It sounds to me like you have a double-row timing chain for a GAS engine, not for the DIESEL. In the photo below, you can see clip-type master links for (left to right) single-row gasser, double-row gasser, and double-row diesel. The chain you install must be identical to the chain you remove or you'll be in deep doo-doo. The current chain part number is 003-997-55-94. BTW, the 300SDL has an OM603.96
1... the 1987 300D has an OM603.96
0. There's almost zero difference between them though.
If the chain you have is not IDENTICAL in size to the one in the car, DO NOT install it.
Here's a photo of the correct chain:
