Head stud replacement - SC

[motogman]

I know this topic has been covered extensively and I have read most of the threads - thanks to all for the vast knowledge sharing. But I haven't been able to clearly answer this question...

If I understand correctly, dilivar was only used on the exhaust studs in the base 911 motor (mine is '81 SC 3L). Is this correct? If so, do the intake side studs need replaced or just the exhaust side?

Bonus question: Can the studs be replaced (that are not broken) without removing the piston & cylinder? For example, as a preventative measure on a motor with Alusil bores, could one preventatively replace the exhaust side stud without removing the bores and then put the top end back on with an acceptable outcome?

[smokintr6]

I think the thing is; you could cut those corners that you're talking about, but the juice isn't worth the squeeze. Personally I would replace all the studs, and would want to pull the cylinders to make sure the base gaskets are fresh. It would be a pretty major annoyance to pull the engine that far apart and create an oil leak "while you're in there".

[targa72e]

couple thoughts and answers. If your studs are not broken ( and have lasted 40years) and you do not need other work, leave them alone and inspect periodically. If the engine has ever been apart they may have been changed to steel, easy to check with magnet.
If you are in far enough to get to the studs I would remove the cylinders. Its not that hard at that point. You can leave the pistons on the rods. Mark the cylinders so you can keep the pistons and cylinders together and reassemble as removed. If the steel studs are in good condition you can reuse otherwise replace. Add new base gaskets.

john

[Trackrash]

Yes, only the lower, or exhaust studs are dilivar.

The uppers are steel and do not need to be replaced, unless they are very rusty.

If you have a broken stud or have the heads off, it is a no brainer to replace all the lower studs. Most will just use steel studs, the same as is on the top. There really is no down side to using all steel studs, IMO, unless it is a race or very high horsepower motor.

Trying to remove the studs with the cylinders in place is a can of worms. The studs are thread locked into the case, which usually requires heat to loosen them for removal. If you try to force them out, with out heating the case, you run the risk of breaking the stud off.

You can remove the pistons and cylinders without removing the piston from the cylinder. At that point you may as well take a look at the rod bearings.

[motogman]

Thanks for the feedback. Abut what I was expecting...

Maybe a follow up...

I understand that Alusil cylinders can not be honed and if the intent is to reuse the piston and bores then the pistons should be removed such that they stay in the bore. The risk and issues are that the rings, pistons or bores may have some wear that reduces the potential life of the rebuild. On the other hand, to replace them is a substantial cost. Let me know thoughts on the Alusil problems.

On the other hand... if the bores are Nikasil they can be honed and reused with the pistons and one would likely replace the rings. However, as I have read, new rings will have higher friction and if there is some wear in the top ring groove, that the add forces and ring banging back and forth can cause the piston area above the top ring to crack. Correct any of these perceptions?

Buy my question really is... for Nikasil bores that you plan to reuse, are you better off keeping the pistons in the bores and not re-honing the bores, using the cylinders, piston and rings as they were before the teardown?

[timmy2]

I re-ringed my Alusils 8 years ago. No problems since on what was an 80,000 mile motor. Everything was in spec for wear.
Light rub with grey scotchbrite lubed with motor oil is all I did to the cylinder walls to take off any deposits.
I’ve read there is a 50% success rate doing that, but I think it is all in how worn the engine is and how you break it in after.
I personally loaded the crap out of it up and down local hills to seat the rings.
I used the Goetz ring sets listed for my car from Pelican.
Only replaced the lower dilivar studs with steel.

[targa72e]

In both cases nikasil or alusil if you remove the pistons from the cylinders the important thing is to keep them together if you plan to reuse as they have warn in together. If all are in good condition they can be put back into service without replacing. The pistons don't have to stay in the cylinders they just need to be keep together. I would measure ring groove clearances, inspect piston skirts for scuffing, measure cylinder bores for ovality and measure piston to cylinder clearance. The important thing is to keep all parts paired. I would clean removed parts before re-installing. I have measured many P&C sets and often find even with 100K miles things are still in new clearance spec.

john

john

[911hutch]

Is anybody replacing dilivar with dilivar?

[safe]

I haven't read any credible explanation to why you shouldn't separate the pistons from cylinders and if so why there wouldn't be a difference between cast iron, alusil or nicasil. I call wife's tale on this.

Anyway I separated my nicasil cylinders from my pistons when I replaced my head studs. I have less than 2% leak down on all cylinders 10 years later after plenty of track miles. Reused the piston rings.

I wouldn't hone or let anyone hone my nicasil cylinders, unless they really, really knows what they are doing and have the equipment to measure the surface roughness before and after.

[safe]

Originally Posted by 911hutch

Is anybody replacing dilivar with dilivar?

993 turbo studs are Dilavar, but better Dilavar.

[Richey]

I replaced Dilavar with Dilavar in 1994 , still ok . I like the equal expansion rate theory , no pulled head studs or cracked cases .

[ahh911]

Richey,
Did you replace all 24 studs with Dilavar? I'd be surprised if the lower half of the cylinder + heads expands more than the top half. Always seemed like quackery to me. But I do understand all 24 being replaced.
Phil

[motogman]

Originally Posted by Richey

I replaced Dilavar with Dilavar in 1994 , still ok . I like the equal expansion rate theory , no pulled head studs or cracked cases .

From my read on the history... when Porsche went to mag cases (in the late 60s I think) all was fine with steel studs. As engine displacement grew (2.2, 2.4, 2.7, +) and more specifically when emissions control systems went on the cars (74 ish), the emissions stuff made them run hotter and the mag cases would get soft and head studs would pull out.

Dilivar, and the equal expansion theory, were put in place to reduce the increased force in the case when things got hot (the aluminum expanding more at temperature than the steel studs) to reduce the stud pull out of the mag cases.

When aluminum cases came back in (I think 78 with the 3.0 SC) the al did not suffer so much from the heat and the head stud pull out issue became a non-issue. But Porsche stuck with Dilivar for a very longtime. Dilivar as we now know, is sensitive to the corrosion pitting from moisture and the stress concentration of the corrosion pits leads to fracture of the studs.

There are many posts where people have put steel studs in the post 78 al case engines with no issues.

I don't know if this is a correct interpretation and I welcome illumination on this history. It is an interesting segment of Porsche engineering history that many of us still live with. Other topics include chain tensioners, IMS bearings and several others I am not remembering right now.

[mikedsilva]

Originally Posted by timmy2

I re-ringed my Alusils 8 years ago. No problems since on what was an 80,000 mile motor. Everything was in spec for wear.
Light rub with grey scotchbrite lubed with motor oil is all I did to the cylinder walls to take off any deposits.
I’ve read there is a 50% success rate doing that, but I think it is all in how worn the engine is and how you break it in after.
I personally loaded the crap out of it up and down local hills to seat the rings.
I used the Goetz ring sets listed for my car from Pelican.
Only replaced the lower dilivar studs with steel.

It's interesting in the 911 world, we are trained to replace or recoat Alusil, with Nikasil... My friend who has a 944 says their entire blocks in 944 are Alusil so they don't have the luxury of swapping! He simply has the block honed and re-ringed and away he goes.. good leakdown numbers and lots of power.

I'm not sure what is right or wrong...

[safe]

Originally Posted by mikedsilva

It's interesting in the 911 world, we are trained to replace or recoat Alusil, with Nikasil... My friend who has a 944 says their entire blocks in 944 are Alusil so they don't have the luxury of swapping! He simply has the block honed and re-ringed and away he goes.. good leakdown numbers and lots of power.

I'm not sure what is right or wrong...

Alusil can be machined and honed, it just needs to be etched the right way after to expose the "sil", like it was done at the factory. They are Alusil straight thru. But there are few places that can perform this process.

Nicasil is a very thin super hard coating. Superior to Alusil if prepared the right way.

[Speedy Squirrel]

I think your history is spot on. I follow this guide:

Magnesium case: all dilavar studs for all bore sizes. The creep strength of the case is poor, so minimizing the tensile load in the stud is important.

Aluminum case and bore size < 95mm: can use steel head studs torqued no higher than 28ft-lbs.

Aluminum case and bore size 95mm and larger: all dilavar head studs.

The reason for use with Magnesium cases is obvious.

For aluminum cases, it is more about bore distortion and cylinder head cracking on the really large bore engines (>100mm). The cylinder walls are made thinner for the 95mm bore, and slightly thinner still for 97mm and larger bore. This makes them prone to distortion due to the clamp load getting too high when they are hot, especially if you are making some power.

Over 100mm bore the cylinder heads can crack from the excessive clamp load. It’s like breaking a stick over your knee.

I’m fairly happy with the status of dilavar and the 993TT studs.

The earliest gold colored studs were plated, but this did not help the corrosion issue. Intergranular corrosion starts at the microscopic level at an open grain boundary at the surface.

The painted studs were little better because the paint flaked off or was damaged on installation (see below).

Originally Posted by motogman

From my read on the history... when Porsche went to mag cases (in the late 60s I think) all was fine with steel studs. As engine displacement grew (2.2, 2.4, 2.7, +) and more specifically when emissions control systems went on the cars (74 ish), the emissions stuff made them run hotter and the mag cases would get soft and head studs would pull out.

Dilivar, and the equal expansion theory, were put in place to reduce the increased force in the case when things got hot (the aluminum expanding more at temperature than the steel studs) to reduce the stud pull out of the mag cases.

When aluminum cases came back in (I think 78 with the 3.0 SC) the al did not suffer so much from the heat and the head stud pull out issue became a non-issue. But Porsche stuck with Dilivar for a very longtime. Dilivar as we now know, is sensitive to the corrosion pitting from moisture and the stress concentration of the corrosion pits leads to fracture of the studs.

There are many posts where people have put steel studs in the post 78 al case engines with no issues.

I don't know if this is a correct interpretation and I welcome illumination on this history. It is an interesting segment of Porsche engineering history that many of us still live with. Other topics include chain tensioners, IMS bearings and several others I am not remembering right now.

[safe]

3.6 litre NA engines had all steel studs.

[Speedy Squirrel]

The above applies to the original case. The 3.6L uses the 964 case. The head bolt spacing was increased. I have never actually worked on a 964 case.

Originally Posted by safe

3.6 litre NA engines had all steel studs.

[ahh911]

"Aluminum case and bore size 95mm and larger: all dilavar head studs."
oops, just put in 24 steel on a 95mm 911 sc. Not going back in now. I'm considerably under the 38 ft/lbs, 24 ft/lbs as originally spec'd, so if it holds, that to me sounds ok

Phil

[motogman]

I am down to removing the dilivar studs and have some more questions...

It seems the Loctite on the studs needs heat to loosen them up. How much heat and what is the best way to do the heating. What Loctite product is used for the head studs?

My first try was an electric heat gun but this did not do much to warm the aluminum case. I think I need to use the propane torch but I am a bit concerned to over heat the case. What has worked for others - please share you experience.