What makes this galvanic corrosion?


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1983 123.133 California
- GreaseCar Veg System

i have one --housing to head bolt that is going to break in half--hows the best way to prevent it from doing so?

You can try my technique above. After you get the bolt off ( hopefully without breaking it), liberally coat it with anti seize before reinstalling.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Broke one many years ago on my 240D. In fact that may have been the first broken bolt I ever had to remove. Now I try to remember every year or so to loosen, remove, clean up and retighten the thermostat bolts in my cars to hopefully avoid this in the future.

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Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles

I insert them with loads of grease or copper paste. The problem is corrosion due to disimilar metals (aluminium & steel).

OK ill try again--- if anyone has broken a housing bolt off in the head---
Were you then screwed?

I busted a water pump to housing bolt while fixing my car at Vstech's. Luckily he had a spare housing available. Needless to say I was much more careful removing bolts after that.

I had to change out my thermostat a few months ago, I feel really lucky nothing broke!

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

I have removed two thermostats in 61x's and never snapped bolts. But I did use new cad plated bolts and a small dab of anti seize

You can see in the pictures there was very little good metal left on the Bolts.

Either they Rusted trough, some one else attemped to remove them and cracked them and stopped or they were over tightend and cracked.
After that rust developed in the cracks.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel

I broke one or two of those decades ago and since have been most careful in removal of them. I have gotten them out without breaking them when nearly half the shaft of the bolt is eaten away.

Lots of penetrating oil and very very careful wiggling of the bolt back and forth to get it started is the key to success.

Of course any that are eaten away get replaced on reassembly.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I wonder if Diesel911 is onto something.

Over torqued bolts will micro crack in the location where they appear to be failing. I'm sure M-B used plated hardware originally due to the environment the bolt is in. Micro cracking would have compromised the plating giving a chance for corrosion to set in. Mix that with improper coolant and you have a plausible scenario.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

coolant shouldn't reach those bolts unless the gasket is leaking. those bolts are often overtightened. I think they should be around 5 foot lbs.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Depends if you can get the broken bits out.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

yes.

I have had three incidents like this happen so far in life. twice on customer vehicles while I was working in shops, and once on my own car.

Two of the incidents the risk of damaging the head was great enough in the car trying to extract the bolt, that the heads had to be fully removed, and in once circumstance taken to a machine shop and extracted via them.

On the other one I was able to somehow make it work, despite every single thing breaking.

On a 6.5 diesel school bus, I broke a corroded glow plug trying to remove it, then drilled out the center, and tried to back it out with an easyout, which ALSO broke off. then I had to grind out the easy out, then a hellish time later, I had managed to get all the bits out, only partially damaging the threads. I got away with a helicoil, red locktite, and a new glow plug. The whole think had to be done outside in about 15 degrees as well, probably helping along the failure of components.

The other two times, the risk was too great, so I completely removed the heads in order to get an appropriate angle on the broken off bolt.

On a VW wagon I had, no less than 6 of 12 exhaust studs were broken off, and on that I just said the hell with it and took the head to a machine shop. fortunately it was my own car

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Looking closer at this pic. The 2 broken bolts have threads with no rust or less rust than the unbroken bolt. On one of the broken bolts I see some whitish substance which is aluminum oxide. Was it packed solid with it? That would act like red loctite and the bolt head would be prone to snap in such circumstance.

Probably whoever worked on it last:

1. didn't use anti seize

2. over torqued and damaged the bolt (only needs to torque it enough to compress the gasket to seal and so the bolts don't back out from vibration- that what that captive lock washer is for)

3. didn't keep mating surfaces clean, which allowed moisture into the space of the shoulder of the bolt (note the captive lockwasher), promoting rust on the bolt and growth of aluminum oxide around the bolt.

Lot's of penetrating oil, before turning the bolt back and forth by small increments (to break the aluminum oxide bond) may have helped to avoid breaking it. Try that next time.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked