OM617 Flywheel ring gear teeth inspection, Video
 

My starter engages and starts to turn the motor, however it "disengages" before completing a revolution. The starter spins freely like it hit a flat spot on the flywheel ring gear or the starter gear is retracting but still spinning. Here is the dental work on my flywheel. Do you think these teeth are the culprit? Thanks in advance.

https://youtu.be/Wx70c_UgX70

they don't look good, but why are they messed up? is the starter hitting square? ...don't think so.

Loose starter bolts?

This depends on how deeply you want to dig into it. Those teeth do NOT look happy! However, from your description of the issue I'd guess a bad starter drive (what us old-timers call a "Bendix") is the immediate issue. Unless you want to dig in there I'd pull the starter and take it to your local rebuilder (most towns have someone who does this) for an inspection and repair as necessary. If it were mine I'd have them rebuild the whole deal - it just makes sense once it's apart and on the bench. So that'll be the drive, bearings, brushes, and whatever else it might need.

BTW - IIRC these are the only bolts on the engine that use 12mm heads. Not an issue for me 'cause I've worked on lots of Japanese cars and have a good stock of 12mm wrenches, universal sockets, etc. Been a while since I had my starter out but that's my recollection.

Dan

Could the teeth be in poor shape due to the starter drive plunger not staying plunged?

This appears to be the case of a long time defective starter drive gear chewing up the flex plate ring gear teeth for an extended period of time. Proper repair is replacement of the ring gear on the flex plate (or a new flex plate with a new ring gear) plus the replacement of the entire starter with new or rebuilt (including new starter drive).

Shouldn't the starter engage flush or nearly flush with back of the flywheel? If so, wouldn't (mightn't) the current starter work fine if it were rebuilt and the bendix were working correctly?

That's what I hope.
Any other opinions?

Yes, if the front of the ring gear is not rounded off like in the video. A new or rebuilt starter may cure the problem or may not, won't know till you try it, a lot less work than r&r the ring gear. A new ring gear teeth has sharp edges Mercedes W123 220D 240D 300TD Genuine Ring Gear New 180 032 06 05 | eBay

IIRC, you can heat the ring gear on these and reverse them. Guys - do I have that right? But the effort to pull the trans and flywheel is considerable, hence my suggestion to try the starter rebuild first and see if he gets lucky.

Dan