Author Topic: Thermostat housing problems - corrosion, pitting and shot threads  (Read 456 times)

Jan S

  • Classic
  • **
  • Posts: 132
  • W116 Enthusiast
  • Location: Norway
Engine is 1977 US M100.985 (6.9), from a 450 SEL

I've started the job replacing the thermostat and a few cooling system hoses.

Not surprisingly, I was met by a few challenges:

Problem #1: the threads for one of the four bolts connecting the thermostat housing cover is shot (upper left thread hole), and a previous owner has solved (?) the problem by filling the threads with silicon and using a screw instead of a bolt. My goodness!

My plan is to talk to a professional workshop and see if they can fix it by inserting new threads for a smaller bolt. I guess I have one try to fix this properly. Any other suggestions?


Problem #2: the lip on the cover is severely corroded/pitted. I'm not sure to which degree this will cause a leak.

My plan is to use the thicker of the two rubber seal rings (the thermostat came with two rubber seal rings, one slightly thicker than the other). The lip on the cover plate will push the seal down and out - i.e. seal against the thermostat and the outer wall of the housing. In addition I plan to use a thin layer of sealer/silicon on the outer surface between cover and housing. I will clean all the relevant surfaces and may try some JB Weld where the pitting is unacceptable. Any suggestions/ comments?


Problem #3: the water return pipe - connected to the water pump with a hose - is severely corroded (see pic).

My plan is to clean and sand as much as possible, before I put on the new hose. How about sealing the corrosion with some Miracle Paint? Will that stop the corrosion getting worse? Any other suggestions?

(https://www.ebay.co.uk/itm/Bill-Hirsch-Miracle-Chassis-Underside-Rust-Killer-Specialist-Coating-Paint-Black-/323364698182?var=0&mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338268676&toolid=10044&customid=Cj0KCQiAhMOMBhDhARIsAPVml-GfbhkR7Twxy9xQ3j-P-5ff95KZauP-opbVpSogiPnlQmREEcWw8isaAi3oEALw_wcB? 


In general - I've read some acids can remove corrosion. Any product tips and/or specs?
1975-mod W116 450 SE with 6.9 engine

ptashek

  • W116 Addict
  • ****
  • Posts: 2,209
  • W116 owner
  • Location: Ireland
    • History of ZV9485
Re: Thermostat housing problems - corrosion, pitting and shot threads
« Reply #1 on: 14 November 2021, 08:29 AM »
#1 you could drill out the hole, and have a thread repair coil inserted. That's a permanent fix if done right. One issue might be finding those in aluminium.

#2 maybe time to get a replacement? Or if you know someone who can weld aluminium that'd be another route - weld the pits, and file them down flush.

1993 "Pearl Blue" W124 280TE
1988 "Arctic White" W124 200T
1979 "Icon Gold" W116 450SE (history, resto)

Alec300SD

  • Classic
  • **
  • Posts: 206
  • W116 Enthusiast
  • Location: San Mateo, CA
Re: Thermostat housing problems - corrosion, pitting and shot threads
« Reply #2 on: 14 November 2021, 09:42 AM »
Regarding problem #3, see my attached post for my workaround  for the damaged iron heater pipes.

http://www.peachparts.com/shopforum/4037206-post1.html

79 W116 300SD 'Stormcloud' new as of 10/22/2017
78 W116 300SD 'Desert Rose' new as of 01/26/2014
83 W126 300SD 'Rena 2.0' with engine from 82 W126 300SD Rena' (SOLD and crushed 11-20-2017)

PosedgeClk

  • Enthusiast
  • *
  • Posts: 74
  • From Sea to Shining Sea
  • Location: Atlanta
Re: Thermostat housing problems - corrosion, pitting and shot threads
« Reply #3 on: 14 November 2021, 11:33 AM »
#1 you could drill out the hole, and have a thread repair coil inserted. That's a permanent fix if done right. One issue might be finding those in aluminium.
A helicoil is a good route, and there is not any good reason to need aluminum on aluminum. You aren't passing current through this junction. A steel helicoil will work beautifully in an aluminum body.

If you wanted to spend money and make a DIY job out of it, you could use McMaster's site for reference. I would call around though and just find someone with the tooling already.

https://www.mcmaster.com/helicoils/helical-inserts-with-installation-tools-7/
1979 450SEL 6.9

Jan S

  • Classic
  • **
  • Posts: 132
  • W116 Enthusiast
  • Location: Norway
Re: Thermostat housing problems - corrosion, pitting and shot threads
« Reply #4 on: 14 November 2021, 03:26 PM »
#1 you could drill out the hole, and have a thread repair coil inserted. That's a permanent fix if done right. One issue might be finding those in aluminium.

#2 maybe time to get a replacement? Or if you know someone who can weld aluminium that'd be another route - weld the pits, and file them down flush.

#1 Yes, thread repair coil might be the way to go. I need to remove the silicon to assess the thread damage.

#2 My first thought was JB Weld, but alu welding is a good idea! I will check with a local firm that welded my rims a few years back.

Thanks!
1975-mod W116 450 SE with 6.9 engine

Jan S

  • Classic
  • **
  • Posts: 132
  • W116 Enthusiast
  • Location: Norway
Re: Thermostat housing problems - corrosion, pitting and shot threads
« Reply #5 on: 14 November 2021, 03:40 PM »
Regarding problem #3, see my attached post for my workaround  for the damaged iron heater pipes.

http://www.peachparts.com/shopforum/4037206-post1.html

Nice job! Is this the pipe for return water from heater box (under the dash) to water pump? Is it possible to disassemble this pipe without removing too many parts on engine top?

Would be great to remove it for a thorough clean and paint job.

Thanks for the advice!
1975-mod W116 450 SE with 6.9 engine

Jan S

  • Classic
  • **
  • Posts: 132
  • W116 Enthusiast
  • Location: Norway
Re: Thermostat housing problems - corrosion, pitting and shot threads
« Reply #6 on: 14 November 2021, 03:47 PM »
#1 you could drill out the hole, and have a thread repair coil inserted. That's a permanent fix if done right. One issue might be finding those in aluminium.
A helicoil is a good route, and there is not any good reason to need aluminum on aluminum. You aren't passing current through this junction. A steel helicoil will work beautifully in an aluminum body.

If you wanted to spend money and make a DIY job out of it, you could use McMaster's site for reference. I would call around though and just find someone with the tooling already.

https://www.mcmaster.com/helicoils/helical-inserts-with-installation-tools-7/

Thanks, yes I need to find someone for the repair coil job - I guess I have one try and I haven't worked with this technique before.
1975-mod W116 450 SE with 6.9 engine

Alec300SD

  • Classic
  • **
  • Posts: 206
  • W116 Enthusiast
  • Location: San Mateo, CA
Re: Thermostat housing problems - corrosion, pitting and shot threads
« Reply #7 on: 14 November 2021, 06:37 PM »
Yes, that is the heater return pipe.
It was easy to remove on my 300SD with a OM617A engine
Just a couple of  8mm sheet metal screws attach the brackets on the pipe to the chassis.

Access to the mounting screws might be a little tighter in your case due to the bigger M100 engine in the engine bay.
« Last Edit: 14 November 2021, 08:15 PM by Alec300SD »
79 W116 300SD 'Stormcloud' new as of 10/22/2017
78 W116 300SD 'Desert Rose' new as of 01/26/2014
83 W126 300SD 'Rena 2.0' with engine from 82 W126 300SD Rena' (SOLD and crushed 11-20-2017)

TNNBENZ

  • Classic
  • **
  • Posts: 172
  • Vintage Enthusiast
  • Location: America
Re: Thermostat housing problems - corrosion, pitting and shot threads
« Reply #8 on: 14 November 2021, 07:52 PM »
#1 you could drill out the hole, and have a thread repair coil inserted. That's a permanent fix if done right. One issue might be finding those in aluminium.
A helicoil is a good route, and there is not any good reason to need aluminum on aluminum. You aren't passing current through this junction. A steel helicoil will work beautifully in an aluminum body.

If you wanted to spend money and make a DIY job out of it, you could use McMaster's site for reference. I would call around though and just find someone with the tooling already.

https://www.mcmaster.com/helicoils/helical-inserts-with-installation-tools-7/
     You must be very, very careful when joining 2 different metals !!! Galvanic corrosion, or something to that effect.  Has nothing to do with electricity.  I do believe I read many times installing the wrong bolts in aluminum and it corroding the bolt & almost impossible to remove.........
Neglected ~ Rescued ~ 1977 450SEL 4.5L  U.S.A. ver.
Project of Compassion ~ Respect
Silver Green Metallic / Toast
Left Hand Drive
Sliding Roof

PosedgeClk

  • Enthusiast
  • *
  • Posts: 74
  • From Sea to Shining Sea
  • Location: Atlanta
Re: Thermostat housing problems - corrosion, pitting and shot threads
« Reply #9 on: 14 November 2021, 08:57 PM »
Has nothing to do with electricity.
It has EVERYTHING to do with electricity. The steel that they use isn't going to react with aluminum enough to matter. Find a standard potential chart, and try to make the potentials as close as possible.

Why would a steel helicoil cause corrosion when a steel screw would not?

If you pass current through this junction, more variables arise, and it is still usually not enough to matter with the currents and time scales involved in a car.
1979 450SEL 6.9

TNNBENZ

  • Classic
  • **
  • Posts: 172
  • Vintage Enthusiast
  • Location: America
Re: Thermostat housing problems - corrosion, pitting and shot threads
« Reply #10 on: 15 November 2021, 09:21 AM »
     All I was saying PosedgeClk is you can have corrosion without electricity.  You mentioned electricity. 
  Last night I saw another post on W116 forum that stated the same thing as I about 2 dissimilar metals.
     I agree with ptashek. I would contact a professional and at least ask if there is a corrosion possibility with a helicoil or anything with aluminum !!!
     Maybe they make aluminum helicoils.  This is why I join forums , or call professional I do not know everything...  Ha  ... lol......." I know enough 2 get me started or in trouble " ha,  lol
« Last Edit: 15 November 2021, 09:41 AM by TNNBENZ »
Neglected ~ Rescued ~ 1977 450SEL 4.5L  U.S.A. ver.
Project of Compassion ~ Respect
Silver Green Metallic / Toast
Left Hand Drive
Sliding Roof

TNNBENZ

  • Classic
  • **
  • Posts: 172
  • Vintage Enthusiast
  • Location: America
Re: Thermostat housing problems - corrosion, pitting and shot threads
« Reply #11 on: 15 November 2021, 11:05 AM »
     There are many thread repair kits @ Autohauaz
Neglected ~ Rescued ~ 1977 450SEL 4.5L  U.S.A. ver.
Project of Compassion ~ Respect
Silver Green Metallic / Toast
Left Hand Drive
Sliding Roof

PosedgeClk

  • Enthusiast
  • *
  • Posts: 74
  • From Sea to Shining Sea
  • Location: Atlanta
Re: Thermostat housing problems - corrosion, pitting and shot threads
« Reply #12 on: 15 November 2021, 11:48 AM »
     All I was saying PosedgeClk is you can have corrosion without electricity.  You mentioned electricity. 
  Last night I saw another post on W116 forum that stated the same thing as I about 2 dissimilar metals.
     I agree with ptashek. I would contact a professional and at least ask if there is a corrosion possibility with a helicoil or anything with aluminum !!!
     Maybe they make aluminum helicoils.  This is why I join forums , or call professional I do not know everything...  Ha  ... lol......." I know enough 2 get me started or in trouble " ha,  lol
It's a redox reaction, so it necessarily involves electricity. The reaction itself is like the cell in a battery. The more dissimilar the metals, the higher the potential (also known as voltage) and the more rapidly that you have electrons being swapped, ions are being formed, and therefore, you have corrosion. Passing current through hastens the process. I am saying to not worry about it because there is no difference between a steel helicoil and a steel screw that was in the hole to begin with. If you are passing large amounts of current through the screw for years on end, you have to consider it. To take a ground strap as an example, the current is usually going to bypass the screw and go straight from the terminal and into the metal body because the steel of the screw has a higher resistance than the short path to the metal body. The corrosion would occur at the surface between the metal body and terminal. After many years, one would only have to sand down the terminal to shiny metal or replace the ground strap altogether.

Use a steel helicoil.
1979 450SEL 6.9

Jan S

  • Classic
  • **
  • Posts: 132
  • W116 Enthusiast
  • Location: Norway
Re: Thermostat housing problems - corrosion, pitting and shot threads
« Reply #13 on: 15 November 2021, 11:54 AM »
Thanks to all of you for insightful and valuable comments/advice!

Still searching for a local workshop that can do the job .....

1975-mod W116 450 SE with 6.9 engine

Jan S

  • Classic
  • **
  • Posts: 132
  • W116 Enthusiast
  • Location: Norway
Re: Thermostat housing problems - corrosion, pitting and shot threads
« Reply #14 on: 15 November 2021, 04:04 PM »
Alec300SD, too tight at the engine top - no chance in removing the return pipe (without disassembling a whole lot of stuff).

Had to sand and clean it where it sits. Put some Miracle Paint on to seal the remaining rust and stop further corrosion.

Before and after pics.
1975-mod W116 450 SE with 6.9 engine