Adjusting the valves on a OM617 engine involves some tight angles to get your wrenches in. Mercedes recommends a special set of bent-handled wrenches to accomplish the task. You can purchase a pair of Hazlet wrenches... But they are pretty pricey.
Alternatively, you can bend your own wrenches to do the job. If you do a search (valve-wrench) in Peachparts Mercedes Diesel Discussion forum, you will find some pretty well done examples. Some guys buy a separate pair of 14mm wrenches, heat them with a torch, and bend them. Another alternative is to do what I did: go to your local bike shop, and buy a pair of their thin handled 14mm bike wrenches. I then put the wrench in a sturdy bench vise, put a piece of pipe over the handle for some leverage, and then bent it to the angle I wanted. They are not elegant, but they are simple to make. They work great, and they won't cost you a small fortune.
I had made a pair of these bike wrenches 10 years ago when I first got Marlene on the road, but, over the years, I seem to have misplaced one somehow. So I went out to the bike shop this week, and bought another 14 mm thin wrench for about 10 bucks.
Today I put it in the bench vise, slipped piece of pipe over it, and bent it. I have added some pics of the process. The first pic shows the bent and the unbent wrenches together.
The next pic shows the wrench in my bench vise.
I now have placed a piece of pipe over the handle of the wrench to get some leverage to bend it to the angle I need.
This pic shows the two wrenches again ..... Both bent now.
I have added a link to the Diesel Giant Valve Adjustment Procedure in the sidebar. Please check it out if you are thinking that it is time to adjust your valves.