"Don't turn the engine backwards!" - PeachParts Mercedes-Benz Forum

cscmc1 · #1 02-02-2009, 09:45 AM

Just curious -- I see the warning "don't turn the engine backwards" in almost every "how to check your timing chain for stretch" thread. What happens if your Do turn your engine backwards? I saw the warning in time to NOT turn my engine backwards, but what is the harm that is done? Thanks!

babymog · #2 02-02-2009, 10:11 AM

The only thing I can think of is that the camshaft will be driven via the tensioner side, potentially creating lots of slack/retard or guide forces beyond the design limit.

Others might know better.

Stevo · #3 02-02-2009, 10:56 AM

I always though is would cause slack in the chain, which could then "jump" a tooth on the cam or crank sprocket.

jt20 · #4 02-02-2009, 12:04 PM

this is referencing 61x model engines.

the force from a tensioned chain will push the tensioner into its bore (only on non-ratcheting models) - as everyone said, this will cause slack and the IP side will get bungled up and skip or get caught.

There is nothing dangerous about doing this if your rocker arms are removed and you have a ratcheting tensioner. Never do this unless you are certain you have a ratcheting tensioner! When you spin backwards, you allow your chain stretch to accumulate on the opposite side - I would be concerned that valves might hit if it is excessive, but I have no proof.

Judging from diagrams of the IP, spinning backwards does not appear to be an issue.

mplafleur · #5 02-02-2009, 12:21 PM

We were moving the 240 we just rebuilt the engine on out of the hanger. I was towing it and it was very hard. I thought the brakes were frozen, but apparently the tranny was stuck in reverse. At one time when I got going the 240 started bucking and belching. It was trying to start in reverse, IN THE FORWARD DIRECTION.

So the engine was turning backwards trying to start.

We quickly realized what was going on and pushed in the clutch. Later the car started just fine. No damage.

jt20 · #6 02-02-2009, 12:34 PM

in that case Michael, you had oil pressure removing slack from the chain so it would not accumulate on the wrong side.

shocking nonetheless.

wgilmore · #7 02-02-2009, 01:29 PM

Quote:

Originally Posted by jt20

this is referencing 61x model engines.

the force from a tensioned chain will push the tensioner into its bore (only on non-ratheting models) - as everyone said, this will cause slack and the IP side will get bungled up and skip or get caught.

There is nothing dangerous about doing this if your rocker arms are removed and you have a ratcheting tensioner. Never do this unless you are certain you have a ratcheting tensioner! When you spin backwards, you allow your chain stretch to accumulate on the opposite side - I would be concerned that valves might hit if it is excessive, but I have no proof.

Judging from diagrams of the IP, spinning backwards does not appear to be an issue.

I have always wondered about this too.
My engine is a 603.961 and sometimes when trying to get TDC I go a degree or so past TDC and turn back 1 or 2 degrees. I have never had a problem, but then I think my tensioner is hydraulic non-ratcheting type - I think. I would not think that a couple of degrees would harm anyting - maybe I'm wrong.
Anyway , I have done this a number of times without any incident - Yet....

babymog · #8 02-02-2009, 01:41 PM

Quote:

Originally Posted by wgilmore

I have always wondered about this too.
My engine is a 603.961 and sometimes when trying to get TDC I go a degree or so past TDC and turn back 1 or 2 degrees. I have never had a problem, but then I think my tensioner is hydraulic non-ratcheting type - I think. I would not think that a couple of degrees would harm anyting - maybe I'm wrong.
Anyway , I have done this a number of times without any incident - Yet....

I have done this on other engines. My method in this case is to turn the engine backwards past the TDC point a few degrees using both the crank and the camshaft (to keep tension on the forward side of the chain), then come back forward to my setpoint.

I would be concerned about arriving at a setpoint going backward as you described because of possible errors due to slack in the chain or timing devices etc.

winmutt · #9 02-02-2009, 04:16 PM

Its not the end of the world, however when measuring lift or what not it will give an invalid result. Anyone with a stick knows the eng can roll back a bit without killing it. To be avoided if possible.