DaveF wrote:Great pics, especially with one of those old Mavicas. But now you've got me wondering about that bushing. No matter how many pictures I take there's always one more I wish I'd taken.
Here's the cam in my valve. It's upside down, inserted in the cover plate with the O-ring (part 64-7) in its land, hidden under the washer. I don't remember a bushing, but from the marks on the cam I assume the vertical alignment is ok. The valve balls seem to be running right down the middle.
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DaveF...you're gonna hate me for this....but I just was reviewing this thread and contemplated your post....and here's my response:
Yes...it appears your actuation balls are running near the middle of your cam-lobes, just as they are supposed to do. (They might actually be running slightly-high. Unfortunately this might indicate even less tolerance for the lack of a lower bushing...and therefore a slightly greater risk of the type failure about which I'm concerned.)
But, the washer which is beneath your top-cover O-ring actually prevents the cam from rising upwards toward the top cover while simultaneously providing a "floor" for the O-ring. This is just as intended, and so-far, so-good.
The problem is: The tension spring (which is beneath the "housing" (pinned on top of the cam, and depicted in your photo clamped in the vise with the detent ball resting upon it) and which presses the detent-ball into the detents to give you a "feel" when clicking the valve into each position of right/both/left/off.... also pushes UPward on the housing pinned to the cam, therefore pulling the cam UPward against the top cover. This also holds that washer against the O-ring and keeps it inserted into the top cover, thereby preventing fuel from leaking past the cam shaft and out the top of the valve. And, yes, that upward pull on the cam also prevents the cam from descending downward into the valve-body. WHOOPS! That means....the small detent spring is what keeps the cam lobes raised so the actuation balls ride along the middle of the cam lobes! There is NOTHING to prevent you from PUSHING DOWNWARD on the cam, which might let the cam lobes slip BELOW the actuation balls and allow them to fall INTO the valve body...
... which would result in your check balls slamming SHUT and CUTTING OFF YOUR FUEL! There would be nothing you could do in-flight to restore fuel flow.
This is what I think killed the 172 pilot in January 2009 and this is what I think the NTSB failed to notice. They were so preoccupied in their use of the IPC to illustrate their observation of the O-ring error ...they totally MISSED their own reliance upon the IPC ....which ALSO failed to depict the missing bushing. They offered NO EXPLANATION as to why the actuation balls were found down in the valve body, presumeably because they totally missed the lack of the bushing illustration. (In fact, the NTSB also erred in thinking that the failed/misplaced check-ball O-ring is what caused the accident fuel valve to shut off the supply of fuel. IF the O-ring were indeed installed on the wrong side of the retainer {virtually IMPOSSIBLE} or slipped and fell into the valve body as NTSB described in their report..then the check ball would have LEAKED and NOT prevented fuel from reaching the engine!)
That bushing sits on the lower end of the cam shaft similarly to the washer sitting on the top of the shaft beneath the top cover O-ring. The bushing blocks the cam lobes from descending within the valve body...thereby locating the cam lobes for the actuation balls in the OPPOSITE direction the upper washer serves.
DaveF... be certain to perform the "test" I suggested in the article: Vigorously press DOWN upon your operating handle to attempt to depress your cam even-lower within your valve body. This operation is intended to deliberately induce a failure of your cam to remain in-position. While depressing the handle, operate your valve several times thru ALL positions.
The intent is to see if you can cause your cam to descend and release your actuation balls into your valve body. Next, see if your valve delivers fuel to your gascolator (or engine while running on the ground) while the handle is selected to each tank, left/right. If the cam has released the actuation ball (because you were able to descend your cam) then the check ball will be CLOSED and prevent fuel from flowing from that tank ...or perhaps from ANY tank.
Yes, this will really tick you off if you perform this test on the ground and it shuts off your fuel because it will mean you have to remove your valve and reassemble it correctly with a bushing beneath the cam lobes. But.... it will be far better to discover this on the ground than in flight. (And the only other option is to remove your valve and inspect and/or do the bushing installation anyway, which should be done in any event at the next opportunity to have that fuel valve out of the airplane.)
EVERYONE: Perform this test. AND...even if after the test your valve still works fine off of either tank... FOR ALL FLIGHTS...whenever operating on BOTH tanks (which is what most of us do) BE CERTAIN to check/confirm that both tanks feed fuel/drop in fuel-level thru-out your flight. If one tank appears to be feeding all the fuel for an excessive period of time*** then find a place to land, or consider that one tank-selection of the fuel valve may have failed in the manner discussed.
*** Most operators have reported a tendency for the left tank to feed the first hour more than the right tank when operating on BOTH. I have also observed this phenomenon. But if either of your tanks fail to indicate supply-of-fuel to the engine after an hour of flight...then consider the possibility of your actuation ball(s) having slipped past the cam and dropped into the fuel valve thereby shutting off the flow of fuel from the suspect tank. Should this be the case, avoid in-flight testing of the fuel valve, and consider the possibility that switching to the failed-to-feed tank may result in engine stoppage. Also, consider the possibility that such tank-switch-selection may result in the failure of the opposite actuation ball, resulting in the inability to return to the former tank.
I hope everyone who has the opportunity to rebuild their valves will inspect for excessive cam-movement in the upwards/downwards directions to preclude this sort of failure. It may be possible that manufacturing tolerances of these valves allow for some valves to be correctly/safely assembled without the lower bushing. This might explain why some valves have been found without. But several valves have been disassembled and found to contain the bushing on TOP of the cam along with the top O-ring washer ...and NO MANUAL depicts this condition as being correct.
It is a good idea, when inspecting your valve, to see if the cam, while resting on it's lowest possible position (whether or not a lower bushing was found existing, to determine if it is possible for actuation balls to slip above the cam lobes with the cam in it's lowest position. If so, then a bushing/washer should be installed as a shim to prevent that cam from reaching that position. Likewise, the washer above the cam should also prevent the cam from rising so as to allow any loss of actuation balls.) Hope this helps all to understand the potential problem.