1st the crank has too much clearance between it and the seal so that in a very short time of run in the seal leaks.
2nd the thrust bearing has excessive play and allows the crankshaft to "walk" too much which varies the point that the seal rides on and changes the seat too much. The seal then wears excessively and has no life. This is not a wear problem exclusive to any one engine, but virtually every engine I have ever worked on, at one point or another. 3rd possibility is that you have insufficient lubrication of the seal when installing which before it seats it wears excessively. Also typically the surface where the seal rides must be polished in order to re-new the surface for good seal life and wear. ++++++++++++++ One more possibility is some existing damage (scuff or tool marks) to the crank that wears out the seal in the time mentioned. Have a good look next time. Steve J
