How to install a Rotax engine on a kart chassis: step-by-step guide and torques
In short To install a Rotax engine on a kart chassis, bolt the engine plate under the engine, place strips of abrasive paper on the chassis tubes so the engine cannot slip, and then fix the clamps. Fit the chain, move the engine forward with the chain tensioner until there is about 5 mm of play and check with a straight edge that the sprockets are aligned. Then connect the fuel line, wiring and battery, the radiator and the exhaust, mounted on rubbers and springs and never rigid. Finally fit the spark plug tightened to 25-27 Nm and check the transmission oil before starting. The DD2 is different: it has no chain, the rear axle passes through the engine, which drives it through a coupling, and it only goes on chassis prepared for it.
Before you install a Rotax engine: what is in the boxes
The Rotax 125 MAX evo engine comes in two boxes. The first holds the assembled engine, already filled with transmission oil. The second holds the accessories: radiator with hoses, mounting plate with ignition coil, solenoid valve and spark plug cap, battery box and wiring harness, battery charger, intake silencer, fuel pump with hose, exhaust system, manuals and engine identity card, small parts with the spark plug, exhaust valve hoses, ECU and carburettor.
Check that everything is there before you start. The engine serial number is stamped on the clutch side crankcase half: it is the same number that must appear on the engine identity card, which you need for the warranty and, if you race in the Rotax MAX Challenge, for sealing.
One last thing before you begin: these instructions cover the engine. For the fixings on the chassis also follow the instructions of your chassis manufacturer.
How to install a Rotax engine on the chassis: engine plate and clamps
On chassis for centrifugal clutch engines the Rotax engine is fixed to the chassis tubes with the engine plate, bolted under the engine, and the clamps that lock it on the chassis tubes. The sequence is:
Bolt the engine plate under the engine.
Put strips of abrasive paper on the chassis where the engine will sit: they stop the engine from moving when you race.
Place the engine on the chassis and fit the clamps without tightening them: the engine must still be able to slide.
Push the engine as far back as possible, fit the chain, first on the front sprocket and then on the rear sprocket.
Move the engine forward to tension the chain and, when the tension is right, lock the position with the chain tensioner.
Tighten the clamp screws.
Check with a straight edge that front and rear sprockets are aligned.
Engine plate dimensions: the engine is bolted to the plate with 4 M8 screws of minimum grade 8.8, tightened to 24 Nm, in 4 holes of 8.5 mm on a rectangle of 80 x 102 mm. The Rotax can run inclined forward from 0° to 15°. The engine is only locked on the chassis once the chain is fitted, aligned and tensioned.
The chain: alignment and tension
The chain is not in the box, because its length depends on the gear ratio. The 125 MAX evo engines need a 219 pitch chain, 7.75 x 4.6 x 4.5 mm, such as the Panther 219 Racing chain.
Alignment: lay a straight edge across the front and rear sprockets. If they are not in line, slide the sprocket carrier along the axle. A 219 pitch chain aligner makes the check quicker.
Tension: the chain play, how far it moves up and down when you push it with a finger halfway between the sprockets, should be about 5 mm. You set it by moving the engine with the chain tensioner, such as the Maranello forged chain tensioner.
The clutch cover only protects against grease from the chain: it does not protect against contact with the moving clutch and drive. Keep your hands away when the engine is running.
For chain maintenance see our guide on kart chain tension, lubrication and replacement.
The fuel system: pump, filter and lines
The fuel pump comes already fitted on its plate, with the 230 mm and 1,800 mm hoses. The pump bracket is fixed with 3 M6x30 screws at 6 Nm.
Short 230 mm hose: from the pump to the carburettor.
Long hose: from the pump to the filter and from the filter to the tank. The arrow on the Rotax fuel filter must point towards the pump.
Routing: the line from the tank to the filter runs on top of the chassis tubes and must not touch moving parts or the track.
Cable ties: they must not pinch either the fuel line or the vacuum hose: the pump works with the engine vacuum.
Engines without exhaust valve: on Junior, Mini and Micro the vacuum hose that drives the pump is 400 mm long and is secured with a cable tie.
The carburettor goes into the socket in a vertical position and is tightened with the hose clamp. The throttle cable runs on top of the chassis tubes. If you need a spare, the shop has the Rotax Dell'Orto fuel pump and fuel hose by the metre.
Electrical system and battery
Wiring harness: lay it on the chassis and start connecting from the engine side, so it is not under tension. Extra length is coiled in loops and must never touch moving parts.
Connectors: unplug them by pulling the connector, never the cable. On pick-up and ignition coil press the catch first.
Battery: the holder goes on the left side pod, next to the seat, with two clamps for 30-32 mm tubes. Do not overtighten the screws: the clamps can break.
Terminals: the battery terminals face the ECU. Connect the positive, red, first, then the negative, black, and avoid any short circuit.
The shop has the Rotax battery and the Rotax evo battery holder.
The radiator
On the 125 MAX evo the radiator is fixed to the engine with an M8x75 screw and its washer, then the two hoses are connected with 16-25 mm hose clamps.
Large radiator: about 0.7 litres, for Senior, Junior and Mini.
Small radiator: about 0.5 litres, for Mini and Micro.
Hoses: they come out of the box the same length, but the lower hose must be shortened.
Air: the air stream must cover the whole radiator.
To keep the water temperature within limits you need a temperature sensor, such as the Alfano water temperature sensor. The complete spare is the Rotax MAX evo radiator.
The exhaust
The exhaust must be mounted flexibly: a rigid mounting makes it crack.
Front: plate with rubber buffers and springs.
Rear: a heat-resistant M8 rubber mount on the two lugs under the exhaust.
Routing: set the chassis supports so the exhaust follows the straightest possible line from the socket to the silencer, without stressing the ball joint.
Ball joint: it is held by two springs. To fit them there is the Rotax spring hook, part number 251680. Do not stretch the springs more than needed.
Gasket: it is the only seal between engine and exhaust, no sealant is needed. An exhaust leak costs power.
The exhaust socket diameter changes with the category: 37 mm for Senior and Junior, 22 mm for the Mini from 2020 (20 mm on earlier Minis), 18 mm for the Micro. Spares: complete Rotax MAX evo exhaust, exhaust spring, exhaust rubber mount and exhaust support.
Spark plug, coil plate and transmission oil
Coil plate: the plate with ignition coil and solenoid valve comes pre-assembled and is fixed to the crankcase with 2 M6x20 screws, washers and lock nuts.
Transmission oil on the MAX: it comes already filled, but must be checked before installing the engine. With the engine level, drain it from the drain screw and measure it: it must be 100 cc of SAE 15W-40 engine oil. The screw is refitted at 10 Nm with a new washer.
Breather: remove the cap from the vent screw and connect it with a hose to a collecting container, without the hose reaching the bottom.
The Rotax DD2: what changes when you install it
The 125 MAX DD2 evo has a two-speed gearbox and no chain: the rear axle passes through the engine, which drives it through a coupling that grips the axle itself.
Chassis: the DD2 only goes on chassis prepared for this engine.
Axle: in the area of the four coupling bushes it must be smooth, with no keyways.
Engine mounts: the 4 M8x25 screws of the upper mount are tightened to 28 Nm, the 4 M8x30 screws of the lower mount to 22 Nm, with medium threadlocker. The screws must go into the crankcase by at least 16-20 mm.
Axle coupling: the clamping ring is tightened to 10 Nm, the 6 coupling screws to 7-8 Nm, repeating the round until all of them are at the right torque.
Radiator: on the DD2 it is fixed to the chassis on rubber mounts, tilted back by 25° plus or minus 5°. The hose must not rub against the seat and an overflow bottle is connected to the filler.
Gearbox oil: check it in the sight glass and top up through the vent until it reaches the middle of the glass. The magnetic drain plug is tightened to 20 Nm with a new washer. The DD2 exhaust has its own spare, the complete Rotax DD2 evo exhaust.
After installation: running in
A new engine must be run in before you push it hard: long gear ratio, main jet two sizes richer and, for the first 10 litres of fuel, a 3% mix. Then 15 minutes up to 10,000 rpm, 15 minutes up to 12,000 and 15 minutes at full load, going down one jet size at a time. The water must reach at least 55 °C: in cold weather tape over part of the radiator.
The full procedure is in our guide on Rotax engine running in.
Our advice
Install the engine calmly and with a torque wrench: almost every problem after an installation comes from a misaligned chain, an exhaust mounted rigidly or a pinched fuel line. Before the first outing check everything again: clamp screws, chain tension, hose clamps, exhaust springs and oil level.
If you are missing a part to complete the installation, the shop has original Rotax spare parts and chassis accessories. And if you have a doubt, message us on WhatsApp with a photo of your kart.
Frequently asked questions about installing a Rotax engine
How do you install a Rotax engine on a kart chassis?
With the engine plate bolted under the engine and the clamps on the chassis tubes, with strips of abrasive paper between clamps and chassis. Fit the chain, move the engine forward to the right tension, lock it with the chain tensioner, tighten the clamps and check sprocket alignment with a straight edge.
How tight should the Rotax chain be?
The chain play, halfway between the sprockets, should be about 5 mm. You set it by moving the engine with the chain tensioner.
Which chain does the Rotax 125 MAX evo need?
A 219 pitch chain, 7.75 x 4.6 x 4.5 mm. The length depends on the gear ratio chosen.
What torque is the Rotax spark plug tightened to?
25 to 27 Nm, after checking the electrode gap against the regulations.
Why must the Rotax exhaust not be mounted rigidly?
Because with a rigid mounting the exhaust cracks. It is mounted on rubber buffers and springs and must follow the straightest possible line from the socket to the silencer.
Is the DD2 installed like the other Rotax engines?
No. The DD2 has no chain: the rear axle passes through the engine, which drives it through a coupling that grips the axle, and it only goes on chassis prepared for it. The engine mounts are tightened to 28 Nm and 22 Nm with threadlocker.
Sources: BRP-Rotax, Installation Manual 125 Senior, Junior, Mini and Micro MAX evo, edition 03/2021; Installation Manual 125 MAX DD2 evo, edition 03/2021.
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: Rotax engine running in → · Kart chain tension and lubrication → · Rotax MAX torque settings → · Rotax engine exploded view → · Rotax engine maintenance →
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Author: Sergio, TKS RaceLab | Published: 22 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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