The "late-style" fuel selector valve, as found in the serials prior to 20286 were simple "Weatherhead" brass valves with a "spigot" drilled internally to provide passageways formed as a "T".... the short tops of the T provided access from each tank in the Both position... and the long bottom of the T fed those tanks toward the gascolator. Rotating the handle, rotated the T, which allowed only ONE tank at a time to feed the gascolator...and when rotated further such that the long bottom of the T faced AFT... the gascolator did not receive fuel,... however the two wing tanks were still connected to each other and fuel could transfer between the two tanks.
The later, square, machined-aluminum fuel selector valve (as found in B-models subsequent to 20285) utilized a triangular "cam" which upsets the seat of check-balls, thereby allowing fuel to flow similarly to the earlier brass valve... EXCEPT that the common complaint of fuel transferring between tanks was eliminated, since the cam did not upset the check-balls when the valve was in the "off" position. THIS WAS THE PURPOSE of Cessna's decision to change to the machined-aluminum valve.... to end complaints of that fuel transfer while the valve was in the Off position.
UNFORTUNATELY... Cessna mistakenly illustrated the machined aluminum valve assembly in their Illustrated Parts Catalog (IPC)... and in that mistaken illustration they indicated the O-ring which provided the seat for the check-ball in an incorrect position.... which actually PREVENTED the check balls from seating correctly. This led to the popular belief that even the LATE valve would allow transfer between the tanks when the valve was actually selected to "off". (It also allowed an empty tank to re-fill, albeit at a slower rate than normal, when the valve was in any positon other than "Both".)
See another thread for the descriptions of that incorrect assembly...THIS LINK is where I posted several pics and detailed description of my overhaul of my fuel selector valve overhaul...scan down the page if you only wish to view my article: http://cessna170.org/forums/viewtopic.p ... e&start=45 ....
The Cessna 172/175 models used the same valve and so anyone shopping junk yards for the later machined aluminum valve should not overlook those airplanes as possible sources.
The specific serials of those airplanes with interchangeable valves are:
C-172... SN 28000 thru 17249544
C-175... SN 55001 thru 17557002
Later 175 aircraft also have an interchangeable valve but may be accompanied with different attaching hardware SN 17557003 thru 17557119.
Hope this helps.
Let me add something which has been commented on previously...about one tank draining faster than the other while operating on "Both" (this is a paste/copy of one my previous posts)
Most high wing Cessnas, when fed from both tanks simultaneously, will actually appear to use fuel more rapidly from one tank than the other. This is because the tanks share a common vent line. Actually both tanks are feeding equally toward the engine, but one tank (the left in most 170A and B's) appears to feed more slowly because it is also being replenished by the dual-vented right tank (remember the vented fuel cap) via the vent line. In other words, a certain amount of fuel will transfer while both tanks are relatively full from the right tank thru the vent line to the left tank. This occurs for two reasons. One is: the right tank is vented not only by the vent line gooseneck, but also by a vented cap. Therefore it's head pressure is potentially higher. The second reason is because at the time of highest fuel consumption (takeoff and climb) p-factor causes the pilot to keep the airplane straight by increased right-rudder application. This keeps the fuel level in the right tank pressed against the vent line further encouraging fuel transfer toward the left tank.
Since the end result is that fuel is leaving the left tank only toward the engine, but is leaving the right tank toward the engine AND toward the left tank, the result is a right tank that empties intially somewhat faster than the left. After approximately the first hour of flight, this transfer naturally stops, and both tanks will continue to feed the engine equally, but the left tank (no longer be replenished from the right) will read higher because it is indeed containing more fuel.
I hope this explanation helps.
(I run my airplane from takeoff to touchdown on Both, and live with it.)
Let me also add that if I were the owner of a 170A model and needed a new brass valve... I think I'd make an "owner produced part" by taking a Spruce PN 05-01032
http://www.aircraftspruce.com/pages/ep/ ... hutoff.php and drill the spigot/stem all the way thu... thereby copying the original part. This might also meet the requirements of AC27-23.
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