OK, in an effort to get these online before I lose them, here are pics I took of my fuel selector valve rebuild. I will be editing this msg as I develop commentary I hope will be found useful.
Firstly, I want to Thank Bruce for instigating this thread, as I think it is likely that many of our members/owners will eventually discover to their chagrine that this is a project they will probably suffer.
As already mentioned in this thread (Thanks to all who contributed and Thanks for allowing me to plagiarize a bit), the Illustrated Parts Catalog, B-model, Fig 64 has an error in the check-ball retainer/O-ring relationship. ALL THESE PICS MAY BE ENLARGED BY CLICKING ON THEM. FURTHER ENLARGEMENT MAY EXIST BY A SECOND CLICK.
- This IPC Illustration is WRONG, showing retainer/O-ring reversed, and NO CAM BUSHING.
The IPC has the retainer (Item 17) shown as next to the large check-ball (Item 18) instead of the O-ring (Item 16). Ok, so that's now well-known that they are actually reversed so that the O-ring is NEXT TO the check ball in order to provide a soft sealing surface for the ball. The retainer merely provides a seat in which the O-ring resides.
As already mentioned by Gary, DO NOT ATTEMPT TO REMOVE THE RETAINER from the valve-body. Instead, use a dental pick or other sharp tool to remove the O-ring from the retainer and valve body. Toss that O-ring in the trash and replace it with new.
When reinstalling the check-ball O-ring...DO NOT USE ANY TOOLS which might injure the new O-ring. Instead, use a drop of clean engine oil on your finger and pick up the O-ring and place it into position in the retainer USING ONLY YOUR FINGER. One side of the O-ring will enter easily (with the O-ring becoming momentarily deformed) then your finger can easily press the protruding side of the O-ring into position.
In fact, use clean engine oil when installing all the O-rings in this valve.
NEWLY DISCOVERED POSSIBLE-ERROR in the IPC versus the Service Manual Illustrations: Compare the IPC Fig 64 with this Service Manual illustration (Fig.13-9, Item 22) of the same valve (also used in 172/175 aircraft.)
- This Service Manual illustration is CORRECT...but no part number supplied for the (perhaps necessary) bushing.
See that BUSHING BENEATH THE CAM? (Item 22) It MAY be necessary in your valve...it may not...but ... That bushing is totally missing in the IPC illustration. It is used to elevate the cam so as to properly engage the smaller fuel valve balls in their outermost circumference. (I suspect a missing cam bushing might be the reason for the NTSB report*** previously mentioned in this thread.) The bushing was found ERRONEOUSLY PLACED in my valve on TOP of the cam (between the cam and the washer just beneath the shaft O-ring. Was this what the NTSB thought was a second washer in the accident aircraft? We'll likely never know.)
- Notice the bushing improperly placed beneath the AN960C416L shaft-seal-retaining washer. IT SHOULD HAVE BEEN BENEATH THE CAM!
Except for the generous thickness of the operating lobes of my cam, it's possible that my valve could have suffered a similar failure, only needing sufficient vibration to eventually wear the cam to descend below the circumference of the smaller balls.
SUGGESTION: There is not available any detailed overhaul/rebuild manual for these valves. I suggest that a digital micrometer be used to measure the depth of the cam's upper ring versus the lower, thickened surface of the top-plate, where the shaft seal resides. The thickness of the cam-bushing will be determined by the need to keep the operating lobes of the cam directly at the largest circumference of the small balls (which in-turn press the larger check-balls away from the retainer o-ring seat, to allow fuel to flow thru the valve.
The bushing which was mis-placed in my valve appeared to be slightly rough, which led me to believe it was locally manufactured. (Which made me wonder even more as to why it was even utilized inside the valve. I think I now know. I believe the previous rebuilder of that valve thought the shaft seal o-ring needed more support due to the increased distance between the cam upper-ring and the washer (item 8 in the IPC), otherwise the shaft seal o-ring might be allowed to fall below it's residence in the top-plate. Up/Down movement of the cam might eventually dislodge that shaft seal O-ring unless the washer(s) beneath the shaft seal adequately took up the available space between the cam upper-ring and the seal.)
My bushing measured .050". My washer (item 8 ) measured .030" for a total thickness of .080". The total distance between the top-plate and the cam upper-ring was .082", therefore .002" of free-play exists in my valve. YOUR valve may differ....therefore I suggest that after assembling the cam on it's bushing to adjust the cam-lobes relationship with the small balls... use whatever additional AN960C416L washers necessary to take up the distance between the cam top-ring and the base of the top-plate. (DO NOT forget to consider that the cork gasket (item 10) which will then provide sufficient clearance to avoid binding of the fully-assembled valve.) IF YOUR VALVE REQUIRES A BUSHING to elevate the cam to fully-contact the balls ... you will not find a part number for it. But a common AN960-416L (replace the hyphen with a "C" for a preferable stainless version) washer or two will suffice. They typically measure .025 to .029 and your digital caliper can assist you in determining the necessary thicknesses or number of washers req'd. Additionally....keeping in mind that not all valves may require such a bushing... you can visually inspect the contact between the cam lobes and balls to confirm correct engagement prior to final assembly.
Finally,...I used fuel lube to lubricate the cam-shaft between the shaft seal o-ring and the shaft to insure against premature failure due to friction. No other lubrication (other than the aforementioned engine oil on o-rings) was used. No gasket compound or sealant is used.
Removing the valve from the airplane required 2 hours. RE-installing the valve was harder than the removal. It took 3.5 hours assisted by some choice vocabulary. Rebuilding the valve itself was a walk in the park compared to the labor of removing/re-installing that valve. The pics will illustrate why. The two nuts which secure the AN3 mounting bolts (IPC Fig 63, Item 25) not shown in this msg-thread are the DEVIL to remove and reinstall. The "closeup attach nut grrrr" pic only displays (barely) the right/aft nut.
- The red arrow points to the right/aft mounting nut. It cannot be accessed with an open-end wrench because of the bulkhead just aft (out of view.) The left/aft mount nut is completely invisible and must be manipulated blindly.
It's a BEAR. That nut is so close to the valve body that an open-end wrench cannot simply be slid over it, but must approach it from the right-rear of the valve...problem is...there's a bulkhead just aft that prevents getting a wrench in position, ...hence my twisted-up, bent-over-double little wrench I made (see special tools.)
But the real challenge is it's partner on the aft/port side of the valve which is totally non-viewable and requires a Midget-Superman to access, remove and install, using the "special tools" I made for the purpose. The control cables which pass above/beneath/around this valve do not help.
I did this alone (except for my properly-certificated supervisory mechanic, of course), but if you have a helper to open the tunnel plate (beneath the flap handle) and use a 1/4-drive socket set with extensions and universal-driver... it might be a bit easier.
I personally like the nut-plate arrangement that has been shown here previously by DaveF, but I think that it may not be necessary to actually fasten them to anything, if all one wishes is for a nut that doesn't require a wrench (special tool) to hold it from turning. All that is probably necessary is to use two-lug MS21069/NAS697 (10-32) ...or MS21075/NAS1068 (10-32) nutplates. The rivet-lugs alone will resist turning when they contact the valve body. (However being the originality-NUT that I am, I kept the AN365-1032 fiber lock nuts. Dang, I hated them in this job!)
Addt'l NOTES: The valve top-plate bolts BENEATH the fuel mount bracket in the airframe. Be CERTAIN to support the ball-and-spring housing containing the roll-pin while removing/re-installing the roll pin. Do not merely use a hammer and punch to drive it in/out or you will possibly stress the shaft seal and cam. Suggestion: Lay the housing on a vise or block to drive the pin out with a punch. When re-installing the pin, place the entire housing and pin inside the vise jaws and squeeze it back into position by closing the vice jaws upon it...or using a C-clamp to accomplish the same act.
Lubricate the housing/spring/ball with graphite-based grease, or with Lubriplate and dry graphite mixed with each other.
As mentioned by Mike, the BEST SOURCE for the fuel selector valve complete rebuild kit is:
http://www.mcfarlane-aviation.com/Produ ... r=FSS-KT-2&
It's street-price is about $28. Cessna charges that just for the gasket.
McFarlane also offers an overhaul kit for a bunch more should any of your hardware be unserviceable.
And they'll do all the overhaul/rebuild on YOUR valve for $500 appox
(the easiest of this entire task, by the way. You still have to do all the hard work of removing/reinstalling the valve.)
And finally, they'll even exchange your valve for a rebuilt/overhauled one for only $1250.00 exchange. Right.
FINAL NOTE: Triple antibiotic cream will help the backs of your hands heal from the scratches.
- Here's what you see when you remove the inspection cover beneath the valve...not nearly sufficient to access and work on this valve.
- This is the view obtained by sticking my camera up thru the forward inspection cover...NOT a view available to the naked eye.
- Check-Ball retainer O-ring in position.
- Check-Ball retainer with O-ring removed (by dental pick.)
- Notice the bushing improperly placed beneath the AN960C416L shaft-seal-retaining washer. IT SHOULD HAVE BEEN BENEATH THE CAM!
- Looking up at the tunnel/flap handle from beneath. Not much access.
- Another view of the tunnel from below. What a rat's nest to work in.
- Cheap chineese ignition wrenches were bent to hold the nut from spinning while turning the bolt from above. The dental pick was used to remove the check-ball retainer o-ring. Nutplates are possibly more convenient to use in lieu of the original fiber-lock nuts. Note the valve top-plate is mounted BENEATH the mounting bracket.
***NTSB report excerpt, C-172F accident:
"The fuel selector valve was sent to the NTSB Materials Laboratory for further examination.
According to the Materials Laboratory Factual Report, the damage to the cam shaft was consistent
with mechanical overload. Two washers were observed on the cam shaft below the o-ring that would
seal up against the valve cover when the valve is assembled. The illustrated parts catalog for the
airplane indicated only one washer in the illustration and accompanying parts list. The additional
washer would locate the actuating cam slightly lower within the valve body. (bold emphasis =mine, gh)
The actuating cam displayed a black colored smear on its lower portion, consistent with material transfer from the
erroneous presence of a loose o-ring within the valve body. The illustrated parts manual
incorrectly depicts the o-ring installed ahead of the retainer. To correctly assemble the valve,
the retainer should be installed ahead of the o-ring. The o-ring found in the valve body was
examined under stereo microscope and no discernible wear was observed. The condition of the valve's
internal components and the loose o-ring found within the valve body did not provide adequate
information to establish whether or not the valve was functioning properly prior to the accident."
I do not believe the NTSB is correct to say that two washers will relocate the cam lower within
the valve body. This would depend upon whether or not the cam was already resting on the bottom
of the valve body (as it was in mine) and also would depend on whether or not the two washers actually
had sufficient combined thicknesses to take up more-than the available space between the cam top-ring
and the valve top-plate (mounting plate or valve-cover as they like to call it.)
Simply because two washers exist below the o-ring that seal the "valve cover" does not automatically
force the cam lower in the valve body, expecially in light of the ordinarily-considerable space in that area
of the design. (My own valve had operated so-assembled for many years without any complication whatsoever
and with adequate engagement between cam and ball(s).
In fact, the housing spring-and-ball actually pull UP on the cam after the valve is assembled, and as long as the
two washers do not exceed the dimensions of the distance between cam top-ring and "valve cover" then the cam
cannot be forced "lower in the valve body" than the bottom of the valve body allows. No mention of that, or
the lower cam bushing is made by NTSB.
However, this is an important point to consider when inspecting your valve. Be CERTAIN YOUR CAM FULLY
ENGAGES the small engagement balls in the Left/Both/Right positions...before final assembly.