Piston assembly for hydraulic cylinders: the piston seal, the wear rings around it, and how to prove the seal holds
Hydraulic cylinder piston assemblies · Single- or double-acting piston seal, trapped pressure between two single-acting seals, wear rings that carry side load and keep metal off the bore, and the mid-stroke bypass test that also finds tube ballooning
This page covers the piston assembly inside a hydraulic cylinder: the piston on the end of the rod, the seal or seals that hold pressure across it, and the wear rings that keep it centred in the tube. Ask three things first: whether the cylinder is pressurised from one side or both and which seal arrangement follows from that, which wear ring material and width carry the side load, and how the finished assembly will be bypass-tested.
Piston assemblies for hydraulic cylinders: pistons with their piston seals, back-up rings and wear rings, as replacement parts or for new cylinders. Complete cylinders are covered on the Hydraulic cylinder page, rods on the Hydraulic cylinder piston rod page, and leather packings on the Leather hydraulic piston seals page. Drilling mud pump pistons, engine pistons, pneumatic actuator pistons for sanitary valves (see Pneumatic actuator for sanitary diaphragm valves) and accumulator pistons are outside this page.
CNFX editorial research · Sources checked October 12, 2026 · 5 sources
One double-acting seal, not two single-acting seals that can trap pressure
Fluid Power World distinguishes single-acting piston seals, which seal dynamic pressure from one side, as in a car jack, from double-acting seals, which seal it from both sides, as in an excavator arm that is pushed out and pulled back under load. [1] Hallite states that a double-acting cylinder seals best with a double-acting seal, and that using two single-acting seals on the piston of a double-acting cylinder can easily cause a breakdown because a very high pressure can be trapped between them; its piston seals of both kinds can be used with integrated or separate back-up rings and guide rings. [2]
Martin's Rubber notes that in a single-acting cylinder the side away from the pressure may hold air rather than oil, so the piston seal should leave a minimum oil film on the bore as it passes, and that air drawn in on that side can carry moisture and contaminants that damage the seal; double-acting seals have a symmetrical cross-section with the same sealing function in both directions. [3]
Give the supplier the bore and rod diameters, single- or double-acting duty, working and peak pressure, speed, fluid and temperature range, side loads from mounting, and the groove drawing if the piston is to be refitted.
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Wear rings that take the side load and keep the piston off the bore
Trelleborg states that a hydraulic wear ring guides the piston and rod, absorbs transverse forces and prevents metal-to-metal contact, which keeps the sealing system working; it grades its Slydring materials by duty: Turcite for low to medium duty with limited radial forces, HiMod for medium to heavy duty, and Orkot for heavy duty with high radial forces. [4]
Editorial note: ask the supplier for the permissible radial pressure of the wear ring material at your oil temperature, and check the ring width against the side load that the cylinder mounting puts on the piston.
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Bypass testing: end of stroke against mid-stroke with a pressure ratio
Brendan Casey, writing in Power & Motion, describes the end-of-stroke bypass test, in which the cylinder is pressurised at the end of its stroke and leakage past the piston seal is measured, and notes that it does not reveal ballooning of the tube under hoop stress. A mid-stroke test does, but the cylinder's force must then be resisted, which is impractical for large high-pressure cylinders; instead, with both sides filled and bled, the rod mid-stroke and the piston-side valve closed, the rod side is pressurised to rated pressure and the two gauges watched: at a 2:1 area ratio, 3,000 psi on the rod side shows 1,500 psi on the piston side, and a falling differential points to the piston seal or the tube. He warns never to direct flow to the piston side with its ball valve closed, because cylinder failure or personal injury could result. [5]
Editorial calculation: a 100 mm bore with a 70 mm rod has an area ratio of 100² ÷ (100² − 70²) ≈ 1.96, so 210 bar on the rod side should hold about 107 bar on the closed piston side in this test. [5]
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Send the cylinder duty and the test you will accept
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Cylinder: [bore; rod; stroke]
Duty: [single- or double-acting; pressure; speed]
Seal: [type; material; back-up rings]
Wear rings: [material; width; side load]
Fluid: [type; temperature]
Test: [end-of-stroke or mid-stroke bypass]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
Can two single-acting seals replace one double-acting piston seal?
Hallite warns that two single-acting seals on the piston of a double-acting cylinder can easily cause a breakdown because very high pressure can be trapped between them. [2]
What do piston wear rings do?
Trelleborg states they guide the piston and rod, absorb transverse forces and prevent metal-to-metal contact. [4]
Why test a piston seal mid-stroke?
Power & Motion notes that the end-of-stroke bypass test does not reveal tube ballooning under hoop stress, which a mid-stroke test does. [5]
Sources and what still needs a direct check
Sources checked 2026-10-12. Trelleborg and Hallite describe their own seal and wear ring products; Fluid Power World, Martin's Rubber and Power & Motion explain seal types and testing.
