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Oil leaks today are judged by Japanese bikes which all split cases horizontally. Like a cup holding all fluids. This makes them wide, perfect for multi fours.
Vertical split cases are susceptible to leaks. All of the English bikes except Nortons leak oil like a sieve because they have lousy machining. Pre AMF HDs had excellent machining and the cases had no gaskets, just two machined parts joining. They NEVER leaked unless some idiot put them together. K model flatheads didn't even have head gaskets, they simply used silver paint. AMF in their first '72 Sportsters revived a 30's style Indian clutch release and a host of other stupid ideas to make them cheaper. Early Sportsters and big twins would sump up the cases if let sit for weeks and a squirt of oil would come out the breather when starting, beyond that they were definitely not oil leakers. Triumphs, BSAs and Vincents were all big oil leakers, their fasteners were too soft and would stretch too.
Vertical split cases are susceptible to leaks. All of the English bikes except Nortons leak oil like a sieve because they have lousy machining. Pre AMF HDs had excellent machining and the cases had no gaskets, just two machined parts joining. They NEVER leaked unless some idiot put them together. K model flatheads didn't even have head gaskets, they simply used silver paint. AMF in their first '72 Sportsters revived a 30's style Indian clutch release and a host of other stupid ideas to make them cheaper. Early Sportsters and big twins would sump up the cases if let sit for weeks and a squirt of oil would come out the breather when starting, beyond that they were definitely not oil leakers. Triumphs, BSAs and Vincents were all big oil leakers, their fasteners were too soft and would stretch too.