I'm trying to finish the head gasket job on my '91 300E 2.6 today and one of the parts I bought was a "timing cover O-ring". It's small, about a 1/4" in diameter and thin. I came across no such o-ring when disassembling. I only took the top cover off. I replaced the fan bearing bracket and while I was at the MB dealer the parts guy said I would need an o-ring when I installed the bracket. I assumed this was the o-ring I had since the bracket fits to the timing cover. There was none when I took it out, unless it fell to the ground un-noticed. However, the bottom right bolt hole for the bracket is larger than the others with a short step and a chamfer. The o-ring fits it nicely, but it's too thin to be proud of the timing cover surface and contact the mating surface of the bracket and too large of an I.D. to snug against the bolt. In other words, I don't see that it can seal anything.

I'm guessing the o-ring I have is for somewhere else and the bracket gets a thicker o-ring? Anybody know anything about this?

Thanks for any help!

One of the upper timing cover bolts does see oil,that O-ring is to seal the bolt to prevent oil from seeping past the head of the bolt, the o ring fits into a groove machined into one of the bolt holes on the upper timing cover,and that bolt has a different shank on it compared to the other upper timing cover bolts, It is one of the bolts on the right side[intake side] of the timing cover as you are standing in front of the vehicle, looking at the fire wall. [left side proper]

Thanks for the info workerunit.

I've already put the cover on. I examined it pretty well and I didn't see any indication of any machined difference between the bolt holes on the mating surfaces of the head or cover, except the very much enlarged bottom left hole on the cover which corresponded to a steel plug in the head which was tapped for the cover bolt, much too large though to have anything to do with this small o-ring.

Is the machined groove you refer to on the outside of the cover, where the bolt head snugs?

Also, and hopefully not to confuse things, I came across a tech article regarding serpentine belt tensioner replacement and it mentions that proper procedure calls for sealer to be applied to both sides of the tensioner bracket at the hole that gets bolted to the fan bearing bracket (which in turn gets bolted to the lower timing cover). The same bolt hole I mentioned in my opening post that I thought the o-ring might go to.

It's surprising to me that in all the many M103 head gasket threads and timing cover leak threads I've read here and on other boards, that this o-ring has never been mentioned!

Does anyone have an exploded upper-lower timing cover view they can post?

On my 1990 190e 2.6 the upper timing cover bolt hole for [I Believe] the lower right side [intake side] was machined with a groove on the inside diameter of the bolt hole in the cover, maybe a 1/4 inch inside the hole, I used a small hook tool to replace the o-ring. The o-ring does not go between the bolt head and the cover or the cover and the block, rather it seals the shank of the bolt to prevent oil from migrating past the o-ring and leaking past the bolt head. There was a difference in the bolt shank for that particular bolt as compared to the other three bolts. The o-ring is not obvious unless you are looking for it. I looked for it because I noticed that three of the timing cover bolts came out dry and one bolt had engine oil on it when I removed it. I looked at my M/B service manuals and they do not mention the o-ring at all. If you start to develope a oil leak from your timing cover bolt head, then likely the o-ring is missing or damaged.