Crankshaft main bearings/ big end bearings

[Eminent]

Is the only way to know if the shells from the main bearings and big end bearings are original or undersized by measuring the crankshaft?
There is a number on the back of the shells, 130 0002 for the mains and 114 0010 for the big ends, but the Mercedes dealer don't recognise these numbers.
So far i only measured #6 and it has a clearance of 0.051mm. One of the main bearings has 0.076mm clearance.
I hope to eliminate the knocking sound by replacing the shells.

[ja17]

Hellmo Eminent,

To identify bearings you can measure the thickness of the bearing to compare with the thickness of others. The oversize bearings will be around .005" thicker than the standards. I can give you the base measurement for the standard bearing thickness.  If the crank is still installed, use plasti-gauge to measure the crankshaft to bearing  clearance.

Most of the original std. bearings are not identified except for a part number.  The oversize bearings are usually marked with the oversize.

[Eminent]

Thanks for your answer Joe.
Today i measured the crankshaft.
The diameter of the rods are still original. So it good be that the shells are 43 years old, if the were never changed.
Another question. Is it possible to change the (front) main bearing with the crankshaft kept in place (with no tension on the chain)?
Thanks again.
Rini.

[ja17]

Hello Rini,

Yes, you can slide a new main bearing shell in place with the crankshaft in place. Be sure to use some good assembly lube on the friction side and keep the back side dry. Naturally, notice which side the index tab on the bearing is and slide the bearing out so it comes out first.

This extreme measure usually isl not recommended. If the crankshaft journal is rough  or worn, the new bearing will not break in properly.

Knocking is usually caused by rod bearing failure. Once the rod begins to knock, the pounding damages the rod big end, in addition the round big end opening becomes oval shaped instead of round. Just replacing the bearing will nor normally last.

What is the condition of the bearings you have removed? Are they heavily worn or damaged (copper base metal showing through)? You really need to determined where the knock is coming from  before you begin.

[Eminent]

The knock is not vey loud and is there for a few months now. I only hear it when i accelerate, when i release the throttle it is almost gone. With other words, only under load it's there.
I discussed it with a person who know's a lot from Mercedes engines form that area and he was very sure that i would be a crankshaft bearing.
We only measured a few bearings. The shells are not heavily worn or damaged, but the shells we measured (with plasti-gauge) are to the max of clearance (0.055) or just a tiny more.
The shell from cilinder#1 is (so far as i can see) most worn in the middle but also not heavily and definitly no copper shining trough.
We did some test before we start this maintenance. The compression from all the 6 cilnders is about 11 bar. Accelerating on the road and than untouching the throttle gives no smoke at all. The engine runs smooth and nice and only give a bit smoke after starting causing by the valve stem seals.

[Benz Dr.]

I tried new bearings without sizing the connecting rods once and the engine ran, but not very long. Sometimes knocking under load is a collapsed piston skirt - there's no cure for that. Any half assed attempt to fix this will lead to a lot bigger repair just down the road.

Right now your engine runs. If it's never been apart it's probably time for a rebuild. You will find general wear all over the engine - that's normal. If something breaks like a rod bearing or a piston ring then you could be into a larger repair bill if you damage the block or crank. I'm not trying to be an alarmist but my experience tells me to be proactive and consider your options carefully.

Strange noises coming out of an engine means many different things and you need to be able to diagnose all of those different sounds. Clicking, ticking, knocking and pounding all mean different things. Some are normal but most are not.

[ja17]

Hello Rini,

Main bearings don't cause a knock.  A bearing causing a knock will be obviously worn. I do not think you have found the location of your problem yet. It can be other issues as Dan has suggested.

Still sounds like a rod bearing to me. If you pull the plug wires one at a time while the engine is running the knock may go away when you get to the bad cylinder.

Oil travelling from the oil pump goes through the oil filter and then to the main bearings. In turn each main bearing supplies one or two rod bearings with oil. As a main bearing wears, it leaks oil and starves one or two rod bearings of oil. These rod bearings overheat from lack of oil and fail. Excessive clearance between the worn bearing and the crankshaft,  as the piston and rod assembly move up and down, is what causes the "knock".

At some point in time low or decreasing oil pressure will become evident. How is your oil pressure?

Other sources of knocks, as Dan has indicated, can be piston damage or wrist pin problems. Valve train problems are other sources of mysterious knocks. With these items, oil pressure is not effected.

One indicator you have given is that the knock occurs "under load". This is normally an indicator of a rod bearing problem. You need to find the problem before yous start replacing parts.

[Benz Dr.]

Most rod bearings won't make much noise when the engine oil is cold. Once the oil warms a bit they start to knock and get louder as you run the engine. If there is no knocking noise at idle but you hear noise under load it will more likely be a pin bushing or damaged piston.