Mercedes-Benz's M133, M270, and M274 four-cylinder engines power a huge slice of the modern A/B/C/E-Class lineup, and like most chain-driven engines, they'll eventually need their timing chain, guides, and tensioner refreshed. The camshaft locking bars these engines use are specific to Mercedes' design and don't interchange with the pin-and-hole style tools common on VAG engines — using the wrong approach here is how you end up with a cam-correlation code after reassembly.
Tool You'll Need
- Engine Timing Tool Kit — the specific camshaft locking bars, alignment pins, and hardware for M133/M270/M274
You'll also need a crankshaft counter-holding tool for the front pulley bolt, a torque wrench, and the full chain, guide, and tensioner kit for your specific engine code — replace them as a set rather than the chain alone.
Step 1: Establish TDC on Cylinder 1
Rotate the crankshaft, counter-held rather than impacted, to Top Dead Center on cylinder 1 per your engine's timing marks.
Step 2: Fit the Camshaft Locking Bars
Install the locking bars from the timing kit across the camshafts. They should seat flat and square against the reference surface — if a bar is rocking or won't sit flush, the crank isn't precisely at TDC and needs rechecking before you go further.
Step 3: Access the Chain and Tensioner
Remove the covers and accessories standing between you and the timing chain assembly, following your engine's specific procedure.
Step 4: Relieve Tension and Remove the Old Components
Using a proper ratcheting tensioner tool, relieve tension carefully rather than forcing it, then remove the old chain, guides, and tensioner together.
Step 5: Install the New Kit
Fit new guides, a new tensioner in its retracted position, and route the new chain per the factory diagram, keeping the locking bars in place and undisturbed throughout.
Step 6: Release and Verify
Release the tensioner, remove the locking bars, and rotate the engine through two full turns by hand, returning it to TDC. Reinstall the locking bars to confirm they seat cleanly — if they don't, something shifted and needs rechecking before you close things up.
Step 7: Reassemble and Test
Reinstall covers and accessories, start the engine, and confirm no cam-correlation or timing-related fault codes appear on startup.
Common Mistakes
- Using VAG-style pin tools on a Mercedes engine — the locking geometry is different; use the correct kit for the platform
- Impacting the crank pulley bolt — always counter-hold
- Replacing only the chain — guides and tensioner wear alongside it and should go together
- Skipping the two-turn verification — the check that catches a timing error before it becomes a running problem
See our full specialty and diagnostic tools lineup for more Mercedes-Benz service essentials.