Genuine vs Compatible — What the Specifications Don't Tell You

Compatible spare parts are marketed with specifications that appear identical to genuine components — same dimensions, same material names, same operating parameters. What the specification sheet does not reveal is how those numbers were determined, what tolerance bands apply, and what quality control process verified compliance. The gap between stated specification and manufactured reality is where premature failures originate.

What a Specification Sheet Does and Does Not Tell You

A mechanical seal specification might list: shaft diameter 35mm, face material silicon carbide/carbon, O-ring material NBR, operating temperature -20°C to +90°C, pressure rating 16 bar. These parameters describe what the seal is intended to do. They do not describe:

• The flatness tolerance on the seal face — which determines leakage performance under dynamic conditions
• The grade and purity of the silicon carbide — which determines hardness, thermal conductivity, and resistance to thermal shock
• The compression set and chemical resistance of the specific NBR compound — which determines secondary seal performance over time
• The spring force variation across the production batch — which determines closing force consistency under variable pressure
• The test protocol used to verify the rated parameters — whether 100% testing or statistical sampling

Genuine OEM parts are manufactured against internal design specifications that define all of these parameters — not just the headline numbers. Compatible parts are manufactured against the headline numbers only.

Dimensional Accuracy vs Tolerance Band

A nominal dimension of 35mm shaft diameter means different things depending on the tolerance applied. OEM specifications for pump components carry tolerances that are typically tighter than the standard commercial engineering tolerances (ISO h6/H7 fits, for example). Flygt and Lowara bearings, seals, and impellers are specified with tolerances appropriate for the clearances and fits required by the pump design — not the looser tolerances of general commercial manufacture.

A compatible bearing with a nominal bore of 35mm but manufactured to commercial h7 tolerance (±0.025mm) will fit the same shaft as a genuine Flygt bearing specified to h5 tolerance (±0.009mm) — but the contact pressure distribution, heat generation, and load-carrying capacity will be different. This difference may be invisible at installation but becomes apparent as operating hours accumulate.

Material Grade — The Invisible Difference

Material names on specification sheets are generic identifiers. "Silicon carbide" encompasses a wide range of grades from standard reaction-bonded grades to premium self-sintered grades with significantly different hardness, porosity, and thermal properties. "NBR" encompasses dozens of compound formulations with varying acrylonitrile content and additive packages that determine low-temperature flexibility, oil resistance, and compression set behaviour.

Genuine OEM seals specify the exact grade and compound from an approved supplier. Compatible seals specify the generic material name. The grade actually used in production may be appropriate — or it may be the cheapest grade that meets the generic specification. The buyer has no way to verify which.

The Testing Question

Flygt and Lowara parts undergo qualification testing against the pump design before approval for production, and ongoing production quality control against defined accept/reject criteria. Compatible parts may be tested against the generic parameters on the specification sheet — but not against the specific operating conditions of the pump they are intended for.