Running In a Rotax Engine: The Right Procedure Step by Step
A new or freshly rebuilt engine is not yet a ready engine: it is a set of surfaces that have to learn to work together. Running in does exactly that, and doing it well decides how much power and how much life your Rotax will have. Doing it badly, on the other hand, is something you pay for forever.
In short Running in a Rotax is done in three 15-minute sessions: the first up to 10,000 rpm, the second up to 12,000, the third at full load. Long gear ratio, main jet 2 sizes larger, 3% mix for the first 10 litres and water at least 55 °C.
Why it is done: the surfaces must bed in
A new piston, cylinder bore, rings and bearings have machining micro-roughness that must adapt to each other. Running in is controlled friction: progressive loads that smooth the mating surfaces without overheating them. Skipping it means imperfect ring sealing and, in the worst cases, a seizure or an engine that is 'born' already below par and will never recover.
The running-in mix: 3%
Running in a Rotax engine on track
The procedure starts even before you start the engine: a 'long' gear ratio and a main jet 2 sizes larger than the one recommended for altitude and temperature (richer carburation = safety margin). With low outside temperatures you also partly cover the radiator with tape, to make the temperature rise sooner. This is where the common sense of people who live at the track counts: it makes sense in winter or in cold weather; in summer you cover nothing, the water warms up on its own and covering it would only mean overheating. Then the threshold that is not negotiable: during running in the coolant must reach at least 55 °C.
Once the temperature is reached, the heart of running in is three track sessions of 15 minutes each: the first up to 10,000 rpm, the second up to 12,000, the third at full load. Then the main jet is reduced one step at a time (for example 172, 170, 168) down to the right size for the conditions. From there the engine can deliver all its power: the extra protection of the 3% mix, however, stays for the first 10 litres of petrol.
Checks during running in
Three things to keep an eye on: water temperature (it must stay in the correct window; if it rises, stop and find out why), the colour of the spark plug at the end of the session (it tells you how the mixture is burning) and the tightening torques after the first session: the first session settles everything, and going round the critical points with a spanner is standard practice. And listen: an engine being run in that changes its sound is telling you something.
Mistakes that ruin a new engine
Full throttle straight away 'to feel how it goes': the best way never to find out. The straight at constant fixed throttle: a prolonged uniform load exactly when variability is needed. Fake running in at idle, standing in the pits: a long idle lubricates poorly and beds nothing in: you need variable load, not waiting.
Frequently asked questions
Which fuel mix is used for running in?
3% for the first 10 litres of petrol: 0.3 litres of oil per 10 litres, i.e. 30 ml per litre. Then back to 2% (1:50). Only fully synthetic oil, such as XPS Castor Racing Oil.
How long does running in take?
The numbers are precise: three 15-minute track sessions, the first up to 10,000 rpm, the second up to 12,000 rpm, the third at full load, with a long gear ratio, a main jet 2 sizes larger and coolant at least 55 °C. The 3% mix, on the other hand, is kept for the first 10 litres of petrol. After that, the engine can use all its power.
What is the most serious mistake?
Skipping the gradual sessions and going straight to full load, or running with the coolant below 55 °C. Before full load you need 15 minutes up to 10,000 rpm and 15 minutes up to 12,000.
What should I check during running in?
Water temperature, spark plug colour at the end of each session and tightening torques. And keep your ears open: new noises must not be ignored.
Is running in needed after a rebuild too?
Yes: running in is done at first start-up and must be repeated after any work on the crankshaft or cylinder parts (lapping and/or boring, piston, rings). Rebuild done = running in to be repeated.
Conclusion
Running in is an investment: one session of patience in exchange for an engine that performs and lasts. 3% mix, revs that rise in stages, checks and zero hurry. And if we supplied your new engine, you already know who to message with any doubts during the first outing.
Procedure data: official Rotax 125 MAX evo operator's manual (2024 edition).
Questions about this topic?
Message us on WhatsApp: we answer personally, with the experience of people who live at the track. And if you want to drive a real racing kart, with Arrive & Drive you get into one of ours, ready on track. The right spare part? You will find it in the shop.
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Read also: The Rotax engine radiator: the right temperature and how to manage it → · Rotax engine maintenance: routine, rebuild and when to do it → · Kart fuel mix: the right petrol, oil and ratios for Rotax engines →
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Author: Sergio, TKS RaceLab | Published: 6 July 2026 | Updated: 4 October 2026
TKS RaceLab is the Italian online store dedicated to karting: spare parts, accessories and everything you need to get on track. With Arrive & Drive you arrive at South Garda Karting (Lonato), Franciacorta, Cremona or Ala and find your kart ready to go. No hassle, just driving.
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