How to Identify an Unknown Filter
When the part number is missing, worn, or unreadable, identifying a filter requires systematic measurement and documentation. This guide walks through the process of gathering the information needed to find a replacement.
Before You Start
Do not clean or modify the filter before documentation. Oil residue, markings, and wear patterns can provide identification clues. Photograph the filter as found before any cleaning or measurement.
Step 1: Document Visible Markings
Before handling the filter extensively, document any visible information. This is often the fastest path to identification.
What to Look For
- Part numbers: Check end caps (top and bottom), the outer wrap, any labels, and stamped metal parts
- Manufacturer logos: Even partial logos can identify the manufacturer
- Date codes: May indicate manufacturer and production timeframe
- Micron ratings: Often printed or stamped on the element
- Color coding: Some manufacturers use color to indicate micron rating or media type
- Material markings: Seal material, media type, or compatibility warnings
Photograph Tips
- • Good lighting, no shadows on markings
- • Multiple angles (both ends, side)
- • Close-up of any text or stamps
- • Include a ruler for scale reference
Common Marking Locations
- • Metal end caps (stamped or engraved)
- • Plastic end caps (molded text)
- • Paper labels (often damaged)
- • Ink stamps on filter media
Step 2: Measure Critical Dimensions
Accurate measurements are essential for identification. Different filter types require different measurements.
For Cartridge/Element Filters
| Measurement | How to Measure | Why It Matters |
|---|---|---|
| Outside Diameter (OD) | Measure across the widest point of the filter body | Must fit inside the housing |
| Inside Diameter (ID) | Measure the center hole (bore) opening | Must fit on the center tube/core |
| Overall Length | End cap to end cap, including gaskets | Must fit housing height |
| Gasket/O-ring OD | Outer diameter of sealing surface | Critical for sealing |
| Gasket/O-ring ID | Inner diameter of sealing surface | Critical for sealing |
| End Cap Type | Open, closed, finned, threaded, etc. | Must match housing configuration |
For Spin-On Filters
| Measurement | How to Measure |
|---|---|
| Thread Size | Measure thread OD and pitch (use thread gauge if available) |
| Gasket OD | Measure the sealing gasket outer diameter |
| Gasket ID | Measure the sealing gasket inner diameter |
| Body Diameter | Measure the can diameter |
| Overall Height | From gasket face to bottom of can |
| Anti-drain Back | Note if present and valve type |
Measurement Tips
- • Use calipers for accuracy (rulers introduce error)
- • Record in both inches and millimeters
- • Measure multiple times and average
- • Note if filter is deformed or compressed from use
Step 3: Identify Construction Details
Media Type
Look at the filter media (if visible):
- • Paper/Cellulose: Tan/brown, fibrous appearance
- • Synthetic (fiberglass): White to light gray, finer texture
- • Wire mesh: Visible metal screen
- • Pleated vs. wound: Note the construction style
End Cap Material
- • Metal (steel): Magnetic, silver/gray
- • Metal (aluminum): Non-magnetic, lighter weight
- • Plastic/urethane: Various colors, molded construction
Seal Type and Material
- • Buna-N (Nitrile): Black, most common for petroleum fluids
- • Viton (FKM): Brown or black, high-temp/chemical resistant
- • EPR/EPDM: Black, for water-based fluids
- • Flat gasket vs. O-ring: Note the seal style
Step 4: Document the Application
Application information narrows down the possible matches significantly.
Information to Record
- Equipment make and model: Brand, model number, serial number if available
- System type: Hydraulic, lube, fuel, air, coolant, etc.
- Filter location: Return line, pressure line, suction, offline, etc.
- Housing information: Housing manufacturer, model, or part number if known
- Fluid type: Petroleum, synthetic, water glycol, etc.
- Operating conditions: Temperature range, pressure, duty cycle
Step 5: Search or Request Identification
With complete information gathered, you can search filter databases or contact specialists for identification.
Self-Service Search
Many databases allow searching by dimension. You'll need accurate OD, ID, and length at minimum.
Limitation: Dimensional search may return multiple matches. Additional verification needed.
Specialist Identification
Filtration specialists can identify filters from photos, dimensions, and application data using experience and proprietary databases.
Advantage: Expert verification of the match, including performance suitability.
Common Misconceptions
"Any filter with matching dimensions will work"
Dimensions determine fit, not function. Two dimensionally identical filters may have different micron ratings, media types, collapse ratings, or seal materials. Always verify performance specifications, not just size.
"If the old filter worked, any replacement will work"
The old filter may have been incorrectly specified originally, or may have been working poorly without visible symptoms. Use the identification process to verify correct specification, not just duplicate what was installed.
"If we can't identify it, we can't replace it"
Even filters from defunct manufacturers or obsolete equipment can usually be matched. Aftermarket manufacturers produce filters for applications long after OEM parts are discontinued. Don't give up—ask a specialist.
Related Topics
What is a Cross-Reference? →
Understanding filter equivalence
What is a Filter? →
Filter components, types, and construction
OEM vs Aftermarket →
Understanding replacement options
Micron Ratings Explained →
Understanding filter specifications
Need Help Identifying a Filter?
Send us photos and measurements. Our specialists can identify most filters within 24 hours and provide verified cross-references.