Here it is in the section on Valve adjustment.

This has never been about the engine possibly running in reverse. I am sure you can never crank a 616 or 617 in either direction fast enough with a ratchet handle to start it.

This is about the timing chain and engine timing. The first issue is the accurate setup of the timing - you can only accomplish that in the forward direction. If you reverse, there is a certain amount of chain slack that will need to be taken up (not much) and the timing will not be the same as in the normal CW direction.

The second issue is potential chain slip on a sprocket. Especially on a 616 engine, because the 616 does not use a ratcheting tensioner. The 616 tensioner only applies significant tension to the timing chain when oil pressure comes up. That's why you can get the chain "rattle of death" on a 240D when it starts up. You hear that and its time for a new chain to go in. There is some tension available from the 616 tensioner due to the spring, but over time that spring weakens somewhat.

The 617 has a ratcheting tensioner, and is less likely to slip a sprocket tooth if turned in reverse. But if you have reached the end of the tensioner ratchet due to an elongated chain, then you are probably more likely to slip a tooth on reverse.

So why take such a risk? Only turn the engine clockwise.

Ken300D

You could pop the clutch on a manual, into reverse gear with the car going forward. I thought I might have heard something about someone doing that in a 240?

I'd like a bit of data.

Has anyone slipped a tooth on the camshaft while rotating a 616 or a 617 in reverse???

Due to the tooth engagement, the chain would need a lot of slack for this to occur......but, maybe it's possible on a 616. I don't see any possibility on a 617.

Let's see what we get.............

I think the issue is where the slack "piles up" on the chain path when you turn the engine over at the crankshaft with the wrench in each direction. If the chain is pulling through the injection pump drive sprocket, and up over the cams, the slack if the chain tensioner isn't really tensioning a lot, is on the chain tensioning rail side of the crank sprocket. The relative positions of the sprockets is always "locked" by the chain because you do not introduce the opportunity for the slack to accumulate between the sprockets. Turn it the other way and the opposite happens, and it is possible the chain and sprockets won't all line up the same when you change directions again. If the chain is pretty used up, the slack can affect timing of the pump. Check how far you can turn the crank in either direction before the camshaft sprocket turns and you will get an idea of what I am talking about. You will get more than a single link of rotation, meaning slack, in the backwards direction on our older engines with worn tensioning rails and stretched chains. Which means you can loose your timing and that is never good. Jim

I would have hoped that a professional engineer could have done better than that.

Answer
 

Yes, I have experienced tooth skip on the 616/617, generally due to a bad/broken chain tensioner and/or guide rail.
The warning is there for good reason.

'1/4 rotation is a small increment. No harm will be done to the engine. '

What freaks me out is no unit designation. 1/4 rotation could be interpreted as '1/4 of a degree' or '90 degree rotation of the crank'. Big difference there. I did rotate mine back maybe a degree. I had a new chain and new guides and tensioners. I was setting it up to put the injection pump back on. The compression kept bumping the crank forward a bit from 24 degree setting.
Tom

Engines turn backwards a small amount quite frequently. An aborted start attempt, a piston is most of the way up the compression stroke when the starter disengages... what do you think happens? The compression pushes the engine backwards more than a few degrees. Older carburetted gasoline engines FREQUENTLY would run on after they were shut off due to the deteriorating fuel quality at the time. Frequently this "run on" would be with the engine going backwards, resulting in smoke and foul smells coming out of the air cleaner.

Park a manual transmission car on a hill with the tranny in 2nd gear. Steep enough hill... backwards rotation. I bet it has happened to 90% of the manual transmission cars ever made.

I would say that any MB engine without worn out timing chain, tensioner, and rails, can be safely rotated backwards by hand about as much as you'd ever want. Yes, setting timing, etc should only be done with rotation in the correct direction. But if you miss and want to go back 1/4 turn before going forward again, no problem. I always like to rotate an engine several revolutions in the right direction when checking/replacing a timing chain or belt to check my work before calling it "good."

Kent Bergsma of ********** has warned a bunched timing chain as a consequence of CCW rotation. Got it in one or more of my recipe books from him.

I would not do it but as JonL says, it happens in manual transmission cars all the time. I try to always put my trans in the proper gear for the slope its on.

We did have a 240D almost running backwards. It was trying.

We were towing the car forward, not knowing that the car was in reverse.

Detroit Diesel 8V71 two-stroke bus engines which are all gear driven, can and did run backwards and blow up.

Of the MB diesels and gassers, the only ones that can be turned slightly CCW are the W 136 and OM 636, because they have a gear arrangement only and no timing chain.

I have been told the OM 352 can also, but I have no personal knowledge.

I have an OM 621 190D of 1959 vintage that is so loose that the timing can be slightly turned ccw, but that's due to wear on a engine with 520,000 km or so that is on its last legs.

For the most part, don't do it.

Always check the manual to be sure.

Thats what parking brakes are for . . . us it, or fix it if it does not work, then use it!:D

Of course, but in the "real world," this happens ALL THE TIME to standard transmission cars. Unless it's about ready to break anyway there will be no harm. Also, you know that those ATE brake systems with the little vestigal parking brake shoes inside the rear disc hub aren't worth ****. They barely work when they're perfect, which they seldom are.