TL;DR
- What: The M50’s timing drive: two chains, two hydraulic tensioners, the chain guide rails and the sprockets.
- Why: The tensioners and plastic guide rails age. A worn chain or sprockets cause noise and erratic valve timing, and a faulty tensioner causes chain noise too. On an old, high-mileage engine with unknown history, it is worth inspecting before it becomes expensive.
- Time: A big job. On the six-cylinder, chain removal requires taking the oil pan off, which means raising the engine or lowering the front suspension crossmember.
- Difficulty: 4/5. BMW special locking tools (or exact equivalents) are needed, because the camshafts have no reassembly timing marks.
How the M50 timing drive works
Unlike the four-cylinder M42/M44, which use one double-row chain, the six-cylinder uses two chains:
- the primary chain is driven by the crankshaft and turns the primary sprocket on the exhaust camshaft;
- the secondary chain runs from the exhaust camshaft to the intake camshaft.
Each chain has its own hydraulic tensioner. The chains are lubricated by engine oil and need no routine maintenance.
There are two generations to know about:
- Pre-VANOS M50 (engines built up to August 1992): no variable valve timing.
- M50 with VANOS (1993 and later engines, often called M50TU): a VANOS control unit at the front of the cylinder head varies the intake camshaft timing. It changes parts of the procedure and some torque values, noted below.
What wears and how it shows
Bentley’s summary is short: a worn chain and sprockets lead to noisy operation and erratic valve timing, and a faulty tensioner can also cause timing chain noise.
In practice, owners usually notice:
- Chain rattle at start-up that fades as oil pressure builds: the first sign that a tensioner is not holding pressure.
- Rattle that stays when warm: more serious. Chain slack is now constant; suspect worn guide rails or a failed tensioner.
- Pieces of black plastic in the oil pan or on the oil pickup screen: broken guide rail. If you find them, do the job now; fragments can restrict the oil pickup.
Any persistent chain noise is worth investigating before driving the car hard.
Parts for a complete refresh
If you are opening the front of the engine, replace every wear item in one go rather than just the part that failed:
- primary and secondary timing chains
- primary and secondary chain tensioners (with new sealing washers)
- chain guide rails
- sprockets, if their teeth show wear
- crankshaft front oil seal (it sits in the lower timing cover)
- gaskets for the upper and lower timing covers and the cylinder head cover
- oil pan gasket (the pan has to come off)
- new crankshaft hub bolt: it is a stretch bolt and must always be replaced
Part numbers: look them up by VIN on RealOEM under Engine → Timing and valve train. They differ between pre-VANOS and VANOS engines and changed over production, so a number copied from a forum or a generic list can be wrong for your car. Buy chains and tensioners from established brands.
On VANOS engines, the VANOS unit has its own seal kit. Whether to reseal it while you are in there depends on its condition; see the VANOS guides.
The job: what matters most
This is not a full step-by-step procedure: follow the Bentley or BMW instructions for your engine. These are the points that decide whether the engine runs correctly afterwards.
Plan for the oil pan. On the six-cylinder, the chains cannot come out without removing the oil pan, so the engine must be raised or the front crossmember lowered.
Lock the engine at TDC with the proper tools. BMW provides no reassembly marks on the camshafts, so timing is set with locking tools:
- crankshaft locking tool 11 2 300, inserted through the transmission bellhousing into the flywheel, at TDC of cylinder 1;
- camshaft locking tool 11 3 240, fitted to the rear of both camshafts.
On pre-VANOS engines (built up to 8/92) the camshaft tool may not seat correctly as delivered and needs adjusting; Bentley describes how.
The crankshaft hub bolt is 410 Nm. Use a heavy-duty holding device (BMW tool 11 2 150) and a ¾-inch drive. The bolt is a stretch bolt: always fit a new one.
VANOS engines need the dummy tensioner. Before tightening the exhaust camshaft sprocket, BMW preloads the primary chain with dummy tensioner 11 3 390, tightened to 1.3 Nm. The real tensioner goes in afterwards, with the cut-out in its piston engaging the chain rail.
Fit the lower timing cover carefully. Use a new crankshaft seal and gaskets, a small bead of sealer (3-Bond 1209 or equivalent) at the corners where the cover meets the head and block, drive the dowels in flush, and tighten the bolts alternately in stages.
Check before you close up. Remove the locking tools, turn the engine by hand through at least two full revolutions, refit the tools and confirm they still go in easily. If they don’t, the timing is off: redo it.
Torque values (Bentley)
| Fastener | Torque |
|---|---|
| Crankshaft hub to crankshaft (stretch bolt, always replace) | 410 ± 20 Nm |
| Vibration damper and pulley to hub (M8) | 22 Nm |
| Lower timing cover to block, M7 / M8 | 15 Nm / 20 Nm |
| Upper timing cover (M6) | 10 Nm |
| Secondary chain sprockets to camshafts (M7 Torx) | 5 Nm, then 22 Nm |
| VANOS engines: intake sprocket nuts to studs (M6) | 10 ± 1 Nm |
| Primary chain tensioner: M50 / M52 | 50 Nm / 40 Nm |
| VANOS control unit access plugs | 50 Nm |
| Cylinder head cover (M6) | 10 Nm |
What’s next
With the front of the engine open, it is the natural moment to look at the water pump, the thermostat and, on VANOS engines, the VANOS seals. See the M50 vs M50TU vs M52 comparison for which engine generation you have.
