dchang

Hello, That is my question. A machine shop told me i needed to replace my Piston rings, Valve guide seals, etc. To stop the oil consumption. So i did, But he didn't replace my connecting rod bearings. After a couple of weeks it started having a knocking sound, like rod knock. My question is , Isn't it standard procediure to replace the connection rod bearings , when replacing the piston rings?

MarcusF

I’ve rebuilt a number of engines. Never a Mercedes, but the process is the same. By rebuilt, I mean disassembled, dropped everything at the machine shop, let them do their work and reassembled them. Occasionally I'd pay the fee and let them reassemble the bottom end, but yes, if the rods are removed, new bearings typically go back in. While the rods are out, they’re cleaned, magnafluxed, and checked to make sure they’re straight. Then the caps are bolted back on and mic'd. The crank should be mic’d too. Also, the rod bolts should be checked for stretch. Now, you don’t HAVE to check the rods. You don't HAVE to change the bearings. You can assume they’re OK, but if it starts knocking after it’s reassembled, that budget rebuild is going to get expensive. If it’s knocking, the rods should come back out and the crank mic’d (again). The knocking could be a lot of things – including the machinist having taken too much material off when he honed the rods.

That’s not the odd part though. The odd part is the machinist determined that both the top and the bottom end weren’t sealing and that's why you were using oil. If both the top and bottom are leaking on a car without a zillion miles, you may want to check the oil pressure and possibly reconsider whatever you’re using for oil. He did perform a leak down test, right?

Lastly, are you SURE it's rod knock? Not a collapsed lifter, but rod knock?

dchang

Hello Marcus, Thank you for your information. I don't know if a leak down test was done, But the machine shop said it was a bad connecting rod bearing. I got the car back, no more knock, but we'll see how she goes.
Thank you again!
Darrell

Khan 55

A good repair automotive machine shop should have inspected the crank for run out and oil clearances as well as thrust surfaces of the rods to crank on the big end journals and rod side clearance as well as the end float of the crank and if need be a crack test on the crank. We get plenty of customers with their huge mine engines that want to reuse bearings sometimes on an engine with extremely low hours but our tech team knows better and if the customer says otherwise then we not responsible.


Nobody can really remove a bottom end of any engine and look at the bearings and crank and rod journals and say yeah it looks okay put them back. Nope as Marcus said you measure them up with a outside micrometre and use a dial bore gauge or inner micrometre on the block and rods to check the clearances if they within spec. obviously one would also measure the bearing thickness or do the inner measurements with the bearings still in the block and rods for your oil clearances. As far as piston rings are concerned, they needed to measure the cylinder bores for taper and ovality. No use putting new rings when your bores look like vases. I don't know if AMG M113 engines use a dry or wet sleeve/liner.
On a lighter note, how is the oil consumption now?