Fuel valve rebuild and manual mistake WARNING

[JSwift]

During the annual we started just last week we found that fuel does not shut off fully when in the off position.

I think it’s time for a rebuild of the selector valve on my ‘52 B model SN 20746. I’ve already ordered the McFarlane kit with the O rings and gasket and read this entire post plus George’s article twice. I’m heading to the hangar now to do the “push test”. If it passes that we may defer the rebuilding of the valve until warmer weather allows working outside or at least with my hangar door fully open. If anyone has some recent learning to pass on please chime in.

I agree with Karl’s comment that the learning provided here is worth much more than the price of admission.

[MoonlightVFR]

I have never had indication to service my Fuel Valve. B s.n.26433.
If I ever come to that the rebuild will be triple checked.

The subject "manual mistake WARNING" should give us all Pause. I had an incident once (rear seat passenger reached down and shut fuel OFF) while landing . I/A talked me out of safety wiring it ON. There are more stories out there.


I can envision Bruce reading the IPC and realizing the IPC is wrong -- I will do it MY WAY. Thank You Bruce.

Go to YOUTUBE watch Frank Sinatra sing MY WAY 15 million views.

[c170b53]

You will especially if you turn it to off every flight. And likely you’ll be stranded, so I’d recommend getting the gasket and o’ring kit from McFarland aviation

[c170b53]

Found this gasket on my workbench and thought it might give others a reason to look at their installs. Check out the cracks in the rubber, I’m thinking this gasket could be 30-35 years old.
The large hole was intact and tore when the gasket was removed but the bottom edge had totally eroded away between the mating surfaces.

[TCU76109]

I wanted to add to this thread as it is such a valuable resource.

We overhauled the valve in our 1964 172E recently, and I wanted to point out a few parts folks might want to consider if they’re putting a drain on the bottom. Ours was configured this way, but the AN fittings needed to be replaced. This gave perfect clearance out of the bottom while also allowing the valve to work. Threading it directly into the valve assembly doesn’t give it the internal clearance to depress fully.

AN 910-1D coupling
AN 911-1D double end nipple
Curtis CCA-1550 drain

[n2582d]

Thanks for sharing that — it was an excellent catch. I never noticed that the drain valve can’t be fully depressed when installed in the fuel selector valve block. The drain valve core hits the fuel valve nipple (p/n 0513126) before reaching its full travel.

I bought the C-Mods STC for this even though, in my opinion, it’s a minor alteration. (That issue is debated here). This STC is now owned by Saf-Air. I like your solution to this problem except that it really leaves a lot sticking out in the breeze. If a rock, stick or wash brush were to snap that off it would not be good. On mine, the distance from the bottom of the selector valve block to the outside of the bottom skin is roughly 5/8”.

Hose barb will be cut off

Using an 1/8” NPT male to 1/8” NPT female adapter would shorten the amount protruding outside the bottom skin. However, I don’t find such an adapter that is an AN or MS part number.

The other option would be to use a rotary file on the outlet nipple to eliminate the interference… probably not a legal or safe thing to do.

[TCU76109]

Gary,

Thanks for those links.

Exactly - that’s what we saw, and there are some brass fittings which would work to extend it less than these two fittings together. We were surprised when we tried to find an AN fitting male-to-female in these sizes and saw it didn’t exist, but I guess that makes sense as the male-to-female designations are to change threads.

This was done decades ago on this 172E based upon the paint on the parts we removed. Given it’s had no issues through the years, keeping this configuration by overhauling the valve and replacing the old AN hardware & drain seemed like the best path forward. I’ll try to get photos of how far down it extends. When I tested it after install, I remember it being just far enough out to actuate the valve but not sticking out like an antenna. If it does protrude too much, the brass fitting you pointed out is exactly what we would use.

On a related note, any idea how have people documented the retrofit of these valves to a 170/170A? SEB05-4 Fuel Selector retrofit as data? AC 23-27? It looks like the valve assembly part number 9851118-1 in the service kit is different than the IPC’s listed part number. I was looking at these threads, among others, as a reference: https://forum.cessna170.org/forums/vie ... f=2&t=4256
https://www.forum.cessna170.org/forums ... php?t=3537

[GAHorn]

I wanted to add to this thread as it is such a valuable resource.

We overhauled the valve in our 1964 172E recently, and I wanted to point out a few parts folks might want to consider if they’re putting a drain on the bottom. Ours was configured this way, but the AN fittings needed to be replaced. This gave perfect clearance out of the bottom while also allowing the valve to work. Threading it directly into the valve assembly doesn’t give it the internal clearance to depress fully.

AN 910-1D coupling
AN 911-1D double end nipple
Curtis CCA-1550 drain

This is a recent/contemporary post to an older thread…so just to clarify:

Thanks for sharing that — it was an excellent catch. I never noticed that the drain valve can’t be fully depressed when installed in the fuel selector valve block. ….

Gary, I don’t quite understand the issue you describe…. are your fuel selector valve drain plug NPT threads enlarged..and allowing the drain valve to enter farther into the body?

One of the “correct” (according to the original C-mods STC) drain valve is the CAV-110H-4 … which works fine and does not interfere with the internal workings of the valve (as far as I knew, until this particular post by TCU76109).
@ TCU76109 : What difficulty and/or interference did you experience using the CAV-110H-4 valve? …OR…did you substitute the Curtis valve and discover it collided with the internals..?
Is it possible your valve-body has had the drain plug “over-torqued” and the NPT threads enlarged to the point that drain valve enters your vavle body excessively-deep? …OR..did you replace the exit Nipple-fitting and find that the “replacement AN-fitting” enters the valve body too far to accept a drain valve…?

According to the IPC, the selector valve does not use a common “AN” fitting on the outlet of the fuel selector valve. Instead, it uses a “Nipple”,…PN 0513126.

[TCU76109]

When we removed the AN hardware I listed above that had been installed previously with the old Curtis valve, we tried the CAV-110H-4 drain in its place per the info earlier in this thread. However, when pressing in the drain, it was prevented from extending inside the valve housing by hitting the nipple 0513126 (which is what we have in this valve assembly). We tried a couple of different drains and found that they all were restricted. It didn’t seem like a good idea to use washers/spacers for obvious reasons, and we understood why the previous owner/mechanic had installed the AN parts extending the drain out from the valve body. Whether the fuel valve assembly had been worn over the years as you’re describing, we don’t know. I do have another valve I am going to disassemble/overhaul, and I’ll see if it works without interference.

[n2582d]

Gary, I don’t quite understand the issue you describe…. are your fuel selector valve drain plug NPT threads enlarged..and allowing the drain valve to enter farther into the body?

One of the “correct” (according to the original C-mods STC) drain valve is the CAV-110H-4 … which works fine and does not interfere with the internal workings of the valve (as far as I knew, until this particular post by TCU76109).
@ TCU76109 : What difficulty and/or interference did you experience using the CAV-110H-4 valve? …OR…did you substitute the Curtis valve and discover it collided with the internals..?
Is it possible your valve-body has had the drain plug “over-torqued” and the NPT threads enlarged to the point that drain valve enters your vavle body excessively-deep? …OR..did you replace the exit Nipple-fitting and find that the “replacement AN-fitting” enters the valve body too far to accept a drain valve…?

According to the IPC, the selector valve does not use a common “AN” fitting on the outlet of the fuel selector valve. Instead, it uses a “Nipple”,…PN 0513126.

When depressed, the valve stem on my 1/8” NPT threaded CAV-110 valve hits the outlet nipple, Cessna p/n 0513126. This is why an adapter is required between the selector valve block and the drain valve. With the drain valve torqued to 45 in/lbs., I can only open my CAV-110 valve 0.055” before it contacts the inboard end of the fuel selector valve nipple. Full travel — to fully depress the drain valve spring — is 0.135” on my drain valve.

CAV-110 valve open 0.055”

CAV-110 valve open 0.135”

[GAHorn]

I’ll have to get my calipers out and check my installation….but it may be useful to keep in mind that the purpose of that valve is to drain the fuel selector valve…which only contains a few ounces of fuel…. not to drain the entire supply-plumbing…which can be drained via the gascolator… so there’s no need to open that particular selector-valve-drain more than a small amount…and for only a short period of time. IIn fact, Cessna considered it to be so inconsequential that no selector-valve quick-drain was originally provided…only a pipe-plug…same as in the carburetor.)

The selector valve is the lowest point while sitting in the 3-point position…but the gascolator is the lowest point in level-flight.

Other fuel-system low-point considerations might include the carburetor bowl itself…which has a drain plug that should be opened and drained at least annually during inspection.
Also… Beware… that any replacement flexible hoses between the gascolator and the carburetor should not be routed that may allow a “drape” or a low-lying portion of hose that might allow water to collect between the gascolator and carburetor. (I’ve seen a couple of what were obviously replacement-hoses which were too long…and allowed that condition to exist. A pre-manufactured supply hose in that location is a Preventive Maintenance item…and an unknowledgeable owner might fall into that trap.)

[n2582d]

... so there’s no need to open that particular selector-valve-drain more than a small amount…and for only a short period of time. ...

That is probably true if one is only concerned about draining water. If there are any solid contaminants that have made it through the finger strainers in the tank, having that drain valve open fully would be helpful. (But there are two additional screens that hopefully would trap such contaminants; the gascolator and the carb inlet screen).

... In fact, Cessna considered it to be so inconsequential that no selector-valve quick-drain was originally provided…only a pipe-plug…same as in the carburetor.) ...

Originally Cessna also didn't have drain valves in the tanks. But they learned with experience that tank drains were necessary. Later model C-172s added at least three drain valves in each tank and, when they changed to a different selector valve, they added a drain valve in the base of it. My guess is that a rash of lawsuits provided the impetus for these changes. I doubt many A&P's remove the selector valve plug during annual inspections, which is why, if it could fully open, it's convenient to have a drain valve there. But, since it doesn’t fully open to flush crud out, it should be removed during annual inspections just as a plug should be.

... The selector valve is the lowest point while sitting in the 3-point position ...

Splitting hairs here, but I think the lowest point is actually a bit further "upstream," about where the six is in the IPC illustration. I suppose a person could get a field approval to add drains there but the juice is not worth the squeeze in my opinion.

[GAHorn]

…
Originally Cessna also didn't have drain valves in the tanks.
…
Splitting hairs here…

While we’re “splitting hairs”… I believe it’s more accurate to say that Cessna DID provide drains in the fuel tanks…. They just weren’t “Quick-Drains”. Subsequently Quick-Drains were supplied on finished airplanes (presumably at increased cost) …instead of the owner having to add them later.