Products
Hydraulic piston seals
A piston seal closes the clearance between piston and bore, so the pressure on one side drives the piston instead of leaking past it to the other side. It sits in a groove around the piston, next to the wear rings that keep the piston off the bore, and the common types are U-cups, cap seals and T-seals. A worn piston seal shows as a cylinder that drifts, creeps under load or loses force while nothing leaks outside. Conquest Seal supplies hydraulic piston seals in polyurethane, nitrile, FKM and PTFE, in inch and metric sizes, from the manufacturers we source from. Send the bore and groove dimensions, the old seal's part number or the drawing, and a Conquest representative quotes it.
- Where it sits
- A groove on the piston, sealing against the bore
- What it does
- Holds pressure across the piston so the rod moves under load
- Acting
- Single-acting (one side) or double-acting (both sides)
- Common profiles
- U-cup, O-ring energized lip seal, cap seal, T-seal, V-packing
- Materials
- Polyurethane, nitrile, FKM, filled PTFE
- To quote
- Bore and groove sizes, pressure, fluid, temperature, or the drawing
Sources: Parker® Fluid Power Seal Design Guide, Catalog EPS 5370/USA, 2007, 2008, section 1, read 2026-09-25.
What a piston seal does
Hydraulic pressure acts on one face of the piston, and the piston seal stops that fluid from slipping past to the other face. When it does slip past, the cylinder cannot hold a load in place and loses speed and force, and the leak stays inside where nobody sees it. That is the practical difference from a rod seal, whose leak shows as oil on the outside of the cylinder.
Single-acting and double-acting pistons
A single-acting cylinder is pressurized on one side of the piston, so one seal facing that side, such as a U-cup, does the job. A double-acting cylinder is pressurized on each side in turn, so the piston has to be sealed both ways. That takes one bi-directional seal in a single groove, such as a cap seal or a T-seal, or two U-cups in separate grooves, back to back, so no fluid is trapped between them.
Piston seal profiles
Each profile below is a construction, not a brand; seal makers sell each one under their own series names. Which one a cylinder takes is set by its drawing or its existing groove, the pressure and whether it is single- or double-acting.
| Profile | How it is built | Seals pressure from | Notes |
|---|---|---|---|
| U-cup | A U-shaped lip seal; the piston version carries its sealing lip on the bore side | One side | Two, back to back, seal a double-acting piston; polyurethane is the usual material |
| Symmetrical U-cup | Both lips have the same shape, so one part fits a rod groove or a piston groove | One side | Also made to the old Army-Navy AN6226 standard |
| O-ring energized lip seal | A lip seal energized by an O-ring; the symmetrical, squeeze-type versions have a square or rectangular section | One side | Symmetrical versions fit rod or piston grooves |
| PTFE cap seal | A PTFE-based ring that rides on the bore, pressed outward by an O-ring or a rectangular rubber ring in the groove beneath it | Both sides | Low friction; also called a slipper seal; low-profile versions fit a standard O-ring groove |
| Polyurethane or hard-plastic cap seal | A polyurethane, nylon or other hard-plastic ring over a rubber energizer | Both sides | Step-cut hard-plastic caps are made for heavy-duty, high-pressure pistons |
| T-seal | A rubber seal with a T-shaped section, fitted with no, one or two back-up rings to suit the groove | Both sides | Back-up rings in PTFE, nylon or PEEK; a capped version adds a PTFE ring over the rubber |
| V-packing set | V-shaped rings stacked and sold as a set, also called chevron packing; machined, molded or cut to size | One side | Fits rods and pistons; U-cups and O-ring energized lip seals have replaced it in many cylinders |
Where the old seal or the parts list prints a maker's series name or part number, send it exactly as printed and the quote works from it.
Sources: Parker Fluid Power Seal Design Guide, Catalog EPS 5370/USA, 2007, 2008, section 1, pages 1-2 to 1-6, read 2026-09-25; Parker PTFE Seals Design Guide, PDE 3354 GB, 02/2015, section 2.3, read 2026-09-25.
Piston seal materials
Polyurethane is the usual material for hydraulic U-cups and lip seals: it wears well, has high tensile strength and is compatible with mineral oil. Hot water and steam rule it out. In oil above its temperature range, nitrile and FKM rubber are the usual alternatives; for water or steam, the compound has to be one rated for that fluid.
PTFE has the lowest friction of any seal material and resists almost every fluid, but plain PTFE creeps under load, so cap seals are usually made from filled PTFE. Hydraulic rod and piston seals often use bronze-filled PTFE, glass fiber makes it harder wearing and stronger in compression, and the rubber energizer under the cap comes in nitrile, HNBR, EPDM or FKM.
Conquest Seal supplies piston seals in polyurethane, nitrile, FKM and filled PTFE, and quotes the compound the drawing calls out. Name the fluid and the temperature range with the request, because the fluid decides which compounds can be used.
Sources: Parker Fluid Power Seal Design Guide, Catalog EPS 5370/USA, 2007, 2008, sections 1 and 3, read 2026-09-25; Parker O-Ring Handbook ORD 5700, 2021 printing, section 2.2.12, read 2026-09-27; Parker PTFE Seals Design Guide, PDE 3354 GB, 02/2015, sections 1.1, 1.2 and 2.3, read 2026-09-25.
The piston's wear rings and back-ups
Wear rings carry the side load on the piston, so the piston never runs on the bore and the seal is not crushed or scored. With a single wear ring, it goes on the cap-end side of the piston, away from the rod, to keep the piston's bearing point as far from the head's as the hardware allows. Wear rings come in filled PTFE, polyacetal, composite resin and glass-reinforced nylon; nylon swells in water, so it is kept out of wet service.
At high pressure a cylinder barrel can swell enough for the seal to be forced into the clearance between piston and bore, and a thin-wall, lightweight cylinder can do the same at lower pressure. A back-up ring beside the seal blocks that path, which is why T-seals are made to take them and why the radial clearance belongs on the quote request.
Sources: Parker Fluid Power Seal Design Guide, Catalog EPS 5370/USA, 2007, 2008, section 4, page 4-3, and section 9, read 2026-09-25.
What to send for a piston seal quote
Measure the hardware, not the worn seal. The drawing, or the part number on the old seal or the seal kit, replaces most of this list; without one, send:
- Bore diameter, and the groove diameter and groove width on the piston
- Radial clearance from piston to bore
- Single-acting or double-acting
- Pressure: normal working pressure and any peaks or shock loads
- The fluid by name, and the temperature at the seal: lowest, normal and highest
- Stroke length, speed and cycle rate
- What the old seal is, if known: the profile, the material and any markings, with a photo of its cross section
- The quantity
Send the list with any drawing through the request-a-quote form and a Conquest representative replies with the quote. If the drawing calls out a gland standard, such as SAE AS4716, name it, because the standard sets the groove.
Sources: Parker Fluid Power Seal Design Guide, Catalog EPS 5370/USA, 2007, 2008, Appendix A, read 2026-09-25; SAE AS4716: Gland Design, O-Ring and Other Seals, AS4716D, read 2026-09-24.
Certificate of conformance with every order
Every order ships with a certificate of conformance naming the part number, the specification, the manufacturer, the lot and the quantity. Orders are handled under Conquest Seal's AS9100D quality management system, whose certificate scope includes distribution. The compliance documents page lists the rest of the onboarding paperwork.
Parker is a trademark of Parker Hannifin Corporation. No affiliation or endorsement is implied.
Reviewed by Grant Midstokke, Conquest Seal Corporation. Last updated . Statements on this page are drawn from the sources listed above; the seal maker's datasheet and your application conditions govern the final choice.
Industries that buy piston seals
Frequently asked
What does a piston seal do?
It closes the clearance between piston and bore, so the pressure on one side moves the piston instead of leaking past it. A failed piston seal leaks inside the cylinder, so the symptom is drift, creep under load or lost force rather than oil on the outside.
What is a U-cup seal?
A U-cup is a lip seal with a U-shaped section. Fluid pressure pushes its two lips apart, one against the moving surface and one against the bottom of the groove, so it seals from one side only. Symmetrical U-cups fit a rod or a piston groove, and a double-acting piston takes two, back to back, or one bi-directional seal instead.
How is a double-acting piston sealed?
With one bi-directional seal in a single groove, such as a PTFE or polyurethane cap seal or a T-seal, or with two U-cups in separate grooves, back to back. The groove already machined on the piston usually decides which, so measure it or send the drawing.
Is a piston seal the same as a rod seal?
No. A rod seal sits in the cylinder head and seals the moving rod, so its failure shows as oil on the outside; a piston seal rides on the bore, so its failure stays inside the cylinder. Symmetrical U-cups and O-ring energized lip seals fit either groove, but most profiles are made for one position.
What should I measure on a worn cylinder?
The bore diameter, the groove diameter and width on the piston, and the radial clearance between piston and bore. Measure the hardware, not the worn seal, and send a photo of the old seal's cross section with any markings on it; the drawing or the seal kit part number replaces the measurements.
Do piston seals ship with a certificate of conformance?
Yes. Every order ships with a certificate of conformance naming the part number, the specification, the manufacturer, the lot and the quantity.
Related pages
Send the bore, the groove or the drawing
The bore and groove dimensions or the old seal's part number, the pressure, the fluid and the quantity. A Conquest representative replies with the quote.
Request a quote