Glossary
The words that appear on kit listings and on the phone, defined the way they are used in a workshop. Where a term decides whether a kit fits, the entry says so.
Where the seal sits
- Gland
- The machined groove or open pocket that holds the seal, cut into the head or the piston. Gland style — threaded, bolt-on, snap-ring, wire-ring — decides whether a kit physically installs, so two cylinders with the same bore and rod can still take different kits.
- Bore
- The inside diameter of the cylinder tube. Piston seals are sized by it. Measure inside the tube, not around the outside.
- Rod
- The shaft that extends out of the cylinder. Rod seals and wipers are sized by its diameter. Measure a section that has not been running through the seal — the working length polishes undersize.
- Stroke
- Extended pin-to-pin length minus retracted. It never changes which seals fit; it only tells us which cylinder you have.
- Extrusion gap
- The clearance between the moving part and its housing, next to the seal. Pressure pushes seal material into it. The bigger the gap and the higher the pressure, the harder a seal has to be — or the more it needs a back-up ring.
What is in a kit
- Rod seal
- Seals against the rod and keeps the oil in. The one that is leaking when you see a wet rod and a puddle.
- Piston seal
- Seals between piston and tube so pressure on one side does not cross to the other. When this one goes the cylinder drifts under load rather than leaking outside.
- Wiper
- Also called a scraper. Sits outboard of the rod seal and keeps grit, water and mud from being dragged into the cylinder. It is not a pressure seal, and it is the cheapest part in the kit that prevents the most damage.
- U-cup
- A seal that is U-shaped in cross-section, open toward the pressure. System pressure pushes the lips apart and against the sealing surfaces, so it seals harder as pressure rises. Sold for both rod and piston duty. A loaded U-cup has a spring or O-ring in the U to seal at low pressure as well.
- T-seal
- A T-shaped rubber section with a back-up ring on each side, used where a plain O-ring would be pushed into the extrusion gap. A direct upgrade for an O-ring-and-back-up piston groove.
- Back-up ring
- A harder ring fitted behind a seal, on the low-pressure side, to bridge the extrusion gap so the seal cannot be squeezed into it. Also sold as a Parbak ring.
- Wear ring
- Also called a bearing or wear band. A plastic or soft-metal ring in a groove on the piston or in the head, carrying the side load so metal never touches metal. It is not a seal. A worn wear ring lets the piston sit off-centre, which then destroys the seals — which is why a kit that keeps failing on one side usually needs bearings, not more seals.
- Gland seal
- The static seal between the gland and the cylinder tube, usually an O-ring. Leaks here look like rod seal leaks and are not.
How seals are specified
- Durometer
- Hardness on the Shore A scale, and the name of the gauge that measures it. Lower is softer. A soft seal conforms to a worn surface; a hard one resists extrusion at pressure.
- Cross-section
- The thickness of the seal's material, measured across the section — for an O-ring, the diameter of the cord it is made from. An O-ring is specified by inside diameter plus cross-section, which is why two of them can share an ID and not interchange.
- Dash number
- The AS568 standard size code, the three digits in a number like 2-214. It fixes both the inside diameter and the cross-section, so a dash number identifies an O-ring on its own.
- Squeeze
- How much the gland compresses the seal, given as a percentage of its free cross-section. Too little and it does not seal; too much and it takes a set and wears fast. The gland tables are built around it.
- Elastomer
- A rubber-like material that deforms a long way and springs back. The property that makes a seal a seal, and the one that heat and the wrong fluid take away.
- Compression set
- The share of a squeeze a seal keeps permanently after being held compressed. A seal with high set has stopped pushing back on its sealing surfaces and will weep even though it looks intact.
How seals work
- Dynamic seal
- A seal between surfaces that move relative to each other — rod seals, piston seals, wipers. Dynamic duty is what wears a seal out.
- Static seal
- A seal between surfaces that do not move, such as the O-ring under a gland or a port fitting. A material can be fine statically and useless dynamically, which is what the middle ratings on the fluid chart mean.
- Energiser
- A rubber ring fitted behind a seal that has no spring of its own, usually a PTFE cap, to press it against the sealing surface. The energiser sets the real temperature and fluid limits of the assembly.
- Lip
- The thin edge of a seal that rides on the sealing surface. Single lip, twin lip, and so on. More lips generally means better exclusion and more friction.