The chain tensioner looks like a single piece when in fact it isn't. Pull the pointed end with some pliers and the inner ratcheter will come out with an o-ring (detent spring) near the end. Now put it back in the front until you feel a couple of clicks and then install it... then the big ring nut that holds tensioner in place, then cap with spring behind it.

Good luck!

~Paul

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'91 300E, 212K miles