PiratePast40

2002 C320 V-6 engine, 167k miles. Long term rattling noise getting louder and louder and finally sounding like something was about to break. Thought it was rattling from the shrouds under the car but that wasn't it. Finally took off the serpentine belt to confirm that it wasn't one of the accessories. Took it to the shop and they confirmed that it was the timing chain and they wanted $3,000 to replace the chain, $1,000 in parts and supplies and $2,000 for labor. Said no thanks and drove the car home.

Thought I would inspect the tensioner first. It didn't look like it was damaged and had plenty of spring pressure so ordered the new chain, chain breaker, gaskets and a new tensioner just to be on the safe side. Took off the right valve cover to test to see how hard it would be to feed a new chain in from the top. Seemed too loose so decided to try it with the tensioner installed. Very difficult to get it back in but finally got it started by hand. A few turns with the ratchet and it was obviously cross threaded. Now I've got about a 1/4 inch (4mm) of aluminum threads stripped in the timing cover. The tensioner is about an inch in diameter(24mm). It's angled down so no way to actually see the hole. What do I do now? Run an appropriate sized tap? Smooth out the 1/4 inch of threads with a dremel tool? Any other solutions? I can't see it and the angle is pretty close to impossible to determine so not sure if a tap or grinding the threads down can be done accurately. As expected, threads on the tensioner are still fine since it's steel and the timing cover is aluminum. Anyone have a magic spell or potion that will work?

Edit to add - Threads are 24 x 1.5. Total length is about 3" . Tensioner itself about a half inch. Threads are about 1.25" in length but start about 5/8" from the head of the tensioner. Thinking about getting a tap to chase the threads but not sure how to get them aligned correctly. Don't want to cut the beginning of the threads only to find out that they don't match the undamaged threads. Any machinists know any tips or tricks to do this right?

tjts1

In my opinion it's easier and cheaper to swap in another engine. Grab another 3.2 or a 3.7 from an ML for $500, swap the oil pan and drop it in. Your ECU will adapt to the extra 500cc. These engines are dime a dozen. Find one with less miles than yours. Even if you somehow fix the tensioner threads you'll still have the noise to fix. The double row timing chain on these engines doesn't stretch or rattle unless there's an oil pressure problem. Fun fact: the M112 doesn't have an oil pressure idiot light. There's an O ring between the oil pickup tube and pump which is known to fail.

PiratePast40

Ouch!

exhaustgases

Kinda makes the shop cost sound good huh? But then who stripped the threads? Yeah I'd hate it too, but I'd remove the engine so I could do the job right. DIY is not for everyone, if your not a mechanic its best left to a mechanic.

PiratePast40

That was very helpful. Thanks.