Before you start
Bottom end
- Know which Rotax you have before the cases open.
- Photograph the timing marks before a gear can move.
- The 123 clutch is a nut. The 122 clutch is not.
How many steps: 8
Guide 04 · Two-stroke
Bottom end
Crank, main bearings, seals, balance-shaft timing, gearbox and the clutch fastening that differs between the 123 and the 122.
Step 1 of 8
Split the cases only for a real fault.
A tired piston is not a reason to split.
A rumbling bearing, a notched crank, a dry seal, or a gearbox jumping out of gear. Up-and-down play and side play are To verify before you split a healthy engine.
Step 2 of 8
Photograph the flywheel and the stator first.
A puller that nearly fits will strip the threads.
On a 123 the stator is the scarce SEM part. The tool number is To verify.
Step 3 of 8
Replace crank, bearings and seals as a set.
Do not use the other engine's part numbers.
Bearing numbers are To verify. A listed 122 crankshaft is a 122 part. A 123 crank is different.
Step 4 of 8
Fit each seal as the manual shows.
You'll need: Grease for the seal lip.
Lip direction is To verify. A dry seal on first start scores the lip.
Step 5 of 8
Photograph every balance-shaft mark first.
If the gears are unmarked, stop.
Photograph the marks in the position the cases actually sit. Tooth count and mark positions are To verify. One tooth out feels like a carburettor fault.
Step 6 of 8
Keep each gearbox part with its own shaft.
End floats are To verify. If you did not measure them on the way apart, you have no baseline.
Step 7 of 8
Strip the clutch for this casting only.
The 123 clutch is a nut. The 122 clutch is not.
Thread direction, the holding method and the torque are To verify. If the manual and the casting disagree, believe the casting.
Step 8 of 8
Turn the engine by hand before adding fuel.
You'll need: The plug out.
Go through the gears. A tight spot now is a shim. A tight spot on the first ride is a case.
Finished!
This guide is done. Your place stays on this device.
Tools
- A flywheel puller if the manual requires one. The tool number is To verify
- A clutch holding tool only if it matches this engine. The 123 and the 122 are not the same job
- Bearing pullers and a press, or a shop that will press while you watch the timing marks
- Feeler gauges and a dial indicator if the manual asks for crank end-float. The float is To verify
- Torque wrench
- Clean boxes. Gearbox shim stacks are not ‘close enough’
- The manual for this engine, open at the balance shaft before the cases come apart
Parts
- Crankshaft for a 122 or a 123, not ‘an RS125 crank’.
- Main bearings and crank seals for that engine.
- Clutch plates if they are glazed. The basket and the hub may not match the other Rotax.
- Gasket set for the cases. It does not interchange.
Slots for these parts, with empty OEM numbers and empty prices, are in theparts directory.
| Item | Figure | Status |
|---|---|---|
| Clutch fastening | The 123 clutch has a nut. The 122 clutch does not. | Sourced Research brief, from the 122 / 123 differences |
| Crank, mains, seals, gearbox | In scope for this guide. Part numbers and sizes are not recorded. | To verify Workshop manual and the parts catalogue for this VIN |
| Balance-shaft gear timing | The gears have a timing relationship. The tooth marks are not in the brief. | To verify Workshop manual for this engine |
| Case-bolt torque, primary-nut torque, clutch-nut torque | Not in the brief. The 122 may not have the clutch nut at all. | To verify Workshop manual for this engine |
The bottom end is the job that ruins a winter if the timing marks are lost or the wrong clutch sequence is followed. Read engine ID first. This page assumes you know which Rotax is in the frame. It does not apply to the RS4 or the current four-stroke.
When you need to be in here
Crank bearings are a named fault on the bee-tail, and a knocking two-stroke with play at the big end is not a top-end job. Seals that weep oil into the crankcase, or that let gearbox oil towards the intake side, lean the engine out or wet the plug. The brief does not give a leak-down figure. Up-and-down play and side play are To verify against the manual before you split a healthy engine “while you are in there”.
A full split is justified when the bearing is rumbling, the crank is notched, a seal has been running dry, or the gearbox is jumping out of gear. It is not justified by a tired piston.
Crank, bearings, seals
Photograph the flywheel and the stator before they come off. On a 123 the stator is the scarce SEM part. On a 122 it is not. A puller that nearly fits will strip the flywheel threads. The tool number is To verify.
When the cases open, the crank, the main bearings and the seals come out as a set in any serious job. Bearing numbers are To verify. Do not read a number off the other engine and call it close. Pedparts is named in the brief as listing a Rotax 122 crankshaft. That is a 122 part. A 123 crank is a different, and often discontinued, part.
Seals go in the direction the manual shows. Lip direction is To verify. Grease the lip. A dry seal on first start is a scored lip.
Balance shaft
These engines have a balance shaft, and the brief puts gear timing in this guide for that reason. Before the gears can move, photograph every mark in the position the cases actually sit, not the position you wish they sat. If the gears are unmarked when you open the cases, stop. The tooth count and the mark positions are To verify. Do not “line up the dots” from a different year and hope. A balance shaft one tooth out is a vibration you will chase as a carburettor fault for months.
Gearbox
Keep each selector, fork and shim with the shaft it came from. A drawing in the manual beats a memory. End floats are To verify. If you did not measure them on the way apart, you do not have a baseline, and “it felt alright” is not a measurement.
The clutch
This is the difference people break. The brief says the 123 clutch is fastened with a nut and the 122 clutch is not. Do not put a 123 strip sequence onto a 122, or the reverse. Thread direction, the holding method and the torque are To verify in the manual for the engine you have. If your manual and the casting disagree, believe the casting and find the right manual. Plates and baskets are in the list of parts that do not swap.
Refit using the photos. Then turn the engine by hand through the gears with the plug out, before you ever put fuel in it. A tight spot now is a shim. A tight spot on the first ride is a case.
Common mistakes
- Following a 123 clutch sequence on a 122. One has a nut. The other does not.
- Splitting the cases without a photo of the balance-shaft marks.
- Reusing a crank seal because the lip ‘felt soft, not torn’.
- Mixing gearbox shims from side to side.
- Buying a 122 crank, which the brief says Pedparts lists, for a 123 bottom end.
Sources
- Differences between Rotax 122 and 123 (opens in a new tab) — Hartrusion
- Aprilia RS125 parts and engine ID (opens in a new tab) — PJME
- RS 125 (1995–2012) review and buying guide (opens in a new tab) — MCN
Research brief dated 24 October 2026. Where this page says “the brief”, that is the sourced master plan behind the first edition.