motoman

This thread is not to grandstand, but the sticky sub threads are so active they seem to disperse readers (all good) , and the subject here is likely to be controversial. It needs all responders.

Fellow enthusiasts, we all may be blinded by common notions about rust and corrosion. I have found a mind bender variety which is complex and not well understood by the experts. Sifted here (I hope) into lay language and presented as the likely cause of the M 156 head bolt problem, "crevice corrosion."

The subject is presented in many online sites. This type of corrosion has felled bridges and planes and ruptured atomic cooler plumbing. I hope you
will convince me that this mechanism does not apply to our cars, because if you cannot it appears to me that the head design may always produce headbolt failure. Enough preamble.

Bolt heads make narrow gaps and can trap fluid (water). If that fluid is free to circulate a more neutral Ph level is maintained. However, if the fluid is trapped by "drop in" it can become stagnant , and can develop a different chemistry. In this case becoming more acidic. Electrolyte and electrical poles develop (+/-), and create a battery-like condition. This is similar to galvanic corrosion, BUT WITHIN A SINGLE BASE METAL (the bolt head). The result is a miniscule crack which grows (rapidly). This type of corrosion is related to stress.

By now some of your hackles are up. Again only from an amateur peprspective I offer (1)"rust" is found under the bolt heads. (2) the bolts are "sunk" in the head and can trap water. (3) water is present in oil circulating from the sump from blowby and condensation. (4) this mechanism does not involve aluminum and coolant (5) no one has shown that coolant past block threads is a problem. I wish this was all, but it is not...

The corrosion is more severe on harder bolts-those above Rockwell 35. Coatings of galvanize,black oxide, and alloys of chrome and moly are of no help and equally vulnerable. So how do harder, shinier and thicker bolts help?

The engineering guidelines for avoiding crevice corrosion suggest eliminating at least one precursor (1) no drop in , stagnant water-head design (2)softer bolts (3) reduced stress. To me none of these things can be done. I have also visited the ARP site. Corrosion is only mentioned in passing. No mechanical engineer can plead ignorance of this subject. Being a fly on the wall of this company would be interesting, but I doubt any seminars would be offered.

This brings me to ask you all for patterns which might shed light on this problem (1) long commuters-less water in oil=less/no failures? (2)frequent hard usage-more blowby=more moisture and failures? (3)short commutes-more moisture retained=more failures? (4) no failures at all with later oem bolts? Do we need to thread and plug the head above each headbolt?

TELL ME IT AIN'T SO, BUT THEN TELL ME WHY THE BOLTS BREAK!

splinter

Another episode of Engineering Disasters? :popcorn:

PaulWdz

Is it just me or are this guys posts extremely difficult to read ?

morganb

I assume you mean through the head bolt. If that is the case then wrong, my failure was apparently water leaking up through the head bolts and letting water in on both banks. To find it they had to reassemble the engine out of the car and pressurise the cooling system with the heads bolted on with a new set of gaskets reusing the same head bolts.

BerBer63

very difficult

motoman

Paul, What is difficult and I will try to clarify.

motoman

What is difficult?

motoman

So coolant was getting by the head bolt threads in the block and corroding under the top of the head bolts? Any "rust" anywhere?

Jasonoff

Just you...

morganb

Yes, but cant comment on rust as I did not see the bolts after removal from the car . The car started running like crap and was putting out so much smoke I had to stop driving. It was a failure that went from running normally to really bad over 1-2 minutes. One had bolt had broken on one side. Seems at the end a lot of water was being put into one bank. But as I said above there was water in both banks.

ZephyrAMG

THIS is all I thought of as I read that post:

Sincity

Motoman: Does that mean even with the revised bolts and ARP studs that it may still fail due to "Crevice Corrosion?"

motoman

Sincity,
I do not know , just as I don't know if this mechanism is actually at work. If it is, however, stronger and thicker headbolts may not cure. That is why I asked about any know failures with the later headbolts . motoman

KenH

After reading up on crevice corrosion, it does seem to be a plausible cause and effect for the head bolt failures with the bolts seeing failure possibly being those that suffered microcracks from the torque to yield process.

As Motoman stated, if it is the root cause, then there should be some correlation between how the car was typically operated and the tendancy to see this failure. There should also be failures of the new bolts within about the same timeframe as the failures showed up in the old bolts. Does anyone know what that timeframe was?

Cars in cold climates that get stored during the winter where the owners start them up on occasion but don't bring them up to full operating temperature would seem to be prime candidates for this problem. Any cars that get driven frequently with the oil getting to 100C+ regularly should be much less susceptible.

bhamg

Mine would be an ideal candidate for the long-haul, hot temps driven car. Rarely do I drive less than 40 miles at a time and almost always in warm to very hot ambient temps. If my head bolts show advanced corrosion it'll truly be one of those WTF moments.

Merc63

Arp 625 studs won't Corrode. You should see these suckers, they are beefie.


625's are the strongest studs arp makes.

Diesel guys torque these to 150ft lbs and run 40-50psi boost.

motoman

This subject is keeping awake nites. Some radom stuff...

A site mentioned shot peening of metal as beneficial but no detail on why in this case. (we all know the benefits otherwise)

What is the Rockwell hardness of the newer OEM bolts. Is it possible to drop to the (stated) "safe" 35 level for head bolts?

Not a lot I could find on water in oil. But is does have a saturation point where the moisture is no longer in ? emulsion? Is free state water necessary to induce crevice corrosion? I don't know.

In ethanol states does strenuous driving allow more blow- by moisture into sumps which later "drops in" the head bolt holes? Pattern?

MB seems to have carefully tuned the normal oil heat into high readings that make many nervous. Is this related, as in drive out moisture?

Merc we need more from you on the 500 degree silicon sealer prescribed by Weistech for their stud installation. And the cleaning (suction etc) you mentioned. Are they trying to keep moisture out?

Edit: Further reading shows shot peen cannot improve corrosion resistance.

Some one who has the old and new headbolt (preferable new ones) mike the dia's. CEK's pic looks like the new oem is slightly bigger OD. If the same OD do a tone test. Tie string same place on each thread, suspend and strike like tuning fork. Different?

Edit, OK arp seems to use a silicone sealer for the block threads, right?

motoman

Merc, thanks again for the pictures of the 9 headbolts removed. Owners following this thread may find them of interest. Look at the Weistec bolt replacement thread.

cyberorth

I agree with everything...

cyberorth

Motorman, are you the Batman?

motoman

cyberorth, it's motoman...and no, not batman. He would have solved the headbolt problem by now and banned other evil and unreliable things .

cyberorth

Huh, motoman has a sense of humor
I swear it was the effing spellcheck.
Don't lose too much sleep.

bud!

motoman

Who'da thunk...one stinking little drop of water in a crack. Theory could be all wet...

bhamg

My guess is that faulty metallurgy is in play, along with one or more factors that motoman has talked about. I was not totally convinced that the problem would get substantially worse over time but after seeing what Merc's head bolts look like there is little doubt in my mind that many more will be popping as time and mileage builds.

motoman

The evidence on the threads now seems to me that several things may be at
work, not just crevice corrosion (if at all)'

(1) Mercs bolt heads "rusty" at top. (crevice corrosion?)
(2) Tri star tech (Las Vegas) shows a bolt shaft "rusted" as though sitting in water. (bad block thread sealant?)
(3) Mrtazzy describe fissures under the bolt head (crevice corrosion?), and tooling stress riser under bolt head (vendor)
(4) Three of Mercs bolt heads look tilted off axis. (crevice cracks?)
(5) Mrtazzy ?? shows a broken bolt shaft that looks hollow, like a plucked
(yes plucked) grape stem. (Classic "lift off"-under spec , over TTY torque,
corrosion?)


OK re precursor crevice corrosion... the DIN hardness specs for 10.9 is RC 32-39, for 12.9 is RC 39-44. Are head bolts marked? Please report this thread