How to match obsolete
gaskets without
OEM part numbers
An OEM part number references a specific product in a specific catalogue. When that catalogue is discontinued, the number becomes useless. The gasket it described is still specifiable — by its dimensions and by the service conditions it was designed for. Those have not changed. The approach to finding a replacement has.
Why the OEM part number often fails you
OEM part numbers are catalogue references — they identify a specific part in a specific manufacturer's parts list at a specific point in time. When a boiler or heating appliance is discontinued, the spare parts programme typically follows within a few years. The part number becomes unavailable from the OEM channel. Searching for the number returns no results, or returns only overpriced old stock.
The part number is not the whole specification. It was a shortcut to a specification — a catalogue lookup that told a distributor which box to pick. The underlying specification — the dimensions, the material, the service conditions — did not disappear when the catalogue was withdrawn. It can still be used to identify a standard replacement.
For most water-side flat face connections on heating appliances, the gasket is a standard flat gasket shape that can be matched by measurement. There is no unique OEM geometry in most of these parts — the flat face BSP union gasket on an older boiler is typically a standard flat ring, specifiable by OD, ID and thickness. The OEM number referred to that standard geometry in a specific colour packaging. The geometry itself is widely available.
The three common mistakes when matching without an OEM reference
What to measure — the four inputs
Gasket OD — from the removed gasket or the recess outer edge
If the removed gasket is intact enough to measure, measure across the outer diameter with a calliper. If the gasket is too degraded, measure the outer edge of the recess on the fitting face — this gives the gasket OD. Take two measurements at 90 degrees and use the average if they differ slightly.
Gasket ID — from the removed gasket or the recess inner edge
Measure across the inner diameter of the removed gasket. If measuring from the fitting, measure the inner step of the recess — the edge closest to the bore, not the bore itself. The bore is larger than the recess inner diameter.
Thickness — from the removed gasket or the recess depth
Measure gasket thickness with a calliper. If the removed gasket has been in service, it may have compressed below its nominal thickness — compare to the recess depth as a secondary check. The recess depth is a useful guide to the expected gasket thickness for that fitting, but it should be cross-checked against the removed gasket and the joint design.
Service conditions — medium, temperature, approvals
Confirm what the joint carries and at what temperature. Heating system water with corrosion inhibitor is different from potable water, glycol, or gas. For UK potable-water contact, use a WRAS-listed grade within its approval scope, listed conditions and maximum water temperature. For steam or high-temperature service, the grade should be rated for the actual continuous temperature — not just the peak.
Choosing material when the original gives little guidance
| Service | Typical requirement | Grade class |
|---|---|---|
| Central heating — inhibited water, up to 110°C | WRAS optional (sealed system), temp rating adequate | EPDM rubber or cellulose/NBR compressed fibre |
| Potable water — drinking water contact | Verify the potable-water evidence required by the jurisdiction and project; WRAS is one approval route, with defined listing conditions | A product or material with evidence covering the exact drinking-water duty, compound or grade, form, conditions and sizes required by the project |
| Higher temperature heating — up to 140°C continuous | Grade rated for continuous temperature, not just peak | Cellulose/NBR compressed fibre with adequate temperature rating |
| Steam service | Continuous steam temperature rating required | aramid-reinforced or high-compressibility compressed fibre |
| Glycol-based heat transfer fluid | Material compatibility with specific glycol type and concentration | Confirm from grade compatibility data — not all NBR or EPDM grades perform equally with glycol |
| Gas service | Gas-rated grade and competent person installation required | Gas-specific grade — not a general heating gasket. Gas Safe registered engineer required for gas-carrying work. |
Material guidance above is for common water-side flat face BSP union connections in heating and plumbing. Confirm temperature ratings and approval requirements from the specific grade datasheet. For gas service, use only grades documented for gas duty and have the work handled by a Gas Safe registered engineer where UK gas regulations apply.
When a standard flat gasket is a valid replacement
For most water-side flat face BSP union connections on domestic and commercial heating appliances — boiler primary connections, pump unions, valve connections, filling loop connections — the original OEM gasket is typically a standard flat ring with no proprietary geometry. A standard flat gasket of the correct OD, ID, thickness and material grade is a valid replacement. The OEM branding on the original packaging was not part of the sealing function.
The test is straightforward: does the measured OD, ID and thickness of the removed gasket correspond to a standard flat gasket available in the suitable material? In most heating and plumbing applications, it does.
When a standard flat gasket is not a valid replacement
- The joint does not use a flat face union. If the original connection is a flanged joint, an O-ring groove seal, or a compression fitting, a flat gasket is not the correct replacement regardless of what dimensions are measured. Confirm the joint type before ordering any sealing element.
- The gasket has a proprietary geometry — anti-extrusion features, profiled cross-section, or bonded elements. Some OEM gaskets are not flat rings — they may have raised sealing beads, reinforced zones, or bonded metal carriers. These cannot be replaced by a standard flat ring gasket, and the fitting documentation should be checked for the correct replacement type.
- Gas service. For any gas-carrying connection, the replacement gasket should be specifically rated for gas service, and the work must be carried out by a Gas Safe registered engineer. Standard heating water gaskets should not be treated as suitable for gas connections solely because the dimensions match.
Dimensional match is necessary but not sufficient. A gasket that fits the recess correctly will only seal reliably if it is also appropriate for the service medium, temperature and pressure. A correctly dimensioned gasket of the wrong material grade — too low a temperature rating, incompatible with the medium, not listed for potable contact where required — may seat correctly and fail in service for material reasons, not dimensional ones.
The OEM number was a reference. The specification behind it still exists.
Measure the removed gasket — OD, ID, thickness. If the gasket is too degraded, measure the fitting face recess. Confirm the service conditions — medium, temperature, approvals. Select a standard flat gasket grade that matches the dimensions and the service. For most water-side flat face BSP union connections on heating and plumbing appliances, this process produces a valid replacement without any OEM reference. The number that was discontinued was a catalogue entry. The geometry and service conditions that defined it are still measurable and specifiable.
FAQ
How do I replace a gasket when the OEM part number is discontinued?
When the OEM part number is no longer available, the replacement is specified by dimensions and service conditions — not by brand or part reference. Measure the removed gasket: OD, ID and thickness. If the gasket is too degraded to measure reliably, measure the fitting face recess directly — the recess OD gives the gasket OD, the recess inner edge gives the gasket ID, and the recess depth guides the thickness. Then confirm the material requirement from the service: the medium (water, glycol, gas, air, process fluid), the operating temperature, and any relevant approval requirements such as WRAS for potable water. A standard flat gasket that matches these dimensions and service conditions is a valid replacement for a large share of standard water-side flat face BSP union connections in heating and plumbing applications, regardless of whether an OEM reference exists.
Can I match a gasket by thread size if I don't have the original part?
Thread size gives you an approximate reference range — it tells you what order of magnitude gasket dimensions to expect. It does not uniquely determine the gasket OD, ID or thickness. Different manufacturers design the flat face recess on the same nominal BSP size to different dimensions, and different product ranges within the same manufacturer may vary. A replacement specified by thread size alone may be several millimetres off in OD or ID — enough to produce a joint that leaks from the outset. Thread size is a starting point for cross-checking, not the primary specification input.
How do I choose the right gasket material when the original gives no guidance?
When the original gasket material is unknown, base the material selection on the actual service conditions: what medium the joint carries, the operating temperature, and any regulatory requirements. For many common water-side flat face joints in domestic and commercial heating systems, EPDM or cellulose-reinforced compressed fibre grades are typical starting options, subject to actual temperature, pressure and equipment design. For higher temperature service or steam, an aramid-reinforced grade with a higher temperature rating is needed. For UK drinking-water contact, use a WRAS-listed grade within its approval scope, listed conditions and maximum water temperature. For glycol-based fluids, confirm material compatibility. The original gasket's physical appearance — its colour, texture and material feel compared to known grades — can provide a starting indication, but service conditions are the primary guide.