Engine, Type 122 and Type 123
- Same capacity. Different compression and piston codes.
- Ring gaps and clutch plate counts do not match.
- Reed data is not published.
RS125 · 1993–2006
Engine, Type 122 and Type 123
Same capacity, different compression, piston codes, ring gaps, clutch and gearing. Read the casting before you use a column.
Haynes 4298 gives these as Type 122 and Type 123. It does not say which registration years used which engine. This site's existing record still stands for that: Italy moved to the 122 in 1995, and a 1998 bike can be either. To verify on the casting.
| Capacity | 124.82 cc |
|---|---|
| Bore × stroke | 54.0 × 54.5 mm |
| Cooling | Liquid |
| Clutch | Wet multi-plate |
| Gearbox | Six-speed, constant mesh |
| Final drive | Chain |
| Compression ratio | 122: 12.0:1 to 13.0:1. 123: 13.7:1 to 14.7:1. |
Piston, Type 122
Diameter measured 11.5 mm up from the skirt, at 90° to the gudgeon pin. Piston-to-bore service limit 0.08 mm.
| Code | Diameter | Service limit |
|---|---|---|
| A | 53.98 mm | 53.93 mm |
| B | 53.99 mm | 53.94 mm |
| 1st oversize | 54.00 mm | 53.95 mm |
| 2nd oversize | 54.01 mm | 53.96 mm |
Ring end gap, new: 0.05–0.2 mm. Service limit 0.8 mm. Ring-to-groove clearance service limit 0.1 mm.
Bore, Type 122
Measured 15 mm down from the top edge, at 90° to the pin. Out-of-round service limit 0.02 mm.
| Code | Bore | Service limit |
|---|---|---|
| A | 54.00–54.01 mm | 54.06 mm |
| AB | 54.01–54.015 mm | 54.065 mm |
| B | 54.015–54.025 mm | 54.075 mm |
Piston, Type 123
Same measuring height, 11.5 mm up from the skirt. Size codes are Red and Green, not A and B. Piston-to-bore service limit 0.11 mm. Per-code piston service limits are not printed for the 123.
| Code | Diameter |
|---|---|
| Red | 53.98 mm |
| Green | 53.99 mm |
| 1st oversize | 54.00 mm |
| 2nd oversize | 54.01 mm |
Ring end gap, new: 0.15–0.3 mm. Service limit 0.8 mm. Ring-to-groove clearance service limit 0.2 mm.
Bore, Type 123
Measured 15 mm down from the top edge. Out-of-round service limit 0.02 mm.
| Code | Bore | Service limit |
|---|---|---|
| Red | 54.00–54.012 mm | 54.082 mm |
| Green | 54.012–54.024 mm | 54.094 mm |
Crankshaft
| Check | Type 122 | Type 123 |
|---|---|---|
| Axial clearance | 0.09–0.59 mm | 0.20–0.40 mm |
| Runout, maximum | 0.03 mm | 0.03 mm |
| Big-end radial, maximum | 0.05 mm | 0.05 mm |
| Big-end axial | 0.35–0.93 mm, service limit 1.30 mm | 0.59–0.94 mm, service limit 1.30 mm |
| Balancer journal service limit | Left 14.94 mm, right 24.94 mm | Left 14.94 mm, right 24.94 mm |
Clutch
| Check | Type 122 | Type 123 |
|---|---|---|
| Friction plates | 6 | 6 |
| Plain plates | 6 | 5 |
| Friction thickness | Service limit 2.8 mm | Standard 3.0 mm |
| Plain plates | Distortion maximum 0.15 mm | Standard thickness 1.0 mm |
| Further limit | Spring free length service limit 31.6 mm | Plate pack service limit 21.2 mm. Spring free length service limit 34.2 mm. |
Gear ratios
| Ratio | Type 122 | Type 123 |
|---|---|---|
| Primary | 3.315:1 (63/19) | 3.200:1 (64/20) |
| Final | 2.353:1 (40/17) | 2.785:1 (39/14) |
| 1st | 3.000:1 (30/10) | 3.400:1 (34/10) |
| 2nd | 2.071:1 (29/14) | 2.308:1 (30/13) |
| 3rd | 1.588:1 (27/17) | 1.688:1 (27/16) |
| 4th | 1.316:1 (25/19) | 1.316:1 (25/19) |
| 5th | 1.143:1 (24/21) | 1.143:1 (24/21) |
| 6th | 1.045:1 (23/22) | 1.045:1 (23/22) |
Reed valve: Not published. Haynes 4298 records no RS125 reed data. The mounting bolts are on the torque page.
Notes from readers
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Sources
Research brief dated 24 October 2026. Where this page says “the brief”, that is the sourced master plan behind the first edition.