<VV> Broken valve seats (no Corvair)

[Roger Gault]

It's pretty busy in there, especially when the valves aren't doing what
they're supposed to.

When I was growing up, my dad owned a Piper aircraft dealership.  I worked
for him in the summers as a mechanic's assistant.  One of the coolest planes
Piper built in those days was the 400hp Comanche.  Pretty exciting in an
airframe designed for 180hp.  The engine was an 8 cylinder opposed
air-cooled Lycoming - 720 cubic inches.  It redlined at about 2700 RPM and
tended to vibrate the airframe to pieces.

One of our customer's 400 swallowed a valve on approach to landing at an
airport about 30 miles from our place.  He shut it off and glided in.  Dad
(an A&P mechanic) went over, replaced the jug (cylinder and head) on the
offended cylinder and didn't find any other damage.  He ran it up on the
ramp and everything sounded good, so he decided to fly it back home and tear
it down for a complete inspection.  On takeoff, as the RPM rose above where
you can get it sitting still, it started to run rough.  He climbed it up
high enough to glide home and flew home - running rough all the way.

When we tore it down, we found that the valve hadn't gone out the exhaust as
he thought.  It had shattered and fallen down into the intake plenum.  The
plenum is a big box centered under the engine and about a foot below the
heads.  The intake tubes are maybe 2.5-3 inches in diameter.  At takeoff RPM
there was enough airflow to suck pieces up into all 8 cylinders, ruining all
8 pistons and jugs.

Pretty amazing - and expensive.

Roger

----- Original Message ----- 
From: "Clark Hartzel" <chartzel at comcast.net>
To: "Virtual Vairs" <virtualvairs at corvair.org>
Sent: Tuesday, September 12, 2006 10:06 AM
Subject: <VV> Broken valve seats


> How do pieces of a broken valve seat get into other cylinders?  On the
> intake stroke the piston is sucking air into the cylinder and on the
exhaust
> stroke it goes out the exhaust tube.  Sounds impossible that a piece of
> steel could be blown into another cylinder when the intake valve is
closed.
> Unless it happens at the overlap period when both valves might be open a
> small amount.
> Clark Hartzel
>