Trying to install gear on camshaft
 

1991 300e, M103. Finally getting around to putting the head back on. First Benz i've owned, so still learning the tricks. Trying to get the cam gear back on the cam and I don't have enough chain slack. I removed the large allen cap that holds the chain tensioner, the spring came out, but the tensioner shaft only moved back approx 1/2 inch max, and will not move any further.

OK, got the tensioner out. After using the 12mm Hex Key to remove the tensioner cap nut, I had to use an M10 bolt (think it was a m10) and stick the bolt head in to thread out a adjustment ring, then the tensioner shaft came out and I was able to slip the cam gear onto the end of the camshaft. Now another question. It appears that this "adjustment ring" is used to set a certain amount of free play for the tensioner shaft, and the spring takes up the difference. How do I set this free play???

The tensioner shaft, as you call it, is a ratcheting device. You push it all the way through one way, it will not go out backwards.
Without the inner piece, you install the piece in the block all the way tight. Then install the inner piece as far as you can, it will again go in one way, but it will not back out. Then install the spring and the inner cap. It is hard to get the inner cap back on as you can imagine, because of the spring pressure. Don't forget any o-ring (aluminum) seals that were there originally.
Gilly