Before you start
Suspension
- Confirm the fork before you buy a seal kit.
- A notch straight ahead is a worn head bearing.
- Do not invent a sag figure. It is not in this guide.
How many steps: 8
Guide 10 · All generations
Suspension
USD fork seals, the shock, headstock bearings and wheel bearings. Fork oil weight and quantity are in Haynes 4298. The RS125 oil level and the sag figures are not.
Step 1 of 8
Support the front so a leg can come out.
Confirm these are the forks on this bike.
2006 is a new fork. A bee-tail seal kit is not a default fit. The seal number is To verify.
Step 2 of 8
Photograph the yoke height, then drain the oil.
You'll need: A note of how much oil came out.
Haynes lists 430 cc of SAE 10W to 20W in each leg. What came out is a clue, not the fill. The RS125 oil level is not published.
Step 3 of 8
Split the leg only by the manual's method.
If it wants a special tool, wait for the tool.
The tool number is To verify. A homemade slide hammer is how the threads strip.
Step 4 of 8
Drive the new seal square, on the metal.
Do not bear on the sealing lip.
A scored bush needs a new bush, not only a new seal. Replace the dust cap with the seal. Fork tube runout service limit is 0.2 mm.
Step 5 of 8
Refill to the manual, with the spring out.
Use SAE 10W to 20W, 430 cc in each leg, spring out and the leg fully extended. The RS125 oil level is not published. Fork clamp bolts are 25 Nm. A leg that stays down is not a stiff seal to ride in.
Step 6 of 8
Check the shock for leaks and knocking.
Do not compress a spring with screwdrivers.
Shock upper is 50 Nm and lower is 25 Nm. Link arm to swingarm is 80 Nm. Linkage rods are 50 Nm. Eye-to-eye length, spring rate and sag are not published. A shock from another generation may bolt up and sit wrong.
Step 7 of 8
Lift the front and feel for a notch.
A notch in the straight-ahead position is a worn bearing. Grease is lithium multi-purpose. Steering stem nut is 80 Nm. Set the bars so they fall gently, then use that torque rather than a bit tighter.
Step 8 of 8
Spin each wheel and rock it for play.
Bearing numbers are not published. On the RSV-style bike the speed signal is a sensor, not a speedo drive. Do not buy a drive for a PY. Front axle is 80 Nm, the pinch bolt is 10 Nm, and the rear axle is 100 Nm. A bearing pressed through the balls should be replaced.
Finished!
This guide is done. Your place stays on this device.
Tools
- A fork-seal driver of the diameter that matches this fork. The diameter is To verify.
- A seal pick that is not a wood chisel
- A front stand, or a way to hold the front end with the wheel and the fork legs out
- Torque wrench
- Lithium multi-purpose grease for the steering head, swingarm, linkage and bearing seals. Bearing numbers are still To verify.
- A bearing puller for wheel bearings. Driving them out with a drift is how hubs get scored.
Parts
- Fork seals and dust caps for this fork. The 2006 fork is described as new, so do not assume a bee-tail seal fits.
- Fork oil: SAE 10W to 20W, 430 cc in each leg. The RS125 oil level is not published. Do not use a level from the RS50 column.
- Headstock bearings
- Wheel bearings
- A shock only after you know the mount. Lengths are To verify.
Slots for these parts, with empty OEM numbers and empty prices, are in theparts directory.
| Item | Figure | Status |
|---|---|---|
| Fork, 2006 | The RSV-style generation has a new fork, radial brakes and braided lines. The frame is unchanged. | Sourced Research brief |
| Fork oil | SAE 10W to 20W. 430 cc in each leg. Set it with the spring out and the leg fully extended. The RS125 oil level is not published. | Sourced Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| Fork tube runout | Service limit 0.2 mm. | Sourced Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| Steering and fork clamps | Steering stem nut 80 Nm. Fork clamp bolts 25 Nm. | Sourced Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| Shock and linkage | Shock upper 50 Nm, lower 25 Nm. Link arm to swingarm 80 Nm. Linkage rods 50 Nm. | Sourced Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| Axles | Front axle 80 Nm. Front axle pinch bolt 10 Nm. Rear axle 100 Nm. | Sourced Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| Rider sag and spring rate | Not published. | Not published Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| Wheel bearing size | Not published. | Not published Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
The suspension guide in the brief is the USD fork, the shock, and the head and wheel bearings. Those are the pieces that make a bike feel finished or feel dangerous. Spring rates and sag are not in Haynes 4298. This page will not supply a millimetre of sag to aim at. The full chassis list is on the suspension, brakes and tyres page, and the fastener list is on torque.
Fork seals
USD forks are what the brief tells you to service. Confirm you actually have them on the bike in the stand. The 2006 generation is “a new fork”, so seal kits from a 2003 bee-tail are not a default fit. To verify the seal number against the fork, not against the model name on an auction listing.
A weeping seal shows as a ring of oil on the leg and a dirty dust cap. Ride it to the MOT like that and you will also have oil on the disc. Replace the seal and the dust cap together.
The outline, which you then match to the manual:
- Support the front so both legs can leave the yokes without the bike falling.
- Note the position of anything that clamps a leg, with a photo, so the fork goes back at the same height in the yoke.
- Drain the oil. Keep a note of how much came out. Haynes 4298 lists SAE 10W to 20W, 430 cc in each leg. That quantity is the fill. The volume that came out is only a clue.
- Split the leg by the method in the manual. Some forks are topped out against a damping rod, and a homemade slide hammer is how the threads strip. If the manual wants a special tool, the tool number is To verify. Wait for the tool.
- Pick the old seal out. Clean the bush. A scored bush needs a new one, not a new seal on top of a groove. Fork tube runout service limit is 0.2 mm.
- Drive the new seal square, with a driver that bears on the metal, not on the sealing lip.
- Refill with SAE 10W to 20W, 430 cc in each leg, spring removed and the leg fully extended. The RS125 oil level is Not published. The only level figure in the photographed pages sits with the RS50, and it is not used here.
- Torque the fork clamp bolts to 25 Nm.
Pump the fork before you ride. A leg that stays down is a missed oil quantity or a bent rod, not a “stiff new seal” to be ridden in.
Shock
Look for a weeping shaft, a soft ride that has no damping left, and bearings in the linkage that knock. Shock upper is 50 Nm and lower is 25 Nm. The link arm to the swingarm is 80 Nm. Linkage rods to the frame and the link arm are 50 Nm. Linkage lengths, spring rates and sag are Not published. A shock from another generation may bolt up and still sit the bike at the wrong height. Measure the eye-to-eye length before you swap. Do not compress a spring with a pair of screwdrivers. A proper compressor, or a shop, is the tool.
Headstock
Lift the front so the wheel is free. Turn the bars. A notch in the straight-ahead position is a worn bearing, not a tight one. Strip, clean, and fit the new bearings with lithium multi-purpose grease. The steering stem nut is 80 Nm. Set the bearings so the bars fall gently to one side under their own weight, with no notch, and then use that torque rather than “a bit tighter than that”. Overtight bearings notch themselves within a few hundred miles. Bearing numbers are Not published.
The lock stops you checked in guide 1 should still meet the frame stops, not the tank.
Wheel bearings
Spin the wheel and feel for rumble. Rock it at the rim for play. Rear wheel bearings on a bee-tail also live next to a speedo drive that the brief says wears out. On the RSV-style bike the speed signal is a sensor, not that drive. Do not buy a speedo drive for a PY.
Bearing numbers are Not published. Press bearings square. Grease is lithium multi-purpose. A dry sealed bearing fitted with the seals popped out “so you can pack it” is no longer the bearing you bought.
The front axle is 80 Nm and the pinch bolt is 10 Nm. The rear axle is 100 Nm. Wheel runout maximum is 2.0 mm axial and 2.0 mm radial. Axle runout maximum is 0.25 mm. Then spin the wheel again. A new bearing that feels notchy in the hand was pressed on its inner race through the balls. Replace it before it ruins the hub.
Common mistakes
- Guessing a fork-oil level. Haynes publishes 430 cc of SAE 10W to 20W per leg, and does not publish an RS125 level.
- Tightening headstock bearings until the notchiness disappears, and leaving them notched.
- Mixing a left and a right fork spring if the manual says they are sided. Whether they are sided is To verify.
- Setting rider sag to a number from a sports-bike forum.
- Buying a bee-tail fork seal for the 2006 fork without checking the number.
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
- Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) (opens in a new tab) — Haynes
- Modellübersicht RS 125 2T (opens in a new tab) — Hartrusion
- RS 125 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.