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?