assemble it on the engine, like the pictures.

Thank you. What size torx socket fits into the threaded ring? It looks like a 70, but I'm not sure. My torx only go up to 60.

It's been 15 yrs or so since i've done this. I thought it's a 17mm allen bit.

My memory is also 17mm hex.

Like I said in my previous comment, it's important when putting 506 through 507 to hear or feel a couple of clicks from 508 before adding the spring and taking 512 all the way home.

Appreciate the update. I have to buy the correct torx wrench to tighten the threaded ring. It's not an allen, and the 17MM is for the screw plug. The threaded ring is massive. My estimation is a torx 80 or 90. Once I buy I will report back.

I could be wrong but I don't recall that being a torx, thought it used a large hex key. Understand about the cap being a 17 hex. Keep us posted and keep up the progress.

torx does not sound right. i never had a big torx socket until i bought a t60 a few years ago. maybe a 19mm hex. i know bloody well i didn't borrow tool ever to do this. and i've done this job about 10-11 times with head gaskets.

510 is a 17mm 12 point. You might think that it's a torx because it's a 12 point. There isn't anything that's a torx on the m103 or any other car of that era. you want to use a 17mm hex.

Of course the correct tool is the best, to confirm the size needed etc., see what size hex head bolt head fits in there, A 19mm or 3/4" = 19mm. An 11/16" or 17mm = 17mm hex bit. If the six sided bolt head hex fits well, so will a hex bit.

Just came back from the tool store. It was a 17mm Allen. I thought it was torx due to the 12 points, but neither the 90 nor 100 fit. The 17mm was correct. I was incorrect in my assumption and you guys were right. Again, many thanks. This is still my first rodeo with a Mercedes. VERY MUCH APPRECIATE ALL OF THE INPUT!! :)

Been fooled by the same fasteners...been there, now your a club member!
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I re-installed the tensioner. All went very smoothly, almost too smoothly for my taste knowing the stumbling blocks I've been hitting. While dealing with the tension of the spring was something I was wondering if that "detent spring" required any special positioning? It did not appear that there was any particular spot other than where it was. Looking for confirmation if possible.

It will find home once the engine is started and oil pressure rises.

Great! Thank you so much for all of your help. I am getting closer. Waiting for a few more parts from Pelican and then finished. I'd say about a week.