How to Replace the Timing Chain on the Mercedes OM 651 Engine

Replacing the timing chain on the Mercedes OM 651 engine is a complex task that requires special tools and absolute precision in timing. The chain is located on the flywheel side of the engine, making access particularly difficult.

Position cylinder number 1 at Top Dead Center (TDC). On the camshafts, the pentagon-shaped mark must align with the arrows on the supports, and the double pin on the crankshaft must be at the 12 o'clock position (as shown in the video). Use specific tools to lock the camshafts and crankshaft in this position before loosening any components.

The chain can be replaced without removing the engine, using the dragging method: open the old chain and connect it to the new one with the appropriate link. Manually rotate the crankshaft to pull the new chain in, which is pulled by the old one. Once in place, remove the old chain and close the new one using the tool to rivet the master link.

It is essential to replace the entire kit, not just the chain: chain, tensioner, guides, and sprockets. Replacing only the chain on worn sprockets would cause the problem to recur.

Pay attention to a series of critical details: a crucial detail is mentioned in the video. The bolts on the camshaft sprockets have left-hand threads.

To loosen them, you must turn them clockwise, not counterclockwise, to avoid breaking them; strictly adhere to the torque specifications, such as 40 lb-ft for the camshaft sprocket bolts and 59 lb-ft for the tensioner. Before closing the valve cover, generously lubricate the chain and sprockets with engine oil to prevent a dry start.

After reassembly, it is crucial to check the timing with a diagnostic tool to ensure there are no discrepancies between the crankshaft and camshafts.