Fork seals

Fork seals are rubber rings pressed into the top of each fork slider, around the inner tube. Above each oil seal sits a dust seal (wiper) and below it a clip and washer.

What it does

The oil seal has thin lips that wipe the inner tube and keep fork oil inside as the fork moves. The dust seal stops grit and water reaching the oil seal. Dirt, dried insects, rust pits or a scratch on the tube cut the lips, and old rubber goes hard. Once the seal leaks, oil runs down the slider and can reach the brake.

Signs it is worn or failing

• Oily ring on the inner tube or oil running down the slider: Fork leak.
• Oil on the brake caliper or disc: stop riding, clean, and fix the seal first.
• Fork bottoms out after a long leak: Forks bottom out.
• Cracked or split dust seal.

When to replace it

Replace both oil seals and both dust seals when one leaks. First try cleaning under the dust seal; dirt can cause a small leak: Clean fork seals. If the inner tube is pitted where the seal runs, a new seal will leak again.

What to match when buying

• Seal size code: inner diameter × outer diameter × height in mm, for example 37×50×11. The inner diameter equals the fork tube diameter.
• Check the seal style (single lip, double lip, with or without spring); match the old one or the part number.
• Genuine or known-brand seals have clean, even lips. Poor seals leak again within weeks.
• Buy new fork oil at the same time.

Fitting notes

• Use a seal driver of the right size; never hammer the seal directly.
• Cover the tube top with tape or a plastic bag so the seal lips are not cut on the way down.
• Workshop job: Replace fork oil and seals.
• A leg that will not slide out after the bolt and circlip are out may be held by the bush under the seal. Push the leg in and pull it out sharply, several times, to work the seal and bush free.
• If no seal driver is available, a socket or a length of tube of the right size can seat the seal. Work round the edge in turn, not at one spot.
• Most of the oil comes out when the damper bolt is loosened. Stand the leg upright in a pan for a while to let the rest drain.

Reference (K-Light 125)