Size matters!!!
 

Can anyone tell me the mm size hex key that removes the check bolt for the rear end? I remember it's in the 30s, but couldn't remember if that's 33, 35, or 37.

Anything requiring a hex key that large rides on rails and blows a whistle when crossing roads.

I thought it was around 14mm.

I have used the below pictured Rod Couplings and a Socket with Braker Bar on similar situations.

If you read the DIY with Mercedes it is handy to have a Propane Torch to heat up the area around the Plug if you have never had it out before

You also might find the size below:
DIY Repair Links
DIY Links by Parts Category - PeachParts Mercedes ShopForum

PeachPartsWiki: Do It Yourself Articles - Mercedes Vehicles

It's 17mm.
I changed my 1987 300SDL transaxle fluid a couple of weeks ago and had to buy the hex wrench for it.

That would depend on the year/model.

For some reason, folks posting here find it incredibly difficult to start their post with "I am working on a 19XX YZZSD."

my SD was a 14mm

14mm, 1981 300TD, Fill and drain.

W123's have a 14mm allen bolt for the fill and drain plugs

Make sure to remove the fill plug first!!!!

14mm on all my w123s. Wasnt the easiest tool to find when I was shopping for it.

A dog-leg lever with two mounted 14mm hex studs came in the W-115 tool kit. Still have it and it fits the plugs on the 220D and the 83 300D. Great tool, too bad it is no longer in the tool kit !!!

Well if it is a train it could be from 18XX...

didn't get the original hex key or tool box with this car. And it's an '83 300D. Guess I'll be off to Harbor freight later today. Thanks. I do remember the fun I had removing it from the 240D, which hadn't been touched ever, I think. . . .. looks around for a cheater bar . . . .a long one!

If you are buying tools get a 1/2" socket with a 14mm hex in it - and put that on your grolly bar...

Grolley Bar???? Got a pic of that???