OM617 N/A main bearing problem- I'm baffled - PeachParts Mercedes-Benz Forum

ROLLGUY · #1 06-13-2015, 05:23 PM

A little background:
A friend brought me a low mile (less than 100,000) 617 that was going to go back into the car after being repaired. It was run low on oil and developed a rod knock. Both the customer and I thought (after inspection) that I would get away with just replacing the rod bearings. Since I have experience in changing the rear rope seal with the crank still in the engine, I was told to completely reseal the engine while it was apart. I replaced all the rod bearings, seals and gaskets, and put it back together. The crank journals looked acceptable, and no bearings spun. After setting it on the garage floor and hooking up all the necessary things to get it running (starter, battery, fuel, exhaust system), We were able to get it started. Almost right away the rod knock came back, although not near as bad as before (my friend drove the car before pulling the engine). We both decided that I should take it apart again, and inspect it. It appeared that #5 rod bearing had the slightest bit of scoring, but my engine guy (been in rebuilding business for 50 years) said that would not in any way cause a knock. He suggested I bring the crank in for polish, and then plastiguage each rod bearing afterwards. Fast forward to today: After picking up the crank, I lubed up all the main bearings and placed the crank in the engine. I was told to use the plastiguage dry, so i put no oil on the rod journals. I just hand tightened all the rod bolts so I could turn the crank slightly to get all the rods on. I lubed up all the main caps and tightened the bolts to spec (66 ft. lbs). I then went to turn the crank and it would not budge. I did notice that the 3rd main cap felt a little weird when I tightened it up, so I removed that cap. I noticed that the bearing was not seated correctly, and it messed up the locating tab a little. No need to worry, I had a parts engine just 3 feet away that I harvested a main bearing from. Just for grins, I left that cap off, and tried to turn the crank. It would not budge. I then loosened the other caps and with all the bolts loose, I was able to turn the crank. If I tightened only one of the other main caps, the crank was tight. I tried every combination, and the only main cap that is tightened to spec is the rear most one. If I tighten any other caps, the crank will not turn. I am baffled as to why this is happening. Anyone got any ideas?