Cold starts: what really wears out a motorcycle engine

Everyone knows a cold engine is a fragile engine, and almost everyone draws the same conclusion: start it, let it idle until the needle moves, then set off in peace. It feels like the careful thing to do. On a modern bike, it is also one of the worst.

The problem lies in what you think you are warming. What wears an engine at start-up is not coolant temperature, it is the state of the oil film between parts that are touching, and the mechanical clearances that are not yet at their design dimensions. Prolonged idling fixes neither, and throws in problems that gentle riding simply does not have. Here is what actually happens in the first hundred seconds, and then in the first fifteen kilometres.

Where the oil is after a night parked

Nowhere useful. After a few hours, gravity has taken the oil back down into the sump. All that is left on the bearing surfaces is what molecular adhesion and surface finish managed to hold: a residual film a few tenths of a micron thick on the crankshaft and big-end shells, and next to nothing at the top of the head, on the cams and rockers.

Hit the starter and three things happen in sequence:

• The engine turns before it has any pressure. The oil pump is driven by the engine: it takes between one and five seconds, longer in hard frost, to fill the filter and the galleries and bring pressure up to the camshaft bearings, which sit furthest from the pump.
• Meanwhile, the revs climb. A cold-start idle often runs 300 to 800 rpm above the warm idle, because the ECU enriches the mixture and opens the additional air. So the engine spins faster at the precise moment it is least lubricated.
• Surfaces genuinely touch. In normal running a bearing works hydrodynamically: the parts never touch, they float on a wedge of oil. At start-up you pass through mixed contact — metal on metal, cushioned only by the residual film. A disproportionate share of an engine's total wear happens right there.

Practical conclusion: what counts is the first few seconds, and nothing you do afterwards makes up for them. Hence the importance of the next two points.

Cold clearances: noise is not automatically a fault

A cold engine ticks, it rattles, and a twin rumbles in a way that sounds alarming. That is normal, for a simple reason: aluminium expands roughly twice as much as steel or cast iron. Cold, the alloy piston is smaller relative to its bore than it will be at 90 °C, hence a slight rocking in the liner — the famous piston slap that disappears within ten minutes.

Valve clearances follow the same logic, and that is why they are always checked on a cold engine, at the ambient temperature specified by the manufacturer. A check made on a lukewarm engine gives a false reading and a setting that will be tight when hot. The exact figures and the check temperature depend on the model: never infer them from a similar engine.

There are, however, two cold noises that are not normal: a timing rattle that persists beyond a minute, the sign of a tired chain tensioner, and a sharp metallic knock that rises with the revs — that one should not be left running.

The myth of the long warm-up on the stand

This is the most stubborn received idea of all, and it exists for a good reason: on a carburetted engine with a choke, you genuinely did have to wait until the machine would hold its idle without the choke before setting off, or you stalled at the end of the drive. The habit stayed; the context changed.

On a fuel-injected engine, ten minutes idling in neutral produces this:

• The temperature rise is desperately slow. At idle the power produced is tiny, so the heat released is tiny too. Oil warms two to three times more slowly than coolant. A coolant needle at 80 °C does not mean oil at 80 °C: the oil only reaches working temperature after 10 to 20 km of riding depending on the season, and standing still it may never get there at all.
• The mixture is enriched. Until the lambda sensor is live, the ECU runs open loop on a rich mixture. Unburnt petrol washes the oil film off the cylinder walls, then drains down into the sump. That is dilution: an oil carrying a few per cent of fuel loses viscosity and load capacity. Exactly what you were trying to protect.
• Combustion water does not evaporate. You need oil above 90 °C to drive out the water condensing in the crankcase. Below that it accumulates and gives you the beige emulsion under the filler cap.
• The catalytic converter stays cold. It only starts working above roughly 250 to 300 °C. Ten minutes of idling is ten minutes of unburnt hydrocarbons going straight out, and an exhaust that clogs up.
• Splash-lubricated parts stay dry. On many engines the cylinder walls and gudgeon pins are fed by throw-off from the crankshaft, which depends on engine speed. Idling serves them no better than gentle riding does.

The right time on the stand is however long it takes to pull your gloves on and fasten your helmet: 30 to 60 seconds. Oil pressure is up, the idle has settled, and the rest of the warm-up happens far better on the move. One exception: a carburetted machine with a manual choke, which you leave running until it will hold without the choke, then ride away gently.

Viscosity is the first number

In a 10W-40, the 40 describes behaviour at 100 °C, and that is the figure everyone looks at. The only one that matters at start-up is the first: the W rates cold pumpability. A 10W reaches the top of the head faster at 0 °C than a 15W, and the difference plays out precisely during the seconds that do the wear.

That does not mean dropping a grade on your own initiative: bearing clearances, wet clutch pressure and the gearbox are all sized for a given range. Most manuals give a viscosity chart against ambient temperature; that chart decides, not fashion. The concrete differences between grades are set out in 15W-50 or 10W-40 depending on temperature and in decoding oil standards and viscosities.

The first five kilometres

The only rule that holds: moderate load until the oil is hot, not just the coolant.

• Stay below half of maximum revs until coolant temperature has settled, then for a few kilometres more.
• Avoid heavy load at low revs, typically pulling in fifth at 2,000 rpm with a fistful of throttle. That is the configuration that loads the bearings hardest, and when cold the clearances are not at their design figures.
• Vary the revs rather than holding a steady speed: it circulates the oil and brings the temperature up faster.
• Don't forget the rest of the bike. Cold tyres, cold pads, a shock full of thick oil: the grip and braking of the first ten minutes are not those of the rest of the ride.

The real killer: repeated short trips

An engine that never reaches working temperature accumulates everything described above, every single day. Five kilometres to work, engine off, five kilometres back: the oil never climbs high enough to drive off the water and fuel it keeps collecting.

Two fixes, neither of them complicated:

• A longer ride on a regular basis, 30 to 40 minutes of continuous running a month as an absolute minimum, to clean the oil up.
• Respect the interval in time, not only in kilometres. Every manufacturer gives a twin interval, for example X km or 12 months. On short, cold use it is the months that govern, sometimes even less. Oil that looks clean at 3,000 km is not clean chemically.

What to do tomorrow morning

1. Start it, then wait 30 to 60 seconds, the time it takes to get kitted up. Not ten minutes.
2. Set off gently, below half of maximum revs, moderate load, without holding a steady speed.
3. Consider the engine warm after 10 to 15 km, not when the coolant needle moves.
4. Check the oil grade against the temperature chart in the manual before winter, and change it on the interval in months if you only ride short trips.
5. Check valve clearances cold, at the prescribed temperature, at the prescribed interval.

If you are torn between laying the bike up and riding on through winter, the maths is done in ride or lay up your motorcycle — and if the MOT falls in the winter months, annual once the bike is three years old, take it there on an engine warmed up by riding rather than by ten minutes of idling. For the figures that genuinely depend on your machine — recommended oil grade for the ambient temperature, valve clearances, interval in months, sump plug torque — L'Atelier's technical assistant gives them from the make, model and year, without you having to cross-check three forums.