Smart Roadster valve clearances do need periodic checking. The 698cc Mitsubishi-derived 3-cylinder engine uses shim-and-bucket followers with specified clearances of 0.15–0.19 mm (intake) and 0.25–0.29 mm (exhaust) when cold. Clearances can tighten over time, risking burnt valves. Inspection is recommended every 40,000–60,000 miles.
For a car that is often dismissed as a city runabout, the Smart Roadster 452 contains a surprisingly sophisticated little engine. Its mid-rear-mounted 698cc turbocharged three-cylinder shares its architecture with Mitsubishi’s M160 unit and features a dual overhead camshaft (DOHC) valvetrain with hydraulic-free, mechanically shimmed bucket tappets. That design is precise and durable — but it demands attention that many owners overlook entirely. This article explains what valve clearances are, why they matter on this specific engine, how to check them yourself, what the correct specifications are, and when the job becomes urgent rather than optional.
What Are Valve Clearances and Why Do They Matter on the Smart Roadster?
Valve clearance — sometimes called valve lash — is the small gap between the tip of the valve stem and the component that opens it (in this case, a bucket-style follower sitting directly under the camshaft lobe). This gap exists because metal expands when hot. Measure it cold, set it within tolerance, and at operating temperature it closes to the correct working value.
The Smart Roadster’s engine has no hydraulic lifters to self-adjust. Every one of its twelve valves (four per cylinder, two intake and two exhaust) relies on a precisely ground steel shim to maintain the correct gap. Over time, the cam lobes and shim surfaces wear, which typically causes clearances to tighten. A tight exhaust valve is particularly dangerous: it cannot seat fully, heat cannot transfer to the head, and the valve face begins to burn — an expensive failure that can end in a full head rebuild.
Conversely, clearances that are too wide produce a characteristic ticking or tapping noise at idle, increased cam lobe wear, and reduced power due to late valve opening. Neither condition is harmless, which is why this check deserves a place in your long-term maintenance plan alongside choosing the right engine oil and changing it on schedule.
Smart Roadster Valve Clearance Specifications
These are the factory-specified clearances for the 698cc M160 engine as used in the Smart Roadster 452, measured with the engine cold (below 35 °C):
- Intake valves: 0.15 mm – 0.19 mm (target mid-point: 0.17 mm)
- Exhaust valves: 0.25 mm – 0.29 mm (target mid-point: 0.27 mm)
These figures apply to all variants — the 45 kW Lite, 60 kW standard, 66 kW SB2 Brabus, and 74 kW full Brabus. The valvetrain design is identical across all output levels; only the turbocharger boost and ECU calibration differ between them. If you have a Brabus model running higher boost, accurate valve seating becomes even more critical because combustion temperatures are elevated.
Shims are available in increments of 0.025 mm. When you measure a clearance and find it outside specification, you select a shim that brings the gap back to the mid-point of the tolerance band. Record every measurement before disassembly — most clearances on a well-maintained engine will still be within range and require no intervention.
Symptoms of Incorrect Smart Roadster Valve Clearance
Clearances That Are Too Tight
Tight clearances are the more insidious problem because they produce no noise — the engine runs quietly right up until a valve burns. Early warning signs include a slight rough idle, a very subtle misfire at light throttle, and in later stages an unexplained coolant loss as a burnt valve allows combustion gases past the seat. A compression test will reveal low compression on the affected cylinder. By that point, cylinder head removal is inevitable.
Clearances That Are Too Wide
Excessive clearance is noisier and more obvious. You will hear a rapid, mechanical ticking or rattling from the cam cover area, most pronounced when the engine is cold and at idle. Unlike a noisy hydraulic lifter on other engines, this sound will not disappear once the engine warms up — it may actually become slightly quieter as the metal expands, but it will not go away entirely. Power delivery may feel slightly harsh at low revs.
How to Check Smart Roadster Valve Clearances: Step by Step
This is an intermediate DIY task. You will need: a set of feeler gauges (0.05–1.00 mm range), a torque wrench, cam cover gasket (always replace it), engine oil, and a clean workspace. The engine must be cold.
Because the engine sits mid-rear in the car, access requires removing the rear bodywork clam. Make sure you are working on a stable, level surface — if you need to raise the car at any point, consult our guide on the correct lifting points to avoid chassis damage.
- Remove the engine cover and disconnect the breather hose from the cam cover.
- Remove the cam cover bolts (typically eight, tighten to 8–10 Nm on reassembly) and lift the cover clear. Take care not to lose the rubber sealing grommets on the bolt holes.
- Rotate the engine using a spanner on the crankshaft pulley bolt (clockwise only, 19 mm) until cylinder 1 is at TDC on the compression stroke — both cam lobes pointing away from the bucket on cylinder 1.
- Measure the clearance by sliding feeler gauges between the cam lobe heel (base circle) and the top of the bucket. The correct gauge should slide through with a slight drag — it should not fall through freely, nor require force.
- Record each measurement against the cylinder and valve position (intake or exhaust). Work systematically around the firing order (1-3-2 on this engine).
- Replace any out-of-specification shims. Shim removal requires a cam follower depressor tool to compress the bucket while you extract the shim with a magnet or fine pick.
- Refit the cam cover with a new gasket, torque bolts in a cross pattern, reconnect the breather, and refit the engine cover.
While the cam cover is off, it is good practice to check for sludge build-up on the underside — a sign of infrequent oil changes or oil/coolant mixing. If you spot a creamy residue, investigate further before reassembling.
How Often Should You Check Valve Clearances?
Mitsubishi’s original service schedule for the M160 engine recommends valve clearance inspection at 60,000 km (approximately 37,000 miles), with re-inspection every 40,000–60,000 km thereafter. In practice, for a car that may be used seasonally or that has an unknown service history, a first check at 50,000 miles is sensible.
Any of the following circumstances should prompt an earlier inspection:
- Unknown service history on a recently purchased car
- A ticking noise from the top of the engine at idle
- Failed compression test on one cylinder
- Overheating event, even a minor one — heat can accelerate valve seat recession
- Recent spark plug replacement that revealed unusual combustion deposits — see our detailed breakdown of what different plug conditions tell you about engine health
It is also worth noting that the valve clearance check is naturally timed alongside the cam belt inspection. The Smart Roadster 452 uses a timing chain, not a belt — but understanding what that means for your long-term maintenance is covered thoroughly in our dedicated article on the subject.
Smart Roadster Valve Clearance and Engine Performance
Correct valve clearances are a prerequisite for the engine to perform as designed — and this becomes especially relevant if you are considering any tuning work. An ECU remap increases boost pressure and advances ignition timing; if the engine is simultaneously suffering from tight exhaust valves and imperfect combustion sealing, the additional thermal load can accelerate damage significantly. Any reputable tuner should ask about valve condition before proceeding.
For those who have already modified or are planning to modify the car, the valvetrain is part of a chain of components that all need to be in good health. The same applies to the engine’s fuel delivery and evaporative emission system — faults in the latter can affect mixture quality in ways that are sometimes mistaken for valvetrain symptoms. A persistent rough idle that does not improve after a valve clearance check is worth investigating further; our article on the purge valve and its associated fault code explains one common culprit.
For those planning more substantial engine work alongside the clearance check, note that the coolant system will likely need to be partially drained. Consider doing a full coolant service at the same time — the process is straightforward and is explained step by step in our coolant flush guide.
Common Mistakes to Avoid
- Measuring hot: Always measure cold. Clearances change substantially at operating temperature and a hot measurement will lead to incorrect shim selection.
- Reusing the cam cover gasket: These gaskets compress permanently. Reusing one almost always results in an oil leak from the cam cover within a few hundred miles.
- Ignoring the breather system: A blocked or collapsed crankcase breather raises oil vapour pressure in the cam cover area and can mask other problems. Inspect it while the cover is off.
- Over-tightening cam cover bolts: These are small bolts into an aluminium head. 8–10 Nm is correct. More will strip the threads — a repair that is far more expensive than the check itself.
- Mixing up shim sizes: Keep every shim in a labelled container corresponding to its exact position. Even a 0.025 mm error in shim selection moves you outside tolerance.
Closing Summary
Smart Roadster valve clearance is a genuine maintenance item that too many owners neglect, often because the car’s image as a lightweight fun car does not suggest the kind of mechanical depth that is actually present under that rear clam. The M160 engine is robust and long-lived when properly maintained, but its shim-and-bucket valvetrain will tighten silently over time. Checking clearances every 40,000–60,000 miles costs a few hours and a cam cover gasket; ignoring them can cost a cylinder head. For any Smart Roadster with an unknown service history, a valve clearance inspection should be one of the first tasks on your list.
Frequently Asked Questions
What are the correct Smart Roadster valve clearance specifications?
The factory specification for the Smart Roadster 452’s 698cc M160 engine, measured cold, is 0.15–0.19 mm for intake valves and 0.25–0.29 mm for exhaust valves. These figures apply to all variants including the 45 kW, 60 kW, and Brabus models. Shims are available in 0.025 mm increments for adjustment.
Does the Smart Roadster engine have hydraulic or mechanical valve lifters?
The Smart Roadster uses mechanical shim-and-bucket valve followers with no hydraulic self-adjustment. This means clearances must be measured manually and corrected by swapping shims. It is a more precise system than hydraulic lifters but requires periodic inspection to remain within specification.
What does tight valve clearance sound like on a Smart Roadster?
Tight clearances on the Smart Roadster typically produce no noise — they fail silently, eventually causing a valve to burn. The warning signs are a rough idle, a slight misfire at light throttle, or low compression on one cylinder. Audible ticking indicates clearances that are too wide, not too tight.
Can I check Smart Roadster valve clearances myself?
Yes, this is an achievable DIY task for a competent home mechanic. You need feeler gauges, a torque wrench, and a new cam cover gasket. The main challenge is accessing the rear-mounted engine by removing the bodywork clam. No specialist tools are required unless shims need replacing, which requires a bucket depressor tool.
