The truth is, we didn't. As we complete the assembly of an engine, we add a leak detection dye to the oil. After an engine completes it's dyno test we inspect with black light to check for leaks. Only then is all oil, gas and water drained and the engine packed for shipping. If oil leaks are found, the engine never leaves until they are corrected. The major cause of rear seal leakage stems from poor crankcase ventilation. Many times, for appearance sake, a new owner will change the valve covers we supply which sometimes negates the breathers we include with our covers. Be sure to provide sufficient crankcase breathing and rear seals will not leak.