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.

According to mechanics I have talked to, and other posts I have read on this site, the coupling does not always shear cleanly into 2 pieces.

Kevin

Vac. Pump Failure
 

I had my mechanic remove old pump and the plastic coupling was sheared. My ship has de-ice boots so the company with the new style piston pump told me they do not have a pump that will work on planes with boots. So I ordered a new dry pump. That should be in this week. If anyone has a 337 without de-ice boots that new piston pump looks good. The factory told me they have one running on a test bed for 20,000 hours and it did not fail yet. Dale Campbell

Airwolf's 400 series wet pump is intended for use with boots. If you're looking for a premium pump it may be worth investigating. Their advertised guarantee is for 2000 hrs. or 10 years. FYI.

Wet Pump
 

The wet pumps do last much longer. My A/P uses them on rental planes. He also said the oil in pumps goes out the exhaust port and gets all over the plane. I spent a great deal of money getting away from all the oil leaks. So I when back to the dry pump. Dale Campbell