Every other engine job on this site assumes you have done this page. The 122 and the 123 look related and do not share the parts that wear out. Read the casting before you order a barrel, a piston or a reed block.

Read the casting

Clean the left side of the engine before you decide you cannot find a number.

On a Rotax 123 the brief says the words “Rotax 123” are cast at the left rear, by the engine number. The carburettor is a Dell’Orto VHSB 34 and the ignition is SEM. That is the Extrema / GS, and it is also many UK and European bikes up to about 1997–98, including some with the later round headlight.

On a Rotax 122 the words “Rotax 122” are cast on top of the left crankcase, by the cylinder. The carburettor is a PHBH 28. Italy moved to this engine in 1995. A 1998 UK or imported bike can be either. The VIN decoder tells you the model code. It does not tell you the casting. Look.

Write the result on the condition sheet from guide 1. If the casting is unreadable, do not order a piston. Take the cylinder off only far enough to see what you have, and still do not order until the casting or the engine number is legible enough to match a catalogue.

What you are measuring

The measurements are the barrel, the piston, the ring gaps, the reeds, the head O-ring and the base gasket. Sizes are in engine specs. They are not interchangeable between a 122 and a 123. The method, in outline, is:

1. Decarbonise until you can see metal, without digging the plating or the head face. How much plating these barrels have, and whether a given year is plated or iron, is To verify. If you do not know, stop scraping.
2. Measure the bore 15 mm down from the top edge, at 90° to the pin. Out-of-round service limit is 0.02 mm on both engines. The bore size codes are not the same. A single mid-bore number is not a service limit.
3. Measure the piston 11.5 mm up from the skirt, at 90° to the pin. Piston-to-bore service limit is 0.08 mm on a 122 and 0.11 mm on a 123.
4. Square each ring in the bore and measure the end gap. New gap is 0.05–0.2 mm on a 122 and 0.15–0.3 mm on a 123. Service limit is 0.8 mm on both. Do not file a ring to a gap you saw on a forum.
5. Hold the reeds closed against a light. Chips, and a gap you can see, mean new petals. The allowable gap is Not published. Petals are cheap relative to a crankcase full of pieces.
6. Fit a new head O-ring and a new base gasket for this engine. Gaskets do not swap. Thickness of the base gasket changes the squish. The squish figure is To verify, so do not stack gaskets to “be safe”.

Torque the head to the figure for that engine: 30 Nm on the 122 bolts, 20 Nm on the 123 nuts. The tightening order is Not published. A cross-pattern is not a substitute for that order.

Intervals that are sourced

MCN’s note in the brief is the one mileage guide you can quote: rings around every 5,500 miles, piston and barrel around every 10,000. That is a planning figure for the two-strokes, not a wear limit. Measure. And warm the engine to 60°C before you use a fresh top end hard. That warm-up is the same MCN note.

PJME is the UK specialist the brief names for pistons, barrels and 34 mm carburettors. The directory links there. It does not copy their prices.