Exhaust manifold gasket swap prep

[JHZR2]

I started smelling a bit of diesel exhaust in my 1991 MB 350SD (200k+ miles) when stopped and idling with hvac on. Sticking my head under with the engine running didn’t result in any signs of exhaust that I could notice. So I pressurized and found some bubbles.

These are steel studs in an aluminum head. I think I’m going to have to remove my whole turbocharger and manifold. Any hints on what else to do? I’ll put the manifold on a straight edge to verify flat. What about cleaning both surfaces?

This is a job I’ve never done before. Apparently the FSM doesn’t give a torque value for the manifold nuts. What’s the best bet for getting a consistent tightness?

Thanks!

 

[Kira]

Vehicle details, please?

 

[JTK]

Did the suspect area look sooty before you sprayed it for the bubble test? If not, I'd be inclined to leave it alone as it's likely not your source of exhaust smell.

 

[JHZR2]

Vehicle details, please?

Oops. 1991 MB 350SD OM603.97 turbo diesel. Edited OP.

Did the suspect area look sooty before you sprayed it for the bubble test? If not, I'd be inclined to leave it alone as it's likely not your source of exhaust smell.

Not really. But it is a turbodiesel, so the exhaust will be under some pressure I think. It’s the only spot I could see/hear.

 

[bdcardinal]

I would be dousing that with panther pee over a few days and then have new hardware on hand to put on.

 

[JHZR2]

I would be dousing that with panther pee over a few days and then have new hardware on hand to put on.

Which one? It willl get pretty hot under there on those bolts.

 

[bdcardinal]

Which one? It willl get pretty hot under there on those bolts.

All of them. PB Blaster is good, then there are the Kroilheads. I probably wouldn't drive the car after the liberal douching of panther pee.

 

[BrendanC]

soak with PB plaster for a few days then heat up the aluminum head with a torch being careful not to heat the stud. double nut the stud and remove by hand. slow and heat is your friend for steel studs in aluminum.

 

[JHZR2]

soak with PB plaster for a few days then heat up the aluminum head with a torch being careful not to heat the stud. double nut the stud and remove by hand. slow and heat is your friend for steel studs in aluminum.

There are maybe four threads in the aluminum as I understand it. There are maybe two exposed in the above pictures.

I’ll have to look into the torch setup. Propane or something hotter? Just around but not on the studs, right? You’re saying to remove the whole stud, not just the nut, right? Thanks!!

 

[BrendanC]

There are maybe four threads in the aluminum as I understand it. There are maybe two exposed in the above pictures.

I’ll have to look into the torch setup. Propane or something hotter? Just around but not on the studs, right? You’re saying to remove the whole stud, not just the nut, right? Thanks!!

propane should be hot enough just make sure you’re only heating the head. i would replace the entire stud while you’re in there. when you go back together use Nickel based anti seize.

 

[JHZR2]

propane should be hot enough just make sure you’re only heating the head. i would replace the entire stud while you’re in there. when you go back together use Nickel based anti seize.

I was thinking about this some more. What if I don’t want to remove the stud, just the nut? Shouldn’t I then heat the nut?

Also what should I use to clean the two surfaces?

 

[Trav]

MB used copper plated nuts on those so they may remove a lot easier than you are expecting. I would heat the nuts using a small tip with a small ball flame on the oxy acetylene torch then use a small impact on them. If the studs come out just replace them. use nickel anti seize on every nut and bolt. Clean the surfaces carefully with a razor blade scraper as not to scar up the aluminum head.
You can use a Mapp gas torch but it cannot concentrate heat quickly right where you need it, they work in a pinch. I cant work without the oxy acetylene.

 

[SC Maintenance]

I wouldn't remove the stud unless I had to. I presume that is why Benz set them up this way.

You might end up with them coming out anyway, or the thread under the nut is so rusty they have to be removed.

 

[Astro14]

When I replaced the turbo on my 1985 Volvo Wagon - aluminum head, iron manifold - half of the studs came out. Corrosion on the studs from the heat of the exhaust and turbo meant that those nuts were difficult to move on the threads. Despite the heat (no oxy-acetylene- just a propane torch) the studs themselves took less torque to move than the nuts.

Subsequent manifold work on a couple of similar engines had similar results. Despite heat, and Kroil. Like this 2001 Volvo manifold. Ten studs, most of them came out with the nuts.

I just keep new studs on hand. They’re usually corroded anyway. I start the job with a new set of studs and nuts. I’ll second the nickel antiseize. The regular stuff can’t handle the heat.

Chase the threads in the head so that they’re clean before installing new studs. Not with a tap, they’re aluminum, but with a chaser. Like @Trav - I make thread chasers from grade 8 (SAE) or 10.9 (metric) bolts by cutting slots with a dremel that run the length of bolt. They scrape the junk from the threads without cutting into the head itself.

 

[JHZR2]

MB used copper plated nuts on those so they may remove a lot easier than you are expecting. I would heat the nuts using a small tip with a small ball flame on the oxy acetylene torch then use a small impact on them. If the studs come out just replace them. use nickel anti seize on every nut and bolt. Clean the surfaces carefully with a razor blade scraper as not to scar up the aluminum head.
You can use a Mapp gas torch but it cannot concentrate heat quickly right where you need it, they work in a pinch. I cant work without the oxy acetylene.

I had a slight leak in my flex joint which is part of the downpipe after the turbo. I took it to a shop and they used a torch to get those bolts off. They also used some red compound at the joint when they remade it.

I don’t have oxy-acetylene, but I do have propane. I think maybe a more specific and directed torch, than what I have for diy pipe sweating may be in order.

I wouldn't remove the stud unless I had to. I presume that is why Benz set them up this way.

You might end up with them coming out anyway, or the thread under the nut is so rusty they have to be removed.

Agree. Apparently there are only four or so threads in the head, so it’s not considered a strong connection by folks who have done this job before…

However, I will be prepared to replace as needed.

When I replaced the turbo on my 1985 Volvo Wagon - aluminum head, iron manifold - half of the studs came out. Corrosion on the studs from the heat of the exhaust and turbo meant that those nuts were difficult to move on the threads. Despite the heat (no oxy-acetylene- just a propane torch) the studs themselves took less torque to move than the nuts.

Subsequent manifold work on a couple of similar engines had similar results. Despite heat, and Kroil. Like this 2001 Volvo manifold. Ten studs, most of them came out with the nuts.

View attachment 212642

I just keep new studs on hand. They’re usually corroded anyway. I start the job with a new set of studs and nuts. I’ll second the nickel antiseize. The regular stuff can’t handle the heat.

Chase the threads in the head so that they’re clean before installing new studs. Not with a tap, they’re aluminum, but with a chaser. Like @Trav - I make thread chasers from grade 8 (SAE) or 10.9 (metric) bolts by cutting slots with a dremel that run the length of bolt. They scrape the junk from the threads without cutting into the head itself.

Good call on the chasers. I have nickel antiseize. What you show in the photo I anticipate to be pretty close to what I’ll see on mine.

 

[BrendanC]

I was thinking about this some more. What if I don’t want to remove the stud, just the nut? Shouldn’t I then heat the nut?

Also what should I use to clean the two surfaces

i like to clean surfaces with a razor blade at a very sharp angle. that keeps it from gouging anything but will remove anything that shouldn’t be there.

 

[JHZR2]

MB used copper plated nuts on those so they may remove a lot easier than you are expecting. I would heat the nuts using a small tip with a small ball flame on the oxy acetylene torch then use a small impact on them. If the studs come out just replace them. use nickel anti seize on every nut and bolt. Clean the surfaces carefully with a razor blade scraper as not to scar up the aluminum head.
You can use a Mapp gas torch but it cannot concentrate heat quickly right where you need it, they work in a pinch. I cant work without the oxy acetylene.

Is there an Oxy-acetylene that is suitable for small jobs and a DIYer?

I’ll probably do this on my kwik-lift, so the car will be a few feet up, but not like a lift.