Vacuum pump

[SteveG]

And yet another pump failure mode which I don't quite understand. Maybe one of our highly experienced mechanic correspondents can comment. Over the years I have replaced several vacuum pumps on various Lycoming & Continental engines but have yet to see an application in which the pump mounting communicated directly with the interior of the crankcase. Unlike say a Continental starter motor whose drive extends thru the crankcase to mesh directly with the internal engine gear train, the vacuum pump's splined shaft is driven from a corresponding female drive rotating thru a Garlock seal. I do not see any opportunity for crankcase contamination from a sheared pump coupling, in my limited experience. I might hazzard the guess that in 5% of worn pump failures, the final cause of death is vane failure followed by coupling failure. It's hard to imagine engineering the pump mount in such a way as to allow crankcase contamination to occur after such a frequent event. Also, I believe the coupling is designed in such a manner as to shear into two halves, not explode into numerous pieces. I would be interested to know what engines are susceptible to such an occurence. Steve G.