Well, I've been habitually plagued with head gasket oil leaks over the 26 years of fun with my '91 300SE. Got the dreaded rear passenger side leak again after only 5 years and 45K miles since a new Reinz gasket.

So......... I decided to go ahead and re-torque the head bolts. What the heck, what's the worst that could happen (I tried not to think about it).

Finished it up today, I'll report back in a few days or so to report if it actually stopped/slowed the oil leak.

But, I observed some things when I did the job that I thought might be of interest.This was only a minor oil leak, no coolant involved that I could tell, and good compression, so maybe it was a good candidate.

Procedure:
I marked the head of each bolt at 12:00 o'clock with typing white-out, and removed/replaced one bolt at a time, starting at the left center and working out in each direction.
I cleaned and oiled the head surface under the bolt, checked the depth of the hole, cleaned and oiled the bolt, and reinstalled, torqued to 40 ft lbs, then turned it 90 degrees ( after doing all 14, I then went back and turned them all an additional 90 degrees in sequence).

Observations:
These bolts have been re-used 3 times over the years. They measure exactly 102 mm; absolutely no measurable stretch, no indication of necking. Mercedes says to re-use them, i think it's pretty safe.

All the bolts behaved exactly alike. Each bolt was hard to break loose, making that lovely snap sound; obviously none had come unscrewed or loosened excessively.

After breaking loose, they quickly became finger tight after less than 30 degrees of rotation. Obviously not a lot of clamping force left here.

When I re-installed the bolt it returned to exactly 12:00 o'clock with only 40 ftlbs or torque. So, again, not much clamp left

When I turned the bolt the first 90 degrees, I used a torque wrench just to see the approximate force - about 75-80 ftlbs for this first turn.

When i made the last 90 degree turn I set the wrench on 90 and it didn't click. Tried the next one at 85, and it clicked just as I reached the 90-degree turn. So, I ended up with about 85 ft lbs on torque after the last turn. All bolts went down smooth and linear, could feel they were in the stretch zone.

Big interesting thing: I ended up turning all the bolts an additional 180 degrees from where they were when I started. That's .875 mm, or about 35 thousands! That's a lot of gasket compression or bolt stretch, or something.
Probably a combo of gasket, a little bolt stretch, and a little recession of the head under the bolt.

Anyway, I'll see if it works, hope to dispel a few myths about this all-to-common issue.

Oh, here's a little info on the source of these head gasket leaks.
The 103 head receives ALL it's pressurized oil from a passage right next to the RR head bolt. Thus the source of the worst of the leaks. There is no oil passage in the front of the head, but there is a blind one drilled in the block next to the RF head bolt. It's a blind hole covered by the head surface, but oil pressure is there and works on the gasket eventually, just like the rear passage. The front timing cover is, of course, another chapter in the saga.

More later, wish me luck,
DG

S-C G:

As a matter of interest, what is the head bolt thread? And, if you noted it while checking for necking, what is the bolt shank diameter?

Can you characterize/describe the head gasket construction?

Played around with two m103 engines in the JY the other day prior to doing a head gasket job on my own 300SE. With both engines, the head bolts were superficially loose, no resistance when losing them whatsoever with a 2ft breaker bar. Every single head bolt.

__________________
1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

Frank, a short flange portion under the head is 11.8 mm thick, then it necks down to 10.75 mm - this portion is 20 mm long. Then the threaded section is 11.8 MM, measured with a caliper - 41 threads at 1.75 mm pitch. The bolt bites about 12 threads into the block.
So, I can assume that the inner thickness of the threaded section is about as thick as the smooth necked-down section above, meaning that both the necked-down section and the first 30 threads above the block are involved in the stretching???, about 70 mm worth.

The entire bolt length under the head is 102 mm, with an allowed reusable length of 105.4!!!!!

Oh, 50 miles of highway today, not a hint of a leak!!!!!!!!

DG

To add my two cents to these details..I RR'd my head 3 times replacing the bolts at first, then not.

On the first head removal the M104 which was done by Mercedes Certified after a total wreck..(all new glass, headliners and tons of parts.) This bolt had no torque at all on the outside rear passenger side head bolt. Was hand tight..The others were all properly torqued. It had tiny leaks of both coolant and water but no mixing only to the outside of the block above the exhaust. Cause appeared to be very small pitting, possibly not corrected. It had valve work only to the intake valves, not the exhaust (appeared to be a cheap-o shortcut head job)

I replaced properly with full machine work and not a drop since, from the head..I suspect I do have some minuscule leaking from the valve cover gaskets, about one drop per week. Nothing from the timing cover! I feel so accomplished

Frank, forgot about the gasket. been 5 years since I put the current one in, but it's a typical Victor Reinz composite with metal fire rings and a little molded line around the water jackets and bolt holes. No idea on the new or compressed thickness. But a typical gasket of that type would maybe be around .050 -.060 new and maybe compress .010-.015------ just guessing from previous forgotten Chevy adventures.

If you come up with a ballpark number on the strength/stretch characteristics of the bolt, please let me know - of course, I'm all in now.

I found the same bolt looseness on a JY head I bought a few yeas back.
Does this mean we all should have been going against all known convention and retorquing our heads periodically for the last 30 years? Or will my bolts all snap next week? Stay tuned, I'll keep my towing insurance current!

Driving over to ft Worth tomorrow - wish me luck.

Cheers,
DG

S-C G:

Using the above information for the length of the deformable section of the bolt, the compression of the gasket, and the thread lead vs. degrees of rotation, a somewhat quick & greasy calculation suggests a stress level in the bolt of ~200,000 psi. 200K is safely within an ultimate strength of 285-300K, but about 65% above the yield strength. As a consequence the bolt has undergone a permanent deformation/stretch, which is what you felt as a plateau of the torque required to continue rotation of the bolt. Hence the term, "Torque-To-Yield".

Few people are familiar with, or understand stress-strain curves of Grade 8 material and how it relates to torque-to-yield bolts, especially the long bolts used in M103 and M104 engines. The design engineer who specifies a max stretch limit for these bolts understands that this limit is well below the danger point of necking for the bolt. The people who use these bolts find that the max stretch limit isn't reached until after multiple uses. One benefit of having a measurable limit is to weed out the bolts that may have been torqued more times than expected, due to the mechanic losing his place or distraction during the torquing procedure.

It is the people who don't understand how the stress-strain curve applies to head bolts that make blanket statements of single-use only.

Thanks guys. Drove it another 125 miles yesterday. Absolutely ZERO sign of any oil oozing; crawled around with a light and a mirror this morning.

Happy camper so far!!

DG

Any updates? Is it remaining dry? Thanks!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

To update; I have several hundred miles on the gasket re-torque. I did have some remaining ooze from the front timing cover, and re-did it last month also.
Currently we are having cold nights, down to 17 or so (cold for Texas) which definitely exacerbates any gasket issue when the cold engine is fired up.

And........yes I do have just a little mist of oil residue on the block under
cylinder 1. Maybe equivalent to 2-3 drops spreading across the block face. Nothing at all further back near cylinder 6, where it leaked before.
I'm hoping it's just leftovers I missed cleaning up from the previous leak that saturated the edge of the gasket along that area; but it's sure a lot less than I saw before.

I'll keep my fingers crossed - as I ponder my seeping tranny input seal in this cold weather - DARN; only 27 years old and she's determined to try to leave her first blood on the garage floor. Oh well, always enjoy some good winter projects.

DG

Thanks for the update...good news.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Just to update: been over 1k miles since the head re-torque and the timing cover seal. The small mist under Cyl #1 was indeed just left-over residue. I wiped it off, and ABSOLUTELY NO SIGN OF A LEAK HAS RE-APPEARED. Engine is absolutely 100% dry and clean.

Of course, I didn't have much time to bask in the glory, as the frigid weather brought out a seeping tranny on the front seal and B-2 piston.
But, a quick R&R and new tranny seals all around put a stop to that also.

And of course, again I swore that at my age this is the very last time I'll do a big repair like that - sure!

Old Pearl is tight and dry all over again - time to cruise the Bingo Parlor parking lot and count the thumbs-up!

Cheers,
Old Wrench

Nice...thank you for the update!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

One more update on the re-torque.
6 months and 3500 miles: totally dry.
No oil leaks, no coolant loss.
Ran a compression check today and got 170-190 on all cylinders; all plugs clean and normal.

Guess this project was a total success.

DG