you know what kills front bearings or introduce dirt into engine..
1) blowing off your engine with an air compressor, trying to get rid of dirt around the flywheel. the air forces the seal to un seat itself..
2) using brake cleaner on a dirty front bearing without taking off flywheel.. when dirt is mixing with the liquid it seeps behind the shield and thus cause the shield to be less effective..
how i know this you asked.. well i ave been a victim of dirt in engine on many occasions.. and i stopped doing the 2 above things and engine is flawless for 8 plus gallons... original bearing, both steel ..