Hydraulic Seal Measurement Guide
Nearly every wrong seal order comes from one of three things: measuring a used seal that has swollen, measuring the seal instead of the groove, or confusing rod diameter with bore diameter. This guide walks through doing it properly, and explains how to read the rod-by-bore specification we use on our seal kits.
The two numbers that matter most
Rod diameter is the outside diameter of the polished chrome shaft that comes out of the cylinder. Measure it with calipers on a clean, unpitted section, and take two readings ninety degrees apart in case the rod is worn oval.
Bore diameter is the inside diameter of the cylinder tube, not the outside. Measure it inside the open end of the tube with the gland removed. If you can only reach the outside, you cannot infer the bore from it — wall thickness varies.
Together these two figures identify most cylinder seal kits. A kit listed as Rod 2.500 x Bore 6.000 means a 2-1/2 inch rod running in a 6 inch bore.
Measure the groove, not the old seal
A seal that has spent years under heat and pressure is not the size it started at. Polyurethane swells in some fluids, nitrile takes a compression set, and anything that has extruded will read long. Where you can, measure the metal:
- For a rod seal, the ID is the rod diameter and the OD is the gland groove diameter.
- For a piston seal, the OD is the tube bore and the ID is the piston groove diameter.
- Seal height is the axial width of the groove, measured with the depth blade of your calipers or a feeler set.
If the old seal is the only reference you have, note that it is a used part and expect to verify the fit before final assembly.
What is in a cylinder seal stack
A full rebuild is more than a rod seal. Here is what typically comes apart, and what to measure for each piece.
| Component | Where it sits | What it does | What to measure |
|---|---|---|---|
| Rod seal | Inside the gland/head, against the rod | Holds pressure behind the rod | Rod diameter (ID), gland bore (OD), seal height |
| Rod wiper / dust seal | Outboard of the rod seal | Keeps dirt and water out | Rod diameter, groove OD, height |
| Rod buffer seal | Inboard of the rod seal | Absorbs pressure spikes before they reach the rod seal | Same as rod seal |
| Rod bushing / bearing | In the gland bore | Carries side load off the rod | Rod diameter, bore, length |
| Piston seal | On the piston OD | Separates the two sides of the cylinder | Tube bore (OD), piston groove diameter (ID), height |
| Wear ring / piston bearing | On the piston, either side of the seal | Stops metal to metal contact | Bore, groove diameter, width |
| Head / gland O-ring | Between the gland and the tube | Seals the static joint | Groove diameter and cross section |
| Back-up ring | Behind an O-ring | Stops the O-ring extruding under pressure | Same groove as the O-ring |
Profiles you will run into
- U-seal / U-cup — single acting lip seal, the most common rod and piston seal.
- T-seal — double acting piston seal with back-up rings either side, good under pressure reversal.
- Vee packing / vee sets — stacked chevron rings, adjustable, common on older and larger cylinders.
- Piston cup — single acting, often on rams and jacks.
- Piston ring — cast or filled PTFE, used where low friction matters.
- Rod wiper — excluder only, no pressure duty.
- Wear ring — non-sealing bearing element, usually filled nylon or bronze filled PTFE.
Match the profile as well as the dimensions. A U-cup and a T-seal of identical size behave differently, and swapping one for the other changes how the cylinder holds load.
How to read our seal kit listings
Our kit titles are built to be searchable. A typical listing looks like this:
Hydraulic Seal Kit, Rod 2.500, Models 824, 824B, 824C, 824S, 834, 834B, 834C (CTC-7X2656)
- Rod 2.500 — the rod diameter in inches. Where we have it, bore is listed too.
- Models — the machine models the kit is listed against. If your model is in that list, the kit is the one you want.
- The number in brackets — the part number. Paste it straight into our search box to jump to the product.
If your machine model does not appear anywhere on the site, that does not always mean we cannot supply it — a lot of kits are catalogued by dimension rather than by machine. Send us the rod and bore measurements, or the number stamped on the cylinder, and we will match it.
Before you reassemble
- Check the rod for pitting and scoring. A scored rod will destroy a new seal in hours.
- Deburr the groove edges and any port openings the seal has to pass.
- Lubricate the seal with system fluid, not grease.
- Fit lip seals in the right orientation — the lip faces the pressure.
- Do not stretch a seal over a shoulder any further than it needs; use a seal installation tool where you have one.
Shop seals and kits
Browse hydraulic seal kits and rod seals, or use the AS568 O-ring chart to spec the static seals in the same rebuild. Rebuilding hose at the same time? See the hose size chart and build a custom hose. All our reference charts are here.