motoman

HEAD BOLT RESEARCH ...

M 156 ENGINE COUNT: A blog stated that 100,000 M 156 engines have been manufactured. Historians should try to validate the number which is a base number in any customer action.

HEAD BOLT TECH If 100,000 is correct the likely process is "cold heading."
Wire of proper alloy is forced thru dies to form the head, shank and threads.
Critical steps are (1) head forming (2) underhead fillet (3) heat treat/quench, and (4) temper

SPEC CONTROL: ASTM, SAE and ISO impose strict strength, reliability and sampling standards on the head bolts. Batches of 100 and up are machine tested for tensile strength and ductility by threading into a fixture and stressing to failure. Two tests are done: (1) straight tensile pull test (2) wedge tensile pull test: a bevelled 10 degree washer is used under the head bolt in the ductility test.



ANY FAILURE MUST BE AT THE THREADED SHAFT AREA OF THE BOLT. NO FAILURES ALLOWED AT THE HEAD. Specified low failure quantities require rejection of the entire lot.

COMPONENT QUALITY SYSTEM Unknown what system is used at MB to control purchased parts, or if the bolt vendor complies with the euro version of the ASTM spec. It is hard to imagine German metallurgy ignoring the testing , but mistakes happen. Paper certs should accompany each batch received. The supplier should advise MB of bad batches shipped. A purge should be done to capture discrepant head bolts. A business decision is made for damage control. (do nothing, fix as found, secret warranty, no warranty,stone wall ??)

Edit:A company dealing with NASA, for instance, has QA systems imposed by contract which would result in immediate line stop, find and fix a bad bolt. Commercial companies like Mercedes Benz have no such constraints. Customers rely on Mercedes Benz reputation to avoid quality problems and fix them equitably if they occur.

CoolBDPhenom03

Very interesting bit about QA.

bhamg

Damn motoman you are relentless but this is good stuff, thanks!

motoman

As I read I get deeper and deeper. I will wrap with a few words about attempts to DIY replace head bolts. Ahead of that we really need a down load of the star procedure for head replacement.

Hopefully that will reveal an MB thread sealant for the block and any detail about lubricating under the head bolt prior to TTY, as well as "snuggle torque" and the TTY angle.

Regarding corrosion...after reading the subject ad nauseum it is surely like death and taxes . Or like that endearing cultural artifact "s**t happens," so does corrosion and no one can stop it , including ARP , only manage it , IMO.

motoman

The reading revealed a number of companies specializing in "torque audits"
and forensic type post mortems. This stuff is not rocket science and widely practiced in the industry.

A determined group could collect used and (hopefully) unused original oem head bolts and have them tested. My impression is that defects would be found in the original oem bolts, just as many suspect.

Have your indy bag 'em up, whether broken or not. Doubtful MB would show the removed bolts. Yes , it'll take money to initiate any class action . It remains to be seen if the evidence is compelling enough to attract those in the legal business on a contingency basis. I raise this topic as I sense there are affected owners who have the skill to at least consider such action.

Merc63

Its not hard to switch them out to studs, even easier to switch to the improved OEM design bolts.

I can write a detailed DIY if anyone wants to try it themselves.

I have the star info for torque sequence.. Its something like 50nm, then 3 stages of 90 degree turns. There is some sort of white sealant on the bolts that I removed.

motoman

Merc, Once again thanks... you are the man in touch. I have read some interesting stuff on sealing head bolts in water passages. One Jag site states that headbolt washers are the seals for the coolant. It seems that MB may have stumbled on sealing some of the engines well in the block. We do not know the circumstances around "tri star's" fully rusted head bolt, for instance.

Reading suggests that Permatex is best (3 kinds) along with Hylomar, Locktite, and ARP. These sealants are compatible with factory torque , TTY callouts. Lube under the bolt head also important. Traditional "universal" is 30 weight engine oil. It looks like the later oem has a ?phosphate/oil? finish which should be left alone. It seems that wrong sealant inblock and/or lube under bolt head could result in under/over clamping of head and perhaps bolt breakage (but should not break off heads). Please advise.

DimitriZ

Merc63

Could you please write a DIY... Im planing on changing mine with the new merc bolts.

Cheers

Merc63

The process that merc uses to torque the bolts is one that is much more accurerate than just simply measuring torque.

As we know, torque wrenches sense friction. Friction can increase from a ton of variables, threads, cleanliness, lube, etc.

Merc starts of with a small amount of torque where there wouldn't be much friction to disrupt the measurement. I believe its 50nm. After 50nm is reached, the bolts are turned in stages of 3 turns of 90 degrees. It doesn't matter if the bolts are lubed or not, 3 turns of 90 degrees will yield the same clamping force on a bolt with or without lube.

Another issue I see after removing these bolts, is they could leak and allow coolant into the oil if not sealed properly. You are right, the washer is the seal on the bolt. When we installed the ARP studs, we silicone the top threads on the stud, the washer and the bottom of the nut. Once torqued down, the silicone spreads out and seals against the head.

I think its the design of the bolt, the corrosion and a metallurgy issue.

motoman

Merc, thanks but a word on friction control under the bolt head during clamping...Subject sites repeatedly show the classic "elastic" and "plastic" portions of the tightening curve. The elastic part is reached with the torque wrench and "snugs" the head to the block. The plastic portion is reached with the TTY or "angle" tightening.

If the wrong (meaning too slippery ) lube is used under the head torque phase can put the fastener into the plastic part of the curve. Then , when the TTY (angle) tightening is done it could place the fastener out of the safe area and into the failure area (snap off). I guess too dry could promote looseness.

Is this voodoo BS? I don't think so. If the improved oem bolts have been oiled, for instance, they should be used as is ("dry"). Depending on who you read torque readings can reflect up to 40 % friction of the under-bolt-head interface. Cheers and keep up good work. (I expect an 8 second car when you're done), motoman

Merc63

8 seconds lol, good luck. Ill be lucky to hit really low 11s.

brano_co

I would like to know this also

but from what i read here is not so easy to do ...