Spun rod bearing- What caused it? - PeachParts Mercedes-Benz Forum

ROLLGUY · #1 09-05-2012, 05:04 PM

I finally got the time to tear into the engine that was in my first 'Benz ('82 300CD). I replaced the engine with a good used one a couple thousand miles after I got the car. It developed a bad rod knock. #2 & 3 con rod bearings were spun down very thin. In the photo you can see the bad bearings:

The crank is bad too:

Notice the pitting in the main bearings:

The bores look real good, so I will save the block. Is the crank worth saving?

auspumpen · #2 09-05-2012, 05:39 PM

Oil starvation. See if you can find a blockage in one of the oil passages or the pump pickup. The crank needs to be mic'd in order to determine how much it needs to be ground down.

ROLLGUY · #3 09-05-2012, 05:44 PM

Quote:

Originally Posted by auspumpen

Oil starvation. See if you can find a blockage in one of the oil passages or the pump pickup. The crank needs to be mic'd in order to determine how much it needs to be ground down.

No blockage in the pickup, but maybe in the passages (have not checked yet). I think the crank is too far gone, but I have a good one. Is the crank worth any more than scrap value (as bad as it is)?

auspumpen · #4 09-05-2012, 05:52 PM

Yard art. Here's a VW crank I combined with Jag brakes and an old MB hubcap to serve as a bird bath

rscurtis · #5 09-05-2012, 06:01 PM

The second bearing from the left looks like it has a lower insert in the block instead of the cap. Was the engine assembled that way when you took it apart?

engatwork · #6 09-05-2012, 06:04 PM

You know I've got a couple old brake rotors and a 350SDL crank that I could use towards some more yard art. Thanks for the tip.

Looks to me like the brg is missing on the second slot from the left in the photo referenced.

97 SL320 · #7 09-05-2012, 10:24 PM

A few things.

The pictures are way too small to tell much more than , yep there is wear. A good automotive machine shop can give you a OK / NG on the crank. Also get it checked for straightness and cracks. When a bearing over heats, it can cause the crank to bend.

Cranks can be reground or even welded up. If the crank is at all common, welding isn't cost effective.

Oil starvation isn't always the cause of bearing failure. Sometimes the bearing clearance becomes large due to wear. Once the crank journal to bearing clearance gets large enough, a hydrodynamic wedge of oil can't establish it self. This allows the crank to contact and beat on the bearing inserts causing them to thin in the radial direction then grow laterally. ( in normal operation the crank should never contact the bearing. )

Once this occurs, radial tension is lost and the bearing insert starts to rotate. ( The tabs on bearings are more for locating the bearing during install than anything else. ) A combination of lack of oil between the bearing bore and the back of the insert coupled with the now sort of bent flat tabs dragging along the bore causes a downward spiral.

ROLLGUY · #8 09-06-2012, 01:23 AM

Thanks all for the great info. I think the crank is beyond saving, and I have another good one anyway. When I got the car the engine was in, it had been sitting for years. I did not bother to change the oil until I knew it would run. Is there is a possibility that the initial trying to start it after it sitting for years, caused a lack of oil in the system? I must have cranked it for a total of about 10 minutes (cranking, fixing something, cranking again etc) before it fired for the first time.

97 SL320 · #9 09-06-2012, 05:59 PM

Old oil not a problem, but sitting long time will drain off the oil filter. Cranking under compression didn't help matters, the crank would have been very dry and not get oil until the filter fills up.

If you use the other crank, send it to a automotive machine shop for a crack check , size check and polish.