Posted by The Moto Store on Oct 30th 2026
Fork Seals: How to Find the Right Size
Short answer: a fork seal is sized by three numbers: inner diameter × outer diameter × height, in millimeters (for example 43 × 55 × 9.5/10). The inner diameter is the only one you can see from the outside: it is the diameter of the polished inner fork tube, which you can measure with a caliper or a tape wrapped around the tube. The other two come from the old seal, a service manual, or a fitment lookup. The easiest route is a fork seal kit listed for your exact year and model, which gives you both oil seals and both dust seals already matched to the fork.
What the numbers on a fork seal mean
Every fork oil seal carries a size code, usually molded into the rubber face or printed on the box. It reads as three measurements:
| Number | What it measures | How to find it |
|---|---|---|
| Inner diameter (ID) | Diameter of the inner (chrome) fork tube the seal lip rides on | Measure the tube directly |
| Outer diameter (OD) | Diameter of the bore in the outer fork leg where the seal presses in | Old seal, manual, or fitment lookup |
| Height | Thickness of the seal; often two numbers (e.g. 9.5/10) because the lip side is slightly taller than the body | Old seal, manual, or fitment lookup |
Common inner diameters run from 35 mm on small-bore and youth bikes up to 48 mm and 49 mm on modern full-size motocross forks, with 41 mm and 43 mm common on street bikes and older off-road machines. Two forks with the same 48 mm tube can still take a different OD and height, so ID alone is not enough to order a loose seal; fitment-matched kits are the safer buy.
How to measure your fork tube
Nothing comes apart for the inner diameter. Clean a strip of exposed chrome just above the dust seal and close a caliper across it. Without a caliper, wrap a strip of paper snugly around the tube, mark the overlap, measure that length, and divide by 3.14: a 150.8 mm circumference is a 48 mm tube. Round to the nearest whole millimeter; tubes come in standard sizes, so 47.9 is a 48.
For the outer diameter and height, pry the old dust seal up off the outer leg and read the code molded into the oil seal face underneath. If it has worn off, the service manual lists the dimensions, or match by year and model in the kit listings.
While you are there, run a fingernail along the chrome. Any nick or pit that catches your nail will cut a new seal within a few rides; dress it or replace the tube before installing fresh seals.
Oil seal, dust seal, or the full kit?
Fork seals come as two separate parts per leg, and the catalog sells them three ways.
| Option | What is in the box | Buy it when |
|---|---|---|
| Oil seal kit | Two oil seals (the pressure seal inside the leg) | You already have good dust seals and only the oil seal is weeping |
| Dust seal (wiper) kit | Two dust wipers (the outer rubber lip that keeps dirt off the oil seal) | Dust seals are cracked, hardened, or torn but forks are not leaking |
| Fork oil and dust seal kit | Two oil seals plus two dust seals | Any time the forks are apart; the usual choice |
If you are dropping the forks anyway, do both. A hardened dust seal is what lets grit reach the oil seal, and dust seals cost little next to a second teardown.
When to add bushings
Fork bushings are the Teflon-coated rings the inner tube slides on. Worn bushings let the tube move side to side, which works the seal lip and causes repeat leaks. If copper shows through the gray coating, or the forks had play before they leaked, replace the bushings with the seals. All Balls Racing bushing kits include slider and leg bushings, copper washers, O-rings, and fork seal snap rings.
A note on fork boots and gaiters
Rubber fork boots (gaiters) are a different part from seals: they cover the exposed chrome tube to keep rocks and mud off it, are sized by tube diameter (43 mm, for example) and length, and are a cheap way to extend seal life on bikes that see gravel.
What we carry
Our fork seal selection is grouped under Suspension. Kits are listed by brand and part number; use the fitment information on each product page to match your year and model.
- Fork Seals, Psychic and Pivot Works oil seal kits
- Fork Seals & Wipers, All Balls Racing oil-and-dust seal kits and dust-seal-only kits
- Fork Bushing Kits, All Balls Racing bushing kits with washers, O-rings, and snap rings
- Fork Boots, Emgo gaiters in 43 mm and other common sizes
- Fork Bleeders, Motion Pro micro bleeders for WP, KYB and Showa forks
- All Balls Racing Fork and Dust Seal Kit 56-185, complete oil and dust seal set, three-lip seal design
- All Balls Racing Fork Dust Seal Kit 57-168, dust wipers only
- Pivot Works Fork Oil and Dust Seal Kit PWFSK-Z051, oil seals and dust wipers for both legs
- Psychic Fork Seal and Dust Seal Kit MX-08743, complete fork and dust seal kit
- All Balls Racing Fork Bushing Kit 38-6127, slider and leg bushings with hardware
FAQ
Q: Can I just replace the one fork seal that is leaking? A: You can, but it is not recommended. Both seals have the same hours on them, and the second one usually starts leaking soon after. Kits are sold in pairs for that reason, and the fork oil should be changed in both legs at the same time anyway so damping stays even side to side.
Q: My fork seal is leaking but the seal looks fine. Do I need a new one? A: Try cleaning it first. A small piece of grit under the lip is a common cause of a sudden weep. Slide a thin plastic seal cleaner (or a piece of 35 mm film negative) under the dust seal and oil seal lip and sweep around the tube. If it keeps leaking after a clean, or if the chrome has a nick, replace the seal.
Q: What fork oil do I use after changing seals? A: Use the weight and volume your service manual specifies for your bike, and set the oil level with the spring out and the fork fully compressed. Oil weight ratings differ between brands, so if you switch brands, match by the manufacturer's viscosity (cSt at 40 °C) rather than the number on the bottle.
Free shipping on orders $50+ (Lower 48).