Rotax Conversion Kit from Junior to Senior MAX evo: What You Need, Cost and Who Fits It
Updated: 3 days ago
The driver grows up, turns 14 and the Junior MAX evo starts to feel too small. The simplest way to move up to the Senior MAX evo without buying a new engine is the Rotax conversion kit 281377: all the parts to turn the Junior into a Senior, with the E-RAVE exhaust valve.
In short: the Rotax conversion kit 281377 turns a 125 Junior MAX evo into a 125 Senior MAX evo with Plasma cylinder, E-RAVE exhaust valve, piston, pneumatic system, gaskets and small parts. The ECU 666815 is not part of the kit. Price: €1,047 VAT included, €1,258 with the ECU. It must be fitted by a Rotax service centre, because the cylinder and the valve are sealed engine parts.
What changes between Junior and Senior MAX evo
According to the Rotax data sheets, the 125 Junior MAX evo delivers 17 kW (23 hp) at 8,500 rpm and has a cylinder without exhaust valve, part number 413530. The 125 Senior MAX evo reaches 22 kW (30 hp) at 11,500 rpm and has the cylinder with E-RAVE pneumatic exhaust valve. The ECU changes too: 666813 on the Junior, 666815 on the Senior.
This is why the kit works on the top end of the engine: cylinder, piston, exhaust valve and its pneumatic system.
What is in the Rotax conversion kit 281377
Cylinder and piston: Rotax Plasma cylinder Senior MAX, cylinder gasket set 296161, stud 441358, new Rotax piston with ring, piston pin 216135, needle cage 932027.
Exhaust valve flange (Plasma cylinder): gasket 450415, support plate 251337, gasket 250230.
E-RAVE exhaust valve: valve housing kit 253385, valve blade 253721, valve piston 854440, bellow 260723, spring 239952, cover 911559, oil seal 930500, O-rings 230261 and 430220, adjustment screw 241220.
Pneumatic system: solenoid valve 664630, check valve 660543, T-fitting 660550, 280 mm hose 660574, 420 mm hose 201576, 2.5 m tube 974562, impulse nozzle 956305.
Small parts: hose clip springs, screws, nuts, washers and cable tie.
You will find it in the shop: Rotax Junior MAX evo to Senior MAX evo Conversion Kit (281377).
The ECU is not part of the kit: which one you need
In the price list the kit is called "complete without ECU". The Junior uses the ECU 666813, the Senior the 666815: to race in Senior you must fit the Rotax MAX EVO ECU 666815. The RMC Italy 2026 technical regulations require a check with the Rotax tester 276230, which must read 666815MAX on the Senior.
Plasma cylinder and the gaskets of the new flange
The cylinder in the kit is the new Plasma cylinder, with the bore finished with a plasma coating. On the Senior the exhaust valve flange is new and three parts are needed: gasket 450415, support plate 251337 and gasket 250230. They are already in the kit. All the details are in the guide Rotax Plasma Cylinder 2027.
How much the Rotax conversion kit costs
Item | Price VAT included |
Rotax conversion kit 281377 | €1,047 |
Rotax MAX EVO ECU 666815 | €211 |
Total kit + ECU | €1,258 |
On top of this comes the fitting at the Rotax service centre.
Who fits it: Rotax service centre and engine seal
The cylinder and the exhaust valve are sealed engine parts: the kit must be fitted by a Rotax service centre, which reseals the engine and updates the identity card from Junior to Senior. During fitting the centre also checks the squish: under the RMC Italy 2026 technical regulations, measured with 2 mm solder, the minimum is 1.00 mm on the Senior and 1.20 mm on the Junior. What you can and cannot touch is explained in Rotax engine seal and regulations.
When it makes sense to move up to Senior
The Rotax data sheets indicate the Junior MAX evo from 12 to 15 years and the Senior MAX evo from 14 years. In Italy you need the right licence: G from 12 years for the Junior, F from 14 years for the Senior. If the Junior engine is in good condition, the Rotax conversion kit is the most direct way to move up a class while keeping the rest of the engine.
All the differences between the two engines, with the full table, are in Rotax Junior vs Senior MAX evo: technical differences and when to move up to Senior.
How the valve that comes with the kit works is explained in The Rotax Exhaust Valve: How It Works and How to Maintain It.
Frequently asked questions
Is the ECU included in the Rotax 281377 conversion kit?
No. In the price list the kit is "complete without ECU": to race in Senior MAX evo you need the Rotax ECU 666815, sold separately (€211).
Which ECU does the Senior MAX evo need?
The 666815. The Junior MAX evo uses the 666813. The RMC Italy 2026 technical regulations require a check with the Rotax tester 276230, which must read 666815MAX on the Senior.
Can I fit the Rotax conversion kit myself?
No. The cylinder and the exhaust valve are sealed engine parts: the kit must be fitted by a Rotax service centre, which reseals the engine and updates the identity card from Junior to Senior.
Does the kit contain the Plasma cylinder?
Yes. The kit contains the Plasma Senior MAX cylinder with the cylinder gasket set 296161 and the gaskets for the new exhaust valve flange: 450415, 251337 and 250230.
How much does it cost to move from Junior to Senior?
The Rotax conversion kit 281377 costs €1,047 VAT included and the ECU 666815 costs €211: €1,258 VAT included in total, plus fitting at the Rotax service centre.
After the conversion, is the engine a Senior MAX evo in every respect?
Yes, once the Rotax service centre has fitted the kit and the ECU 666815, resealed the engine and updated the identity card. Always check the regulations of your championship too.
Source: Rotax 2026 parts price list, Rotax 125 MAX evo 2025 parts catalogue, Rotax Tech Tip 07, Rotax 125 Junior and Senior MAX evo data sheets and RMC Italy 2026 technical regulations.
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Read also: Rotax Plasma Cylinder 2027 → · Rotax engine seal and regulations → · The Rotax exhaust valve →
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Author: Sergio, TKS RaceLab | Published: 20 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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