"Tom Cummings" wrote in message
hlink.net...
We were doing the annual on a 1973 Cessna 172M today and discovered the
fuel
selector valve would not shut off the fuel - in any postion. The handle is
not slipping on the shaft and there is mechanical connect through the
linkage. The detents can be felt in each position and we can visually see
the component turning on the exterior topside of the valve assembly. So
the
valve has to come out so we can examine it thoroughly.
Anybody experienced this?
Tom Cummings

I just rebuilt one in a '56 which is probably the same or similar. Getting
to it is difficult --- you remove two inspection plates on the belly and
then you must remove the valve. You have to take the u-joint out from the
cabin by pulling a few screws, iirc, and the cotter key.

Just take a look at the manual. The fuel selector is operated by the
movement of a cam on the selector shaft which moves ball bearings which push
another larger ball off the o-ringed seat for the appropriate valve to open.
When the selected valve is closed, a spring pushes the larger ball against
the o-ring and retainer. When the o-rings go bad, the valve will not shut
off the flow of fuel.

Simple repair, and o-rings are cheap, but hell to get to and to put back.