Fuel valve rebuild and manual mistake WARNING

[DaveF]

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.

Also, I have skinny arms and was able to use one of the inspection holes under the copilot seat to get at the valve. It was easier than working from below. I used an angle wrench to get at the nuts on the valve. I reinstalled the valve with nutplates, as I showed before.

[GAHorn]

The marks on the lower shaft of your cam are likely from the valve-body itself, and if your valve was rebuilt previously, it's possible the bushing was discarded if the rebuilder didn't understand it's purpose...or it's also possible the bushing never existed. It 's an unknown at this point because of the lack of good Cessna documentation. (No bushing in the UN-approved IPC...yet there it is in the APPROVED Service Manual. )

Without a manual specific to the fuel selector valve...who knows? (I stopped by an accessory CRS, Hatfield's in Houston, yesterday to see if they had an overhaul manual for those valves and they did not. They also said they never rebuilt those valves as they never considered it profitable seeing as how "simple" the valve was. When I mentioned this dilemna...they were just as puzzled as we about how to pursue it as even Cessna has little info on this matter.)

The evidence of where the balls have previously ridden on your cam lobes may not be definitive.
The housing (IPC Fig 64, item 1) contains the spring (Item 4) and detent-ball (Item 3) which imparts and UPWARD
pull on the cam, which would serve to locate the cam within the valve. If an operator were to simply rotate the selector
thru the various positions...that detent spring/ball would tend to keep the cam in the upper position....and the operating
balls might be located exactly in the middle of the cam lobes.

But what if an operator were to press DOWN on the operating lever while selecting a new position? It's possible
to imagine the cam lobes descending BELOW the balls...and the balls then being released to run around "free-range"
inside the valve body, and the check-balls would then slam shut!

My own valve also displayed similar marks in the exact middle of the cam lobes, yet the bushing was found incorrectly placed on top of the cam beneath the shaft-seal washer. The bushing in my valve ws .050" thick and would therefore relocate the cam that much.

SUGGESTION: It's my recommendation that anyone not certain of their fuel selector valve's assembly make several
attempts to operate the valve through all it's possible selections while energetically depressing/pushing DOWN on the operating lever/shaft in an effort to determine if the operating-balls can be made to jump free within the valve. IF they do, then the fuel will not flow from the tank side which released the operating ball (s).
If there is no change, then your valve is likely safe to continue in operation without the need to remove/inspect it .

[DaveF]

Maybe this was already noted and I missed it, but the bushing in question is called out in the '56-'62 172/175 parts manual. It's a p/n S-1003-9A "bearing". See item 103-11. The 172 fuel valve assembly part number is the same as that for the 170B, so I'd assume the internals should be the same, too. The 175 uses the same valve but with a different outlet nipple.

1956-1962 172/175 IPC

Insert-Note from George: Be aware that Item 16 is the RETAINER and Item 17 is the O-RING .... the parts listing in this pdf is still incorrectly labelled.

[Bruce Fenstermacher]

Where is it leaking? Through the valve? If so them the ball on that side isn't sealing against the o-ring. Simple as that if it is assembled correctly and that can be a big if.

[Bruce Fenstermacher]

***The page above (C172 IPC Feb 13 1963 .pdf) still shows the o-ring and retainer incorrectly. #16 should be the retainer and #17 should be the o-ring, right?

Yes you are correct the C172 IPC Feb 13 1963 .pdf page shown just above does still show the o-ring and retainer incorrectly.

The o-ring must be next to the large ball or the ball will not seal.

[jlwild]

It functioned perfectly for 20 years, so I don't think it's assembled improperly. I was trying to see where it was leaking from but there just isn't much access to get a good look. It's all wet with fuel all over in there, even pooling on the top of the valve (which is why I suspected the top o-ring as the culprit.

Tomorrow should be a fun day draining all the fuel out and seeing how scraped up I get trying to get the valve out.

Aryana, Twice in the past I have had the same type of leak you describe above. Each time it was the "fuel valve cover gasket" Fig.64-10 p. 106 in the IPC. Each time the gasket started leaking along the top aft part of the fuel body. In this area on my fuel valve block the top edge is very narrow (much more than the front edge) and you need to be very careful during installation of the gasket it does not slip. Your issue might be different but this might help you not create another problem. Jim

[GAHorn]

Aryana.... if you read the 170 News in which I wrote about how to rebuild this valve ....(1st Qtr 2011) ...then your question should be answered.
The top O-ring seals the cam-shaft from leaking...along with the top cover gasket. The MOST COMMON LEAK is that top O-ring.

The problem which commonly occurs is: That cam should have only ONE thin washer on the top of the shaft...and should have a thick washer (cam bushing/bearing) on the bottom of the shaft...as depicted in the illustration above.

Maybe this was already noted and I missed it, but the bushing in question is called out in the '56-'62 172/175 parts manual. It's a p/n S-1003-9A "bearing". See item 103-11. The 172 fuel valve assembly part number is the same as that for the 170B, so I'd assume the internals should be the same, too. The 175 uses the same valve but with a different outlet nipple....

WARNING: This "parts" book is NOT A MANUAL. It is a CATALOG.... the difference being that a manual instructs how to assemble or how to perform repairs. A catalog is only for the purpose of ordering parts.
These IPC's (Illustrated Parts Catalogs) are NOT APPROVED documents and are full of errors. Do not use them for definitive assembly or service guidance. Use only the approved Service Manuals.

[jrenwick]

Members can access George's article on-line here: http://cessna170.org/forums/membersOnly ... ws1Q11.pdf

[GAHorn]

Members can access George's article on-line here: http://cessna170.org/forums/membersOnly ... ws1Q11.pdf

Unfortunately, there is a printing error on page 18 of that article: Three photos at top of page: the center and the right-hand photos have each OTHER's caption beneath it. I.E. the center photo should be above the right hand caption.

Also... THIS VERY thread you are reading... I have much of the info posted on an earlier page (page 4?) with the photos that may prove useful : http://www.cessna170.org/forums/viewtop ... 7&start=45

[170C]

Aryana, welcome to the club of those of us who have removed, repaired and reinstalled the fuel valve's on our Cessna's I have been through it twice in 22 years and it isn't any easier the second time except you know what to expect. Seems the most difficult part is removing and reinstalling the two bolts that secure the valve to the bulkhead. Had Cessna installed two nutplates and provided access from above the bolt heads, which would have been easily done, the whole process would have been MUCH easier. Good luck with your project

[bagarre]

Wait 'till you see how many hours it takes to put it back in

I think it's in this thread....but someone made up a small bracket with nutplates. It had ears bent on the sides to hold itself in place.
We used a piece of duct tape to hold nut plates in place...and taped the bolts into the socket head...and taped a universal joint to get the right angle on the bolt.... that worked but, the bracket looks like a better long term solution and you'll be happy if you ever have to get in there again.

A set of stubby wrenches really helped with the fittings. I'd even think about a set of crow's feet and a wobble head extension.

I put the quick drain in mine and had to enlarge the hole in the belly for it to fit. The hole wasn't centered on the drain plug for some reason...and nothing looked bent. IMO it's a good idea. Even if only used at annuals and 100 hours; it's better than taking the plug in and out and risking crossed threads or over torquing.

But, the Association should consider issuing a campaign ribbon for those who have taken on this task

[GAHorn]

There is no reason to use ANY thread locker on this expensive valve, which may risk its future if
those threads are injured. There is no propensity for the screws to work loose.

[DaveF]

That gluing jig probably works, but why spend $25 for it? If you want to glue the nutplates, just do it, no fixture required.

IMO a better use of the $25 would be to pay your mechanic to make a nutplate retainer, like this http://cessna170.org/forums/download/fi ... &mode=view . He'd do a better riveting job, too.

[GAHorn]

The stip with nutplates rivetted to it is a nice idea...but it's more sophisticated than necessary.

The problem is that it's difficult to hold the original self-locking nuts in position while attempting to install the AN3 bolts which secure them. Ordinary nutplates have "ears" for containing the rivets...and those ears alone will prevent the nuts from turning while the AN3 bolts are tightened.

So.... simply replace the original AN or MS lock-nuts with nutplates...no rivetting or aluminum strip required..and you're done.

(Remember...the fuel valve top plate mounts beneath the bulkhead bracket.

Before closing up...it's a good idea to use a mirror and light to confirm that cables, pulleys, and fuel lines are clear of each other and of structure. Some fuel lines come very close to adjacent structure and catipillar-grommet material may be advisable to insure no chafeing of lines.

[GAHorn]

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.

....

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.