Is Crankshaft play ok? - PeachParts Mercedes-Benz Forum

cousindave76 · #1 09-19-2011, 06:37 PM

I have a friend who's 81'(aprox) 300cd with 141K started making loud metal clanking noises. He shut down and had a shop look at it to which they said "the crank shaft in crumbling". An odd assessment beings all they did was remove the crank pulley and balancer(didn't drop the oil pan). I've not been able to hear it run but I took a look at it and found that I could move the crank in and out(not up and down) about 1/8". The seal is not leaking a drop of oil. I've never inspected a 617 with no belts on before. Is this amount of in-out movement normal or should it be rock solid? And if this is not normal, does that mean the crank is shot, or is there a spacer or something may be replaced?

Thank you for you time!

layback40 · #2 09-19-2011, 07:43 PM

Unusual description.
There are thrust washers on the crank. They must be bad.
Unlikely that it can be fixed without the motor coming out.

1960mog · #3 09-19-2011, 08:05 PM

Sounds like your crank is broken into two parts.

cousindave76 · #4 09-19-2011, 08:30 PM

Quote:

Originally Posted by layback40

Unusual description.
There are thrust washers on the crank. They must be bad.
Unlikely that it can be fixed without the motor coming out.

Thanks for the lead! I'll look into the thrust washers and see if that could be it.

cousindave76 · #5 09-19-2011, 08:34 PM

Quote:

Originally Posted by 1960mog

Sounds like your crank is broken into two parts.

I don't think that is the case as it will start and run, just sounds horrible while running. I'll have a chance to hear it in person tomorrow. I suppose there's not much more I can say till I hear it.

1960mog · #6 09-19-2011, 10:12 PM

This engines will run with a broken crank.

Was this engine ever rebuild?

cousindave76 · #7 09-20-2011, 12:23 AM

Quote:

Originally Posted by 1960mog

This engines will run with a broken crank.

Was this engine ever rebuild?

Can't say that it was. I'll have to ask that detail tomorrow. 141k original miles on a car that has near perfect exterior paint and beautiful interior, I can't imagine that it had been ...but stranger things have happened.

The engine does start, and I will be seeing it run tomorrow. Still can't wrap my brain around the idea of an engine running with a snapped crank. I've replaced the head gasket on my own 617 and do understand the crazy robustness of this engine. Perhaps the owner will allow me to drop his oil pan and take a peak of his bottom end, hehe.

Stretch · #8 09-20-2011, 02:56 AM

If you can take some pictures of what you find - I for one would like to know the outcome.

barry123400 · #9 09-20-2011, 09:21 AM

I concur that likely the crankshaft is broken.

cousindave76 · #10 09-20-2011, 02:52 PM

So we started it up today. It started fairly easily. Once running I found that the sound wasn't quite as bad, or loud, as described to me. It also wasn't shaking too bad. With all accessories(belts) removed so as to hear only the engine sounds, I started poking around with my stethoscope(long broom stick) to zero in on the problem area.

The sound seems the loudest nearer the top side of the engine at the back cylinder. I would think right away it's a broken cam lob except for the fact that the sound is not like a ticking clock but a little more intermittent or uneven. Not ruling it out, just saying. Or maybe cam tower? I think a look under the valve cover is in order. You think a valve adjusting locking nut breaking off or getting loose could cause this type of symptom?

I'm starting to think the play I found in the crank while likely not good, is possibly not the source of the sound and problem. Why? Because one of the cylinders is not firing. If the top side is still doing what it's supposed to do, I can't imagine the crank being 1/16" one way or the other causing one cylinder to consistently misfire.

With all that being said, I want to know exactly what you guys mean by "broken crank". Do you mean that the crank is broken in half(two pieces) or do you mean there is a deteriorated crank bearing surface or something like that? Because, the car starts using the starter motor(back of the crank) and the front of the crank spins with it. This to me indicates a solid crank from the back to the front. The 1/8" of in-out movement of the crank concerns me but when I turn over the engine by hand at the crank, there is no crunchy sounds or metal grinding feeling. I have not drained the oil yet because I wanted to hear it run. I did however check the dipstick and did not see anything interestingly odd about it. Oil level is up and no metal grindings on it. hmm.

Another thought. Can a glow-plug break off? And if it does, is it inside the pre-chamber? Any idea what the engine would sound/run like if that were to happen?

Lot's of writing, must stop now before it's too crazy to even respond to.

Thanks folks!

jt20 · #11 09-20-2011, 03:21 PM

Sorry I am not directly answering your questions... they will probably not help.

I am going to address the broken crank idea though then give some hints as to where you should start in your search for noises.

The mention that the crank is broken is not far-fetched. The thrust bearing is more towards the front of the engine than the rear - any break behind the the thrust bearing carrier will not be fully detected by the observer when measuring axial play at the front of the engine.

They mean the crank is actually sheared into 2 pieces. If its really 1/8" than something terrible has happened. The spec is less than .3 mm.... yes, millimeters.

Some ideas assuming you actually measured 1/8" of axial crank play:

Remove the chain tensioner spring, Free the chain from the camshaft and suspend it tightly or have someone hold it under tension. Remove the vacuum pump. Advance the crank by hand then reverse slowly paying attention to changes in resistance. Repeat the advance and reversal at a faster speed... then move about 70 degrees on the crank and do it again.

Try running the engine with the vacuum pump and valve cover removed.

Drop the lower oil pan, wedge something between the crank and the block in different places and re-measure your crank end play again each time. Have someone do the wigglin' while you look at the crank.

cousindave76 · #12 09-20-2011, 03:30 PM

Quote:

Originally Posted by jt20

If its really 1/8" than something terrible has happened. The spec is less than .3 mm.... yes, millimeters.

Thanks for the spec and the ideas jt20! Is there a proper method of measuring it? I simply pushed the crank to the back of the engine as far as it would go, then pulled it forward toward the radiator and observed the distance. Yes, 1/8" is the amount I observed. Not cool.

jt20 · #13 09-20-2011, 03:32 PM

A glowplug (gp) breaking off will eventually destroy a prechamber (pc) and the damage the top of a piston and head surface. You will have metallic sounds and poor combustion. But I think its more common for a PC to fail, then cause that same damage. Removing the injector and visually inspecting will clear suspicion.

Johnt49 · #14 09-20-2011, 03:33 PM

You might try turning the crank a couple degrees in both directions from the front & see if the torque converter moves at the same rate as the front, or there's "play" between the front & back of the crank. Don't go too far counter clockwise or you'll do more damage. I've seen cranks break, but remain "keyed" due to the jagged nature of the break. You've got way too much axial play in there for whatever reason.

jt20 · #15 09-20-2011, 03:33 PM

Quote:

Originally Posted by cousindave76

Thanks for the spec and the ideas jt20! Is there a proper method of measuring it? I simply pushed the crank to the back of the engine as far as it would go, then pulled it forward toward the radiator and observed the distance. Yes, 1/8" is the amount I observed. Not cool.

thats all you can do.... 1/8" is not cool. I would not be surprised if barry and 1960mog are correct.