Choose a seal by four things, not just by size: the material, the hardness, the fluid and temperature it will see, and the pressure it must hold. Identify each seal and its job first, then measure the dimensions correctly, then select the material. Nitrile (NBR) suits general hydraulic oil at moderate temperature, FKM handles higher temperatures and more aggressive fluids, and polyurethane (PU) gives the wear resistance a rod seal needs at high pressure. Hardness, measured on the Shore A scale, rises with pressure and with the need to resist extrusion. A seal that matches the size but not the material or hardness will fail, often within days.
How to confirm it
- Identify every seal and the job it does
A cylinder is not one seal but a set of them: an O-ring or a rod seal to hold pressure, a wiper or dust seal to keep dirt out, a buffer seal to absorb pressure spikes, a wear ring to guide the rod or piston, and back-up rings to stop extrusion under pressure. Each has a different job, and replacing the wrong one or missing one is why a rebuild leaks again.
- Measure the dimensions correctly
For an O-ring, measure the inside diameter and the cross-section, not the outside diameter, because the outside diameter is a result of the two. For a rod or piston seal, read the profile, the inside diameter, the outside diameter and the height. Use calipers, measure an unworn part of the seal, and remember that a used O-ring has taken a set and will measure small, so do not copy a stretched or flattened old seal.
- Select the material for the fluid and the temperature
Nitrile (NBR) is the general-purpose choice for hydraulic oil, with good oil resistance and a moderate temperature range. FKM handles higher temperatures and more aggressive fluids and holds up where NBR would harden. Polyurethane (PU) gives the strength and wear resistance a rod seal needs at high pressure, but it is less tolerant of hot water and steam. Match the material to what the seal will actually touch.
- Select the hardness for the pressure
Hardness is measured on the Shore A scale, and it rises with the pressure the seal must hold. A softer seal conforms better at low pressure; a harder one resists extrusion as pressure climbs. Fitting a soft seal into a high-pressure groove is a common cause of a seal that extrudes and fails early, even though it was the correct size.
- Confirm fluid compatibility, not just temperature
The fluid decides the material as much as the temperature does. Some hydraulic oils, phosphate-ester fluids, glycol brake fluids and biodegradable oils attack NBR. Confirm compatibility between the seal material and the actual fluid in the machine, because a seal that swells or hardens in the wrong fluid fails regardless of its dimensions.
- Decide between a seal kit and individual seals
A kit carries every seal for a rebuild, in the correct materials and hardness, and it is the reliable choice for a cylinder or a motor because the internal seals all share the same hours. Buying individual seals is cheaper per item but risks the wrong material, the wrong hardness, or a missed seal. Use a kit for a rebuild and individual seals only for a known single seal.
- Install and store the seals correctly
Lubricate seals with clean hydraulic oil or the specified assembly grease, keep them away from sharp edges and threads, use a fitting sleeve where one is called for, and never stretch an O-ring over a shaft shoulder to get it on. Most new seals that fail within days were damaged during installation, not defective. Keep spares in their packaging, away from sunlight, ozone and heat, and check the shelf life before fitting.
What to do about it
Leaking cylinder: rebuild with a complete seal kit rather than swapping the one visible seal. The seal that is leaking is usually the one that failed last; the wiper, buffer and wear ring around it are on the same hours and are the next to go. A full kit costs little more than the single seal and avoids a second strip-down.
Seal failed by swelling or hardening: the material was wrong for the fluid or the temperature, not the size. Identify the fluid and the working temperature and select the material again; NBR, FKM and PU are not interchangeable, and the same dimensions in the wrong compound will fail the same way.
Extruded or nibbled seal: the hardness was too low, the clearance was too large, or a back-up ring was missing. Fit a harder seal where the pressure demands it, add the correct back-up ring, and check the groove and clearance for wear that is allowing the seal to be pushed out. Guangzhou Shenyue Machinery Technology Co., Ltd. supplies O-rings and back-up rings by grade.
Wiper or dust seal missing or worn: replace it with the kit, because a missing wiper lets grit reach the pressure seal and the rod. On a machine working in dust, the wiper is the seal that decides how long the rest of the kit survives. Guangzhou Shenyue Machinery Technology Co., Ltd. stocks wipers and wear rings for excavator cylinders.
Choosing between a kit and single seals: buy the kit for any rebuild and single seals only for a known, isolated seal. The saving on individual seals is small, and one missed or mismatched seal turns the whole job into a repeat visit.
For seal kits and individual seals, Guangzhou Shenyue Machinery Technology Co., Ltd. supplies cylinder and motor seal kits, O-rings, rod seals, wipers, wear rings and back-up rings from Guangzhou, holding 3,000+ part types on the shelf and 100,000+ part numbers available to order, and states on every quotation line whether a part is genuine, OEM-quality or aftermarket. We are an independent supplier, formally authorized for five brands: Garrett, Mitsubishi Heavy Industries, TONGMYUNG, NIKURA and Marusan; for other makes we supply through original supply channels or as OEM-quality equivalents.
Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Cylinder or motor seal kit | Genuine; OEM-quality. Supplied as a complete set in the correct materials for the application. |
| O-ring, NBR | Genuine; OEM-quality. The general choice for hydraulic oil at moderate temperature. |
| O-ring, FKM | Genuine; OEM-quality. For higher temperature and more aggressive fluids; confirm fluid compatibility. |
| Rod seal, PU | Genuine; OEM-quality. High-pressure rod sealing with good wear resistance. |
| Wiper or dust seal | Genuine; OEM-quality. Decides how long the pressure seals last on a dusty machine. |
| Wear ring and back-up ring | Genuine; OEM-quality. Guides the rod or piston and resists extrusion under pressure. |
Need the parts this covers?
Send the machine model, serial number and the part numbers you need. We confirm fitment before payment and state the grade of every line — genuine, OEM-quality or aftermarket.
Request a quoteFrequently asked
Why can’t I just match the size and fit any O-ring that measures the same?
Because size is only half of the specification. The material decides whether the seal survives the fluid and the temperature, and the hardness decides whether it resists extrusion under pressure. An O-ring of the correct size in NBR will fail quickly in a fluid or a temperature that needs FKM, and a soft seal in a high-pressure groove will extrude. Match the material and the hardness as carefully as the dimensions.
Should I buy a seal kit or individual seals?
Buy the kit for any rebuild. The seals inside a cylinder or a motor share the same hours and the same conditions, so when one fails the others are close behind, and a kit supplies them all in the correct materials. Individual seals are worth buying only when you know a single specific seal is the problem and the rest are genuinely new.
How do I tell whether a seal failed because of the material or the installation?
The failure looks different in each case. Swelling, softening or hardening that affects the whole seal points to a material or fluid problem. A cut, a nick, a twisted seal or a torn edge points to installation damage, often from a sharp edge or a shoulder the seal was stretched over. Extrusion that has pushed the seal into the clearance points to hardness, clearance or a missing back-up ring.