Walk into any hydraulic shop and you'll find rows of standard seal kits — pre-packaged sets for common cylinder makes and models. For most routine repairs, these work perfectly well. But there is a significant class of hydraulic equipment where standard kits simply don't exist, don't fit, or won't survive the operating environment. That's where custom-machined seals become not just an option, but the only option.
Understanding when you need a custom seal — and what to specify when you order one — can mean the difference between a cylinder that holds pressure for years and one that fails again within weeks.
The 6 Situations That Demand a Custom Seal
Obsolete or Discontinued Equipment
The most common reason customers call us for custom seals is simple: the equipment is old and the OEM no longer makes a seal kit for it. This is especially prevalent with RV leveling systems, vintage construction equipment, older marine hydraulics, and legacy defense hardware. When the part number returns no results and the manufacturer has been acquired or shut down, custom machining is the only path forward. We can measure the worn seal directly — or take dimensions from the cylinder bore and rod — and machine an exact replacement.
Non-Standard Bore or Rod Dimensions
Standard seal kits are built around common bore and rod diameters in round numbers: 2.000", 2.500", 3.000". But custom-built cylinders — especially those manufactured for defense, aerospace, or specialty industrial applications — often have non-standard dimensions. A cylinder with a 2.187" bore or a 1.375" rod won't accept a standard kit. Custom machining allows us to produce a seal to any dimension, in both inch and metric, with tolerances as tight as ±0.002".
Extreme Temperature or Chemical Environments
Standard seal kits typically come in Buna-N (nitrile), which handles petroleum-based hydraulic fluid in normal temperature ranges. But if your system runs fire-resistant fluid (phosphate ester, water glycol), operates above 250°F, or is exposed to aggressive chemicals, Buna-N will fail quickly. Custom seals allow you to specify Viton® (FKM) for high-temperature and chemical resistance, PTFE for extreme temperature ranges and chemical inertness, EPDM for water-based fluids and steam, or polyurethane for superior abrasion resistance in dirty environments.
High-Pressure or High-Cycle Applications
Standard seals are designed for typical hydraulic pressures — usually up to 3,000–5,000 PSI. Aerospace actuators, defense hydraulics, and specialty industrial presses can operate at 6,000–10,000 PSI or higher. At these pressures, standard seal compounds extrude into clearances and fail rapidly. Custom seals for high-pressure applications use harder durometer compounds, tighter tolerances, and back-up rings specifically selected for the pressure range. Similarly, high-cycle applications (thousands of strokes per day) require seals with superior wear resistance that standard kits don't provide.
Large Diameter Cylinders
Standard seal kits top out at relatively modest bore sizes — typically 8 to 12 inches for most distributors. Large industrial presses, marine steering rams, ship hatch cylinders, and heavy lift equipment can have bores of 14, 18, 20, or even 24 inches. Seals at these diameters are not stocked anywhere — they must be machined to order. We produce custom seals up to 24 inches in diameter, covering the full range of large-bore hydraulic and pneumatic cylinders used in marine, defense, and heavy industrial applications.
Unique Groove Geometry or Profile Requirements
Some cylinders use non-standard groove geometries that don't accept off-the-shelf seal profiles. This is common in older European equipment (which often uses metric profiles not stocked in the US), military hardware designed to MIL-spec groove standards, and custom-built industrial equipment. In these cases, a standard seal physically won't fit the groove correctly — even if the ID and OD are close. Custom machining allows us to produce any seal profile: standard U-seals, rod seals, piston seals, buffer seals, wipers, and fully custom profiles specified by drawing or sample.
CNC lathe machining a large-diameter polyurethane rod seal to exact customer specifications.
What Information Do You Need to Order a Custom Seal?
The more information you can provide, the faster and more accurately we can produce your seal. At minimum, we need:
If you don't have all of these dimensions, don't worry. You can send us the worn seal itself — we can measure it directly. Or if the cylinder is disassembled, we can take dimensions from the groove. For completely unknown applications, a drawing or photo of the groove geometry is often enough to get started.
The eight most common hydraulic seal profiles — all available as custom-machined seals in any material and size.
How to Choose the Right Material
Material selection is the most critical decision in custom seal specification. The wrong material will fail quickly regardless of how precisely the seal is machined. Here's a practical guide:
| Situation | Recommended Material | Why |
|---|---|---|
| Standard hydraulic fluid, normal temps | Polyurethane or Buna-N | Best combination of cost, availability, and performance for typical hydraulics |
| High abrasion, dirty environments, RV jacks | Polyurethane (PU) | Superior abrasion resistance vs. rubber compounds |
| High temperature (above 250°F) | Viton® (FKM) | Rated to 400°F, excellent chemical resistance |
| Fire-resistant or phosphate ester fluid | Viton® (FKM) | Resistant to aggressive synthetic fluids that destroy Buna-N |
| Water-based fluid or steam | EPDM | Excellent water and steam resistance; Buna-N swells in water |
| Extreme temperature range or chemical inertness | PTFE | Widest temperature range (-328°F to +500°F), chemically inert |
| Marine or outdoor weathering | Neoprene (CR) | Excellent ozone, weather, and UV resistance |
| High-pressure (above 5,000 PSI) | Hard polyurethane or PTFE back-up rings | Prevents extrusion at high pressure differentials |
When in doubt, describe your application to us — fluid type, operating pressure, temperature range, cycle rate, and environment — and we'll recommend the right material. Getting the material right the first time is far less expensive than a repeat failure.
Lead Times and Same-Day Service
One of the most common misconceptions about custom seals is that they take weeks to produce. For most applications, that's simply not true. Standard profile seals — rod seals, piston seals, U-seals, wipers, O-rings — can typically be machined same day for urgent orders. Complex profiles or very large diameters may require additional time, but most custom seal orders ship within 24–48 hours.
For emergency situations — a vessel that can't sail, a lift that's grounded, a press line that's down — call us directly. We prioritize emergency orders and can often have seals machined and ready for pickup or overnight shipment the same day.
Emergency Custom Seal Service
Equipment down and can't find the seal you need? Call us at (321) 312-0689. We handle emergency custom seal orders for marine, defense, aerospace, and industrial customers — same-day machining available for most profiles.
The Bottom Line
Standard seal kits are the right choice for common, in-production equipment with normal operating conditions. But for obsolete equipment, non-standard dimensions, extreme environments, large bores, or unique groove geometries, a custom-machined seal is the only reliable solution.
The good news: custom seals don't require molds, don't have minimum order quantities, and for most profiles can be produced same day. The barrier to getting the right seal is lower than most people expect.