Measuring Timing Chain Stretch on the 1984 300SD OM617.951 Engine / W126 Chassis Mercedes

The proper way to measure chain stretch.
OM617 used in this example but procedure could apply to other engines as well.

There are two ways to determine chain stretch:

  1. The first method is commonly called the 2mm valve lift method. This is the method that Mercedes Benz specifies. It will give you a highly accurate reading.
  2. The second method consists of aligning marks on the front cam bearing tower and the sprocket thrust washer. This method can give you an accurate reading but it all depends on how accurate the marks are aligned.

It's not necessary to bring out the dial indicator every time you have the valve cover off. Aligning the cam marks is a good way to see where the chain is at. I like to check with the dial indicator once maybe twice a year to get an accurate reading.

Ok lets get started...

1-The first thing to do is remove valve cover. It will help putting things back together if you take pictures.

2-Setup intake valve #1 pointing up in relation to rocker arm. Intake #1 is the second valve from the front.

3-Just remove valve lash.

4-Remove magnetic base from dial indicator post holder and install as shown. Use channel locks to gently make sure it's tight.

5-Setup your dial indicator. It will take some finagling but it will work.

6-Dial indicator post should be on the valve keeper. Make it as straight up and down as you can. This dial has a 1" travel. If you are going to purchase a dial indicator buy one with a 2" or more travel. This will allow you to get it straight vertical.

7-Note that the cam lobe has moved 90 degrees and there is no movement on the dial indicator.

8-Crank engine at crankshaft until dial indicator shows 2mm lift. 2mm = 0.080 inches. No it's not reading 0.020 inches it spun around counter clockwise.

9-Read degrees at balancer. This reading is 11 degrees.

That is not our chain stretch. to determine chain stretch we have to go to the Factory Shop Manual and get specifications.

For engine(s) 615.912/913, 615.940 (40kw), 615.941, 616.916, 616.912 (48kw); Camshaft code(s) 02 & 06, specs are:
With new chain 11.5 degrees ATDC.
With used chain (from approx. 20,000 Km) 13.5 degrees ATDC.

For Engine(s) 615.940 (44kw), 616.912 (53kw); Camshaft code 10, specs are:
With new chain 9 degrees ATDC.
With used chain (from approx. 20,000 Km) 11 degrees ATDC.

For engines 617.910, 617.912 (59kw); Camshaft code(s) 00 & 08, specs are:
With new chain 11.5 degrees ATDC.
With used chain (from approx. 20,000 Km) 13.5 degrees ATDC.

For engines 617.950 (up to year 1979); Camshaft codes 00 & 08, specs are:
With new chain, 11.5 degrees ATDC.
With used chain (from approx. 20,000 Km), 13.5 degrees ATDC.

For engines 617.950 (1980 and after), 617.951, 617.952; Camshaft code 05, specs are:
With new chain, 9 degrees ATDC.
With used chain (from approx. 20,000 Km), 11 degrees ATDC.

Verify engine number by reading the stamp on the engine block. It is located under glow plug #5 near the oil filter.

Note: Cam codes are stamped on the rear of the cam.

You take your reading off the balancer, compare it to the proper specifications for your engine. The difference is the actual chain stretch.

Mercedes Benz makes 4 woodruff keys for the correction of cam timing:
  • Part # 621 991 04 67, Offset 0.7 mm, For correction of about 4 degrees.
  • Part # 621 991 02 67, Offset 0.9 mm, For correction of about 6.5 degrees.
  • Part # 621 991 01 67, Offset 1.1 mm, For correction of about 8 degrees.
  • Part # 621 991 00 67, Offset 1.3 mm, For correction of about 10 degrees.
Choose and install appropriate key.

For this engine, 617.951 the chain is right on at 11 degrees. I installed a 4 degree woodruff key on June 11, 2005. Almost 8 months later the chain is still in great shape.

10-Trying again with cam timing marks.
Align the marks as shown. They have to be PERFECTLY centered! Even a little off will throw off your reading.

Read chain stretch at balancer. Looking at the specifications we see that Mercedes Benz builds their chains to wear in 2 degrees at roughly 20,000 Km. Using this method you should see 2 degrees at the balancer. What we see here is an accurate reading.

11-Reset valve lash back to spec then put it all back together.

Checking and correcting chain stretch is an integral part of maintaining your engine. However you also want to make sure your tensioner, guides and rails are in good shape. Failure of any of these parts can lead to catastrophic failure of your chain and engine.

Also, don't leave your ratchet on the crank bolt!

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