250C timing chain

[magnumrick]

Yesterday I finally replaced my valve cover gasket. I was surprised by the amount of play in the timing chain. The car seems to run fine but the chain was sloppy/loose on the right side and obviously stretched. How do I check to see if it should be replaced? Is this an afternoon job or do I have to pull the head?

[Mike D]

Pull the valve cover back off. Bring the engine to TDC. Check your cam timing marks. I believe under 6 degrees is acceptable. Slop on the right side (passenger side of engine) is normal. The tensioner is oil pressure activated and older ones will leak down quite rapidly when the engine is off.

[magnumrick]

I will try to get a look at it this weekend. Where are the cam timing marks for the cam located? Are they on the front sproket?

[Mike D]

There should be a notch or mark on the front cam tower. There will be one on either the cam gear or the spacer which sits behind it. They line up.

Hand crank the engine to TDC on the #1 cylinder compression cycle and check the alignment. If they do not line up at TDC then crank the engine until the cam notches are aligned. Read the degrees on the dampener pulley to get the stretch. Give it a plus or minus of 2 degrees unless you are "dead square on" the timing marks on the lower pulley.

My M110 has 2 degrees of stretch at 161K miles. The timing chains are pretty durable.

Do you get a clatter when you first fire the engine after a prolonged period of non-use? That is usually the first indication of a tired tensioner.

[magnumrick]

I don't notice any clatter at startup, I was just surprised at how much play there appeared to be when I had the valve cover off. The fact that the tensioner is dependent on oil pressure would explain the looseness. I will try to take a look at it this weekend. Thanks for the information on what to look for. Can you replace the chain without pulling the head?

[senorunimog]

Yup! changing the chain is easy: find the split link in the original chain disconnect it and splice the new chain in using the split link. Hand-crank the engine until the new chain is all the way through. A helper is useful to keep the chain engaged on the cam sprocket so you keep your timing all lined up. when you are finished clip the new chain together using the new split link that came with it. It won't take more than half an hour. New chain should run about $60.

[magnumrick]

That sounds almost to easy. What about the tensioners? Should they be changed at the same time?

[Mike D]

If it is the original chain it may not have a removeable link. Most of them don't. You will need to remove a link to split the chain. Stuff lots of wet rags around the opening and covering the exposed cam area. Use a die grinder or chain splitter to remove the link. The reason for wet rags will become apparent once you begin using the die grinder.

Are you asking about the guide rails or the tensioner? The actual tensioner is a bit of a booger to change if the car has A/C. The lower rail is a long pivot arm with a hard rubber face or a flat asbestos-like "shoe" surface. Check it for grooves. The upper rail is a metal guide with a rubber slide.

The tensioner doesn't have to be removed to replace the lower rail. The lower rail and tensioner can be either of the two types. If you replace either the lower rail or the tensioner you need to replace it with the same type.

[magnumrick]

Mike, I completely understand the wet rag requirements, I have been manufacturing resin bonded cutoff and grinding wheels for a lot of years. PM me an address and cutoff wheel sizes you use and I will forward a selection. What should I be looking for on the guide rails to determine if they need to be replaced?

[Mike D]

The flat shoe style should be exactly that. A smooth flat surface like a leather shoe sole. No deep grooves or metal showing through the pad. It will probably show some wear. Let me run a chain over YOU at about 4800 rpm for 10 years and see how you look.

The hard rubber one may have some regular grooves in it already, kind of a "corrugated" look. It will probably have small grooves from the existing chain. It should not have DEEP grooves from chain wear. No metal showing in the grooved areas