Craig,

I learned about a blocked crankcase vent on the first flight of my first airplane (an Alon Aircoupe).  It had been used as a 15 minute flight commuter causing complete blockage of the vent over time.  On the return home flight, pressure would build in the crankcase until first one accessory gasket after another would blow, after the last one to blow had been fixed.  We did not figure it out until three different gaskets had failed. 

The reason for this story is to explain that the crankcase pressure gage on our Legacy was tapped into the vent line at about the half way point.  I did not block the vent line, just measured the pressure. 

The piston ring land and ring wear was way over tolerance on all six pistons.  Replacing the Performance pistons with new ECI pistons and rings solved the problem.  All cylinders were new with about 50 hours on them, one was so worn it had to be replated, the others honed out OK.  ECI helped me out of a very stressful and expensive week after week search to identify the problem and it was not their pistons that failed. 

Steve Colwell

"I checked my crank pressure by hooking a pressure gauge to the breather (before the separator) and it is high somewhere in the 4-5 psi."

Don’t do this…you either had a leak at your pressure gage or you blew out a gasket.  All engines have blow-by or combustion gases that get by the piston rings and you will continue to build pressure until something breaks.  You must let your engine crankcase “breath”.  The test you wanted to do was a blow-by “flow” test.  I am NOT a IO-550 expert but I would guess .5 CFM at idle and 2-3 CFM at cruise would be normal.  I believe your air/oil separator inlet should be located higher than the engine fill tube vent outlet.  Maybe that is causing your problem.  Since the problem is intermittent, I would rule out piston rings.

Craig Berland

N7VG