300cd,

The cam timing mark is on the side of the cam washer just behind the cam sprocket, not on any chain guides. The other mark is on the cam tower. There is only one set of these marks.

If you are checking chain elongation, you align the two marks and look on the cranakshaft pulley for the degrees. Should be off no more than 4 degrees or it is time to replace the chain or install offset woodruff key between cam and cam gear.

Also check the chain tensioner to make sure it is keeping the chain tight.

P E H