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.
Spring bolts are 7 Nm on both engines. The 123 centre nut is 80 Nm. The 122 has no centre nut in the brief, and Haynes lists that nut for the 123 only. 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. Crank axial clearance is not the same on a 122 and a 123.
- 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 clearances, Type 122 | Axial 0.09–0.59 mm. Runout maximum 0.03 mm. Big-end radial maximum 0.05 mm. Big-end axial 0.35–0.93 mm, service limit 1.30 mm. | Sourced Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| Crank clearances, Type 123 | Axial 0.20–0.40 mm. Runout maximum 0.03 mm. Big-end radial maximum 0.05 mm. Big-end axial 0.59–0.94 mm, service limit 1.30 mm. | Sourced Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| Clutch plates | 122: 6 friction and 6 plain. Friction service limit 2.8 mm. Spring free length service limit 31.6 mm. 123: 6 friction and 5 plain. Pack service limit 21.2 mm. Spring free length service limit 34.2 mm. | Sourced Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| Gear ratios | Primary 3.315:1 on a 122 and 3.200:1 on a 123. The internal ratios and the final drive also differ. Full table on the engine specs page. | Sourced Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| Torque | Crankcase bolts 10 Nm. Clutch spring bolts 7 Nm. Clutch centre nut 80 Nm on the 123 only. Crankshaft pinion nut 70 Nm on the 123. Alternator rotor nut 70 Nm. | Sourced Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| Balance-shaft gear timing | The gears have a timing relationship. The tooth marks are not published. | Not published Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
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.
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. Big-end radial clearance is 0.05 mm maximum on both engines. Big-end axial clearance is 0.35–0.93 mm on a 122 and 0.59–0.94 mm on a 123, with a service limit of 1.30 mm. Crank axial clearance is a different measurement again. The full table is on the engine specs page.
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. Haynes 4298 agrees on the count: a 122 has 6 friction plates and 6 plain plates, a 123 has 6 friction plates and 5 plain plates. Spring bolts are 7 Nm. The 123 centre nut is 80 Nm. Thread direction is Not published. If the manual and the casting disagree, believe the casting. Plates and baskets 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.
Notes from readers
No reviewed note is attached to this page yet.Be the first to send one.
Spot a mistake? Tell usIt is reviewed before anything is published.
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 review and buying guide (opens in a new tab) — MCN
- Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) (opens in a new tab) — Haynes
Research brief dated 24 October 2026. Where this page says “the brief”, that is the sourced master plan behind the first edition.