan to go with the 993 studs to keep things simple. This is a stock engine in a bone-stock car.

I also plan to address the exhaust-side valve guides as I get black soot on the back of the car and burn a little oil.

Question1: By pulling the heads, will I be disturbing the cylinder base gasket? YES
Question 2: Will I need to replace the cylinder base gasket? YES
Question 3: The Bentley manual indicates to snug the head nuts down by hand then torque the cam housing to the heads.... Then come back and torque the head nuts afterward. Is this the correct? YES

Thanks for any and all help!

The process you are contemplating is pretty involved. Removing the heads, replacing the head studs all without disturbing the cylinder base gaskets.
They are copper and compress when the head/cylinder stack is torqued.
It may not leak but why risk it? Pull the cylinders, do your head stud replacement, install new gaskets (we use Curil T as a sealant) and enjoy the peace of mind.

As for cam tower assembly, you cam also assemble the heads and cam towers on the bench. Then drop them on the engine as an assembly. Either way works.
What ever method makes you more comfortable is fine.

dtxscott · 01-22-2021, 08:05 AM

Quote:

Originally Posted by Henry Schmidt

The process you are contemplating is pretty involved. Removing the heads, replacing the head studs all without disturbing the cylinder base gaskets.
They are copper and compress when the head/cylinder stack is torqued.
It may not leak but why risk it? Pull the cylinders, do your head stud replacement, install new gaskets (we use Curil T as a sealant) and enjoy the peace of mind.

As for cam tower assembly, you cam also assemble the heads and cam towers on the bench. Then drop them on the engine as an assembly. Either way works.
What ever method makes you more comfortable is fine.

Thanks Henry! I've been following your "rattlings" on this forum and appreciate your advise and input.

KTL · 02-02-2021, 09:22 AM

When you pull the cylinders, avoid possible piston ring issues (during cylinder reinstallation over the piston) by pulling the cylinders just enough to expose the piston pin. CAREFULLY remove the pin circlip and the pin. Highly recommend you stuff a few paper towels into the engine case bores to prevent a circlip from flying in there. You only need to remove the one circlip on the side to which you'll be knocking out the pin. Once the pin is out you can remove the cylinder and piston together. This way the rings stay in place with no risk of breaking one when re-compressing them for cylinder replacement.

I'll add that you potentially could leave the base gasket alone if you were simply doing the heads and not doing the lower head studs. But it's indeed a risk. My first top end job around 15 years ago in 2006 was on a very nice '87 3.2L. It had moderate mileage of something like 68K miles and a rust free car. I share those details simply to substantiate how I got EXTREMELY lucky in the following ways:

1. None of the base gaskets have leaked since the work was done and engine has 20K more miles now in 2021. It's driven plenty hard and often. I had the car from 2000-2014 and the guy I sold it to is a very close friend who drives it like he stole it.

2. I found no broken lower studs upon disassembly (wow those nuts SCREECHED coming off) and i'm shocked that a dilivar head stud didn't break during disassembly, reassembly or since then. This thread got me wondering what method I used to torque them in reassembly...........

I think the lack of leaking base gaskets in my case is just dumb luck. They may show some weeping now but certainly no legit drip leakage. I also think the survival of the head studs is from the car rarely ever seeing inclement weather and never being in horrible salt-tainted winter weather. That said, it irks me to this day that the engine still has those annoying dilavar studs in place.

Point of me sharing is not to endorse leaving the base gaskets. Just sharing my one example of luck. When I tore down my most recent engine repair job, an '86 3.2L in late 2017 that's been in my family of friends since 1999, it had more of the common issues I would expect to find. Nasty valve guides (cyl. #3 so bad that the oil was washing the combustion chamber clean- not covered in carbon like others) and one broken lower stud.

Number 6 cylinder showing typical carbon on the piston and note the missing end of the stud on the bottom right

Number 3 cylinder with piston washed clean of some carbon due to valve guide leakage (and maybe a leaky fuel injector too?)

Note the pool of oil sitting in the number 3 intake port

Number 3 head with a very clean combustion chamber

Number 6 head with typical carbon buildup in the chamber

Number 6 lower, outer head stud broken and bouncing around in the cam housing

Henry Schmidt · Henry Schmidt's Garage

This is a perfect illustration of why Dilavar studs should be banished from the lexicon of air-cooled engine building. They offer an unstable cylinder to head joint even when they aren't broken.

rcw911rsr · 02-02-2021, 08:19 PM

there in no way i would risk 40-50 additional hours to do it over agian in an effort to save maybe 1-2 hours to switch the base gaskets. IMHO they are one time use and very cheap. like mentioned above, my vote is that you should change them.

rcw911rsr · 02-02-2021, 08:20 PM

and henry can sell you a set of great and affordable headstuds. i just used them on drbrian's motor and they will be going in the 964 motor im building next.

winders · 02-02-2021, 09:00 PM

Quote:

Originally Posted by Henry Schmidt

This is a perfect illustration of why Dilavar studs should be banished from the lexicon of air-cooled engine building. They offer an unstable cylinder to head joint even when they aren't broken.

Except 993 Turbo head studs (dilavar) seem to work really really well. Many of the high output 3.6 to 4.0+ liter race engine builders exclusive use the 993 Turbo head studs. I am not saying the SuperTec studs aren't great, but it seems odd to put all dilavar heads studs in the same bucket when the 993 Turbo parts work great.

Henry Schmidt · Henry Schmidt's Garage

Quote:

Originally Posted by winders

Except 993 Turbo head studs (dilavar) seem to work really really well. Many of the high output 3.6 to 4.0+ liter race engine builders exclusive use the 993 Turbo head studs. I am not saying the SuperTec studs aren't great, but it seems odd to put all dilavar heads studs in the same bucket when the 993 Turbo parts work great.

993 turbo studs are lumped in with "all" other Dilavar studs for one simple reason: Metallurgy. The redesigned 993 stud (all-thread) helped prevent breakage but did nothing to change the underlying flaw in the material. Inconsistent clamping force based on environmental changes. The reason Porsche tried so hard to make Dilavar work had very little to do with cylinder to head stability and everything to do with warranty mitigation. Horizontally opposed air-cooled engines tend to leak oil from the head to cylinder joint, especially during cool-down.
The instability during full temperature running is always appearance on tear-down.
Clamping pressure when cold translates to a higher clamping force at full temperature. Engineers can easily calculate that force when dealing with constant numbers. As the engine expands, clamping forces should/do increase. With an engineered steel stud, that clamping force is magnitudes more consistent than you get with the ever-changing Dilavar. This inconsistency causes the cylinder to head joint to fluctuate creating poor sealing and unnecessary internal stresses.
Two simple questions: Why were the first 993 turbo studs made from steel? Why, if Dilavar is so magical, did Porsche move to steel studs in the GT3 Mezger based engine?

dtxscott · 02-03-2021, 04:49 AM

After tearing the engine down further it became clear that re-using the base gaskets is impossible. Given they are 0.25mm thick, they are very fragile.

Thanks for the inputs and continued dialog.

winders · 02-03-2021, 10:08 AM

This thread says a lot:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/644752-993-head-studs-two-types-3.html

I don't get how you get "Inconsistent clamping force based on environmental changes" with the 993 Turbo dilavar head studs. Dilavar is a metal alloy that expands a certain rate. That rate is more in line with the rest of the engine than other alloys.

Henry Schmidt · Henry Schmidt's Garage

Quote:

Originally Posted by winders

This thread says a lot:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/644752-993-head-studs-two-types-3.html

I don't get how you get "Inconsistent clamping force based on environmental changes" with the 993 Turbo dilavar head studs. Dilavar is a metal alloy that expands a certain rate. That rate is more in line with the rest of the engine than other alloys.

Simple enough. Not all of the head studs operate in the same environment.
Some are always hotter than the others.
If they are different temperatures, they will expand differently creating inconsistent clamping forces.
The greater the expansion rate the greater the inconsistency.
Now explain why Porsche used steel studs on top and Dilavar on the bottom of 911 SC. Only one reason: control seepage during cool down.
One more important question: The static head nut torque (cold) on a steel stud is 32NM and with cylinder expansion that preload becomes considerably higher when running. The engineers knew this and I assume deemed the warm clamping force desirable.
Now, if the Dilavar stud has a static torque of 32NM and the expansion rate is commensurate with the cylinders, the clamping force must be lower. How can both be correct?

BTW: In theory, the Dilavar expands at the same rate as the aluminum cylinders.
If that is true (that is the claim) then clamping forces would not increase as temperatures increase. Who in their right mind believes that 24psi is sufficient clamping force for a 911 head in full song?