M42 Chain Tensioner Replacement
The M42 timing chain is tensioned by a chain guide pushed against the chain by a hydraulic tensioner. A weak or frozen tensioner can cause timing chain noise. Replacement of the tensioner takes about ten minutes and requires only a 19 mm socket wrench.
BMW updated its original M42 tensioner with an improved version that is also used in the M44 (part no. 11 31 1 743 187). The original model is no longer available. The photo below compares the new tensioner (top, next to scale) to the original. Between the two units is a spring found in both tensioners.
The tensioner consists of a cylinder, closed at one end, having a closely fitted piston. Oil under pressure enters the port in the wall of the cylinder and, assisted by the spring, expands the piston against the chain guide. To make it easier to install, the tensioner is sold with the spring compressed and the piston held in place by a snap ring. It takes only a slight force of about 1 newton (1/4 lb-ft) applied to the piston to release the spring. Once released, it can be very difficult to lock the piston back into place for installation. Avoid pressing on the piston or dropping the tensioner.
The tensioner is located behind a 19 mm hex-head cap on the right side of the cylinder block at the front of the engine, approximately 13 cm (5 inches) below the head/block interface. The photos below show its location with (1) the hex cap in place, and (2) the tensioner partially removed from the block.
To install, slide the compressed tensioner into the cylinder block with the piston end facing inward. Use a new compression washer and torque to 40 Nm.
The final step in the installation procedure calls for revving the engine to 3500 rpm for 20 seconds. As this is done, the chain noise will be heard to decrease suddenly as the compressed spring is released and the tensioner is filled and expanded by the engine oil. (The compressed tensioner is released by the chain as it slaps against the chain guide during the revving process.)
Replacement of the tensioner is a simple fix that eliminates one possible source of engine noise.