Before you start
Buying guide
- Read the plate and the casting before you believe the advert.
- A 122 and a 123 do not share parts. A replica is only stickers.
- Write the asking price down. This page does not value the bike.
How many steps: 10
Before you pay
Ten checks, then the awkward questions.
There is no priced market survey in the research brief, so this page does not tell you what a bike is “worth”. It tells you what to look at, and how to write the asking price down when you have seen it.
Step 1 of 10
Read the frame number before you trust the advert.
Use the decoder, then look at the plate yourself. The year in the advert is not the frame number.
Step 2 of 10
Find the casting. 1998 can be either engine.
A 122 and a 123 do not share parts.
Cylinders, gearbox, clutch, gaskets and ignition do not swap.
Step 3 of 10
Read the carburettor and match it to the engine.
VHSB 34 belongs with a 123. PHBH 28 belongs with a 122. VHST 28, with a sensor and a heater, is the Euro 3 bike.
Step 4 of 10
Look for a blade or a blanking plate.
UK bikes were often sold at 15 bhp. That is not the same sentence as the 11 kW A1 rule.
Step 5 of 10
On a 2003-on two-stroke, assume a catalyst.
The Euro 3 bike has two catalysts.
Step 6 of 10
See whether the powervalve moves.
A seized solenoid is a named fault on the bee-tail.
Step 7 of 10
Look for rot, rumble and a tired starter.
Exhaust rot, a worn speedo drive on a pre-2006 bike, crank rumble, and a tired starter freewheel on an early 123.
Step 8 of 10
Check the clocks before you believe them.
The mechanical tacho drive on the Extrema is no longer available. Old digital clocks fail.
Step 9 of 10
Compare the paperwork with the frame number.
A UK V5C, an unfinished Italian import, or a number that does not match. A replica is a decal scheme, not a different engine. A 1998 Chesterfield is not a sourced scheme.
Step 10 of 10
Write the asking price and the date.
This page does not say what a bike is worth.
A scarce 123 is not the same purchase as a 122 with a rotten pipe. The parts directory has no prices on purpose.
Finished!
This guide is done. Your place stays on this device.
The ten checks
- Read the frame number or VIN before you believe the year in the advert. Use the decoder, then look at the plate yourself.
- Find the engine casting. A 1998 bike can be a Rotax 122 or a Rotax 123. Cylinders, gearbox, clutch, gaskets and ignition do not swap.
- Read the carburettor. VHSB 34 belongs with a 123. PHBH 28 belongs with a 122. VHST 28, with a TPS and a heater, is the Euro 3 bike.
- Ask whether it is restricted, and look for a powervalve blanking plate rather than a RAVE blade. UK bikes were often sold at 15 bhp. That is not the same sentence as the current 11 kW A1 rule. Both are on the UK law page.
- On a 2003-on two-stroke, assume a catalyst in the expansion chamber until you know otherwise. The Euro 3 bike has two.
- On a two-stroke, see whether the powervalve moves and whether the solenoid is seized. That fault is named on the bee-tail.
- Look for exhaust rot, a worn speedo drive on a pre-2006 bike, crank rumble, and — on an early 123 — a tired starter freewheel.
- Clocks. The mechanical tacho drive on the Extrema is no longer available. Old digital clocks fail.
- Paperwork. A UK V5C, an unfinished Italian import, or a frame number that does not match the document. A “replica” is a decal scheme, not a different engine.
- Write the asking price in the log below, with the date. The parts directory has no prices on purpose. A scarce 123 is not the same purchase as a 122 with a rotten pipe.
Questions for the seller
- Can you photograph the frame plate and the engine casting, in focus?
- Has the crank been apart, and were the seals replaced?
- Is the powervalve a blade or a plate, and has the CDI been opened?
- For a 2006–2007 bike: does the immobiliser behave, and do you have both keys?
- For a replica: which year’s decal set is on it, and do you have photos of the bike before it was stickered?
- Why is a 1998 Chesterfield called a Chesterfield? The sourced schemes skip 1998.
Red flags by generation
| Generation | Walk away, or budget for it |
|---|---|
| Extrema / GS | Starter freewheel and flywheel ring gear. Mechanical tacho drive, which the brief says is no longer available. SEM stator. Discontinued 123 parts. |
| Round-light, including 1998 | The wrong engine story. 123 parts scarce. Failed clocks. |
| Bee-tail | Catalyst from 2003, CDI flat spot, stuck powervalve, worn speedo drive, crank bearings, rotten exhaust. |
| RSV-style | Immobiliser trouble, exhaust corrosion, plus the bee-tail’s engine faults. The frame is still the MP frame, so a “new chassis” claim is wrong. |
| Euro 3 | Extra sensors and two catalysts. Fussy, though the TPS ignition is well regarded in the brief. The 18 kW figure for the RM may be the exhaust, not a measured output. |
| RS4 and current RS 125 | No fault list was researched. To verify. Do not apply two-stroke advice, and do not buy RS50 D50B0 parts for one. |
Prices, in pounds, without a valuation
Asking prices move with the engine, whether the numbers match, whether the pipe is solid, whether the restriction is honest, and whether the V5C matches the frame. None of those has a pound figure in the brief. When you see a bike, add a row. A forum “worth about” is not a valuation.
| Date checked | Year and engine | Asking price | Where | Notes |
|---|---|---|---|---|
| — | — | Not recorded | — | No priced survey in this edition. |
Decal kits are the one price-ish figure in the brief: Decalmoto at roughly €95–€140. That is euros, it is “roughly”, and it is not what the motorcycle costs.
Sources
- RS 125 (1995–2012) review and buying guide (opens in a new tab) — MCN
- Differences between Rotax 122 and 123 (opens in a new tab) — Hartrusion
- Modellübersicht RS 125 2T (opens in a new tab) — Hartrusion
- Storia e modelli (opens in a new tab) — rs125.it
- RS 125 replica decal sets by year (opens in a new tab) — Decalmoto
- Aprilia RS125 parts and engine ID (opens in a new tab) — PJME
Research brief dated 24 October 2026. Where this page says “the brief”, that is the sourced master plan behind the first edition.