Woodruff key vs chain replacement
 

The engine in my dad's 300D has a good amount of stretch (can't remember how much ATM but I think it was 6 or more degrees). I know Mercedes recommends using a woodruff key to correct the stretch, but what exactly does that do? Would I still need to time the injection pump if I just put in an offset key?

The woodruff key offsets the cam gear to bring it back in time with the rest of the engine.

Shouldn't have to reset the pump timing unless it was adjusted to account for the stretch at some point...even then it would probably be close enough. My advice would be to replace the chain and time the pump if you know how. Otherwise just replace the chain and tensioner, I've seen a few 240D chains break and it is not good.

The chain is relatively easy to replace by (this is over simplifying but...) cutting the old chain, attaching the new chain to the old one, turning the engine by hand until the new chain is in place, separate the old chain and crimp the new one together.

Good luck!!!

Only consider the woodruff key if you used the 2mm method to check the stretch. That allows you to pick the proper sized Key.

+1, dial indicator method is best. Ive never really understood using the keys though... By the time you cut open the chain to insert, youre 50+% of the way there, and could have a known good and new chain... A critical part!

Chain doesn't need to be broken to insert the key.

On the 617.952 you wire or otherwise tie the timing chain to the camshaft gear so it does not jump teeth (look for more complete instructions on this job). Then you remove the end cap on the timing chain tensioner, remove the spring and check it for worn coils, remove the timing chain tensioner and housing with the piston/plunger still in it.

After you do that you have enough play in the chain to pull the crankshaft gear forward enough to get at the Key. You need to be carful.

When you go to put the timing chain tesioner back on you need to pull out the piston in the direction it would move when it applies tension. Install the timing chain tensioner housing. Then re-insert the piston/plunger (don't shove it in hard let the spring do the pushing) followed by the spring and the end cap with crush washer.