If you’re specifying a transformer gasket for a tank cover, radiator flange, or bushing joint, you’ll almost always be choosing between two material families: NBR (nitrile rubber) and rubberised cork. Both are proven, both are used across power and distribution transformers worldwide and both can deliver a 100% leakproof seal, but they’re not interchangeable. Picking the wrong one for a given flange doesn’t usually fail immediately; it fails quietly, months or years later, as a slow oil weep that’s expensive to trace and worse to explain to a client.