Before you start
Engine ID and top end
- Read the casting before you order anything.
- Stop if the bike is a four-stroke.
- Measure only against the manual. Limits are not here.
How many steps: 8
Guide 03 · Two-stroke
Engine ID and top end
Tell a Rotax 122 from a Rotax 123, then measure the barrel, piston, rings and reeds without using a clearance this site does not have.
Step 1 of 8
Clean the left side and read the casting.
Stop if this is a four-stroke.
Rotax 123 is cast at the left rear by the engine number. Rotax 122 is on top of the left crankcase, by the cylinder. A 1998 bike can be either.
Step 2 of 8
Match the carburettor to that casting.
VHSB 34 and SEM ignition with a 123. PHBH 28 with a 122, or VHST 28 on the Euro 3 bike. The VIN does not tell you the casting.
Step 3 of 8
Write it down. Do not order a piston yet.
If the casting is unreadable, do not order.
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.
Step 4 of 8
Decarbonise until you can see metal.
Do not dig the plating or the head face.
Whether a barrel is plated or iron is To verify. If you do not know, stop scraping.
Step 5 of 8
Measure bore, piston and ring gaps.
You'll need: The workshop manual for this engine.
Do not file a ring to a forum gap.
Positions, piston height, clearance and ring limits are To verify. A single mid-bore number is not a service limit.
Step 6 of 8
Hold the reeds closed against a light.
Chips, or a gap you can see, mean new petals. The allowable gap is To verify.
Step 7 of 8
Fit a new head O-ring and base gasket.
Gaskets do not swap between the 122 and the 123.
Base-gasket thickness changes the squish. The squish figure is To verify. Do not stack gaskets to be safe.
Step 8 of 8
Torque the head the way the manual says.
The pattern and the figure are To verify. MCN's planning figures are rings around every 5,500 miles and piston and barrel around every 10,000. Those are not wear limits. Warm to 60°C before hard use.
Finished!
This guide is done. Your place stays on this device.
Tools
- A camera for the casting and the carburettor stamp
- Feeler gauges
- A micrometer and a bore gauge. The sizes they must read are To verify
- A scraper that will not dig the sealing face. Material is your choice; the brief does not name one
- New base gasket and head O-ring for the correct engine. The old ones are not a template to reuse if they are crushed
- Torque wrench. The settings are To verify
- The workshop manual for this engine, 122 or 123, not a general Rotax booklet
Parts
- Piston, rings and barrel for the casting you have. See the parts directory.
- Reed petals
- Head O-ring and base gasket. They do not swap between 122 and 123.
- Small-end bearing, if the manual for your engine lists it as a service item. Confirmation is To verify.
Slots for these parts, with empty OEM numbers and empty prices, are in theparts directory.
| Item | Figure | Status |
|---|---|---|
| Rotax 123 identification | ‘Rotax 123’ cast at the left rear by the engine number. Dell'Orto VHSB 34. SEM ignition. | Sourced Research brief, from Hartrusion’s engine notes |
| Rotax 122 identification | ‘Rotax 122’ cast on top of the left crankcase by the cylinder. PHBH 28, or VHST 28 on the Euro 3 bike. | Sourced Research brief, from Hartrusion’s engine notes |
| What does not interchange | Cylinders, gearbox, clutch, gaskets and ignition. | Sourced Research brief |
| Service interval | Rings around every 5,500 miles. Piston and barrel around every 10,000 miles. | Sourced MCN, via the research brief |
| Ring end gap, piston clearance, bore limit, head torque, base-stud torque | Not in the brief. Leave the fields blank in your notebook until the manual fills them. | To verify Workshop manual for this engine |
Every other two-stroke 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. The four-strokes are not covered here at all. If the bike is an RS4 or a current RS 125, stop. You do not have a barrel and 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 until the Euro 3 bike, which uses a VHST 28 with a TPS and a heater. 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
With the correct manual open, the measurements the brief expects this guide to cover are the barrel, the piston, the ring gaps, the reeds, the head O-ring and the base gasket. The limits are not in the brief. The method, in outline, is:
- 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.
- Measure the bore in the positions the manual names. Those positions are To verify. A single mid-bore number is not a service limit.
- Measure the piston at the height the manual names, and subtract in the direction the manual names. Clearance is To verify.
- Square each ring in the bore and measure the end gap with a feeler gauge. The limit is To verify. Do not file a ring to a gap you saw on a forum.
- Hold the reeds closed against a light. Chips, and a gap you can see, mean new petals. The allowable gap is To verify. Petals are cheap relative to a crankcase full of pieces.
- 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 in the pattern and to the figure in the manual. Both are To verify. A cross-pattern is the usual practice on a multi-stud head and is not a substitute for the number.
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.
Common mistakes
- Identifying the engine from the registration year. 1998 can be either.
- Fitting a 34 mm VHSB piston kit to a 28 mm 122 because both say RS125 on the box.
- Reusing a base gasket because it ‘looked fine’.
- Setting a ring gap to a figure from a different two-stroke.
- Applying the two-stroke top-end intervals to an RS4 or a current RS 125.
Sources
- Differences between Rotax 122 and 123 (opens in a new tab) — Hartrusion
- RS 125 (1995–2012) review and buying guide (opens in a new tab) — MCN
- Aprilia RS125 parts and engine ID (opens in a new tab) — PJME
- RS50 D50B engine (opens in a new tab) — Pedparts
Research brief dated 24 October 2026. Where this page says “the brief”, that is the sourced master plan behind the first edition.