Found the knock in the engine today.Pulled the oil pan down and found #2 rod bearing gone.Here is the question-I have another car with a good motor in it. Should I swap the motors or pull the head and replace #2 rod and piston.I did not pull the cap off yet so I do not know if the crank is hurt.If the crank is not hurt can I get away with just changing the piston and bearing?

If the bearing is gone there is a better than good chance that the crank is damaged as well. With proper care with disassembly you can pull the crank without removing the pistons or the heads. You simply remove all the rod caps and push the pistons back up into the cylinders. then you remove the crank bearing caps and drop the crank. You can then have the crank turned and radiused for undersize bearings and replace all bearing, rod and mains as you replace the newly turned crank. Care is needed to insure that you dont nick the crank with the connecting rods that are hanging down in the block. Sometimes this takes another pair of hands. You can cover the rod bolts with rubber gas line to prevent accidental nicks. After the crank is replaced you simply pull the pistions down out of their bores and slip in the new bearings and replace the rod caps and torque to proper spec, replace oil pan and add oil and drive off. Now the question is, if you have to replace the bearings, what else needs replacing as well. Does the engine have good compression, doesnt smoke or burn oil. Are timing gears in good shape, valves seating proper. In my experience it is often easier, and cheaper in the long run, to just pull the engine and do a complete overhual. Think about it for a minute, you are spending good money for new bearings and having the cranked turned and leaving equaly worn parts in the engine. Might work for a car you are considering getting rid of, or only has a few miles on it, but if you are planning on keeping it, do it right and just do it once.

Why not leave head on engine and just change the rod bearing if crank seems pretty good and rod not damaged. Get a new pair of bearing shells if you cannot find a major problem and make sure the oiling hole in crank is clear. Plastigauge should allow you to check things out. Usually is number one rod bearing gone bad for some reason but at least number two hopefully can be worked the same way if you are lucky.. Muddstopper, it seems the mercedes cranks do not recondition well as they are very hard compared to other cranks. Also his crank may not be damaged just bearing wear. Yes the out of round should be checked to see if it is still within spec. Hopefully he has a serious look before deciding on a course of action.

Major engine work with the engine in the car just sucks. Pull the engine and trans and seperate them. Then you have some room. If your bearing is gone, then your crank journal is gone too. A scratch deep enough to stop your fingernail means regrinding. We send our German and Italian cranks to a specialty shop that welds the bad journal, grinds it to factory specs, nitrides, and balances. That way it uses standard bearings.

Think about replacing your engine with your spare and taking the time to rebuild your sick engine.

You better look hard for the cause of your rod bearing failure.

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You can get farther with a smile and a gun than you can with just a smile.

1984 300D
1985 300CD
1980 300SD
1993 SL500

that changing the one rod bearing will yield a good usable engine... slim and none.

the bottom ends on mb diesels are just so rugged that it is very unlikely that the other bearings are any good. also if the bearings are shot not much chance any thing else is good either.

i wouldnt fool around with anything less than pulling it all apart and doing it right. if your other engine is a known quantity just put it in and go. and you have a spare to rebuild at your leisure.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I'm just a young punk shooting my mouth off, but I've heard stories from old farts in the depression using a piece of belt leather as a main bearing on their model A ford when the babbit bearing took a dump...

Just drive the knocking engine nicely until it gets too loud!

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1974 240 D (4 spd!)
1980 Olsmobile Cutlass Supreme Brougham 350-DIESEL
1969 VW Bus
1964 Chevy Pickup, severly Kustom... SOLD!
'69 Yamaha AT-1, '71 CB350, '79 XS1100
'81 XS650, '82 XJ650, '83 GR650
'83 KZ1100, '88 GSX600, '03 RSX-S
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Thanks for the comments-I am thinking I will put my spare engine in and rebuild the other one this winter. I am thinking the guy I got it off of ,ran it a little to fast on the highway.He has a lead foot. What are some of the other reason you would loose a rod bearing?

thing i would think would be to have run it low on oil or low on oil pressure or water in the oil.

if the oil is full with good oil and the pump isnt broken the bearings just dont lay down. i have had several benzes apart with 250,000 miles on them and no measureabl wear on the bottom end.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

ABSOLUTELY, I would choose to swap motors over trying to fix it in place. Especially if you plan to rebuild the old one, degrease everything first - you'll be glad you did. There are a few things like motor mounts and oil cooler lines that are SO much easier to do during the swap compared to later that it's worth considering.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.