leaking fuel tank drains

[n3437d]

Assistance please... I cannot find my parts manual. I have tow quick drains in my 1955 170B, serial number 26980(?). The leak is very minor, mostly seepage. They are both "damp." I need a part number. I also have a question...is it best to simply replace the "o" rings or the whole unit?
Thanks.
N3437D

[cessna170bdriver]

I don't have any parts books handy so I can't help with the part number. As to whether to replace the unit or just the o-rings, remove the drain and visually inspect it. If the area exposed to fuel is pitted (probably due to standing water in the bottoom of the tank), replace the whole thing. If not, it you can sucessfully stop the leak with new o-rings. There are two o-rings: one on the valve core, and one at the base of the body where it seals against the tank fitting. I've always had good luck with Viton o-rings from local auto parts stores.

Miles

[GAHorn]

The fuel drain valves do not actually use O-rings for the seal. The seals look like O-rings but they are actually shaped seals, and O-rings can slip off with not-so-good results in flight.
The genuine fuel drain valve seals are available from the manufacturer, usually SAF-AIR or Curtis, depending upon which valves you have.
But,.... the problem with replacing seals on fuel drain valves is that the valves sit in the lowest part of the tank where water, if any, collects and corrosion will weaken the "tip" and "seat" of the valve. Add that to the fact that pilots typically operate the valves by allowing them to "snap" shut...and you have the potential (and the valves have a history) of snapping the tip off the valve. The guts then fall out and the fuel pours out of the airplane into the slipstream or onto the hangar floor. 42 gallons of fuel on a hangar floor, airport ramp, or slipstream is not a good procedure.
An Advisory Circular issued about this subject about 10 years ago. I'll try to locate a copy.
I suggest you consider replacing the valves with new, SAF-AIR valves from Aircraft Spruce (877-477-7823), their PN CAV-170, priced at $10 each.

[David Sbur]

Saf-Air valves do in fact take 0-rings, but the Curtis valves are the ones with a 'shaped' o-ring and are not servicable nor should you attempt to service them. I think that is the service letter or advisory that George might be thinking about. The Saf-Air valves are servicable and use common MS29513 0-rings, that are dirt cheap.

[GAHorn]

The advisory was indeed concerning the Curtis-brand valves (which have an additional construction feature involving riveted tips that are subject to snapping off), but the advice I offered regarding the replacement of both brands of valves is valid, in my opinion.

(There is ample opportunity for error when replacing the seals with O-rings, as well. The material of common automotive O-rings is not always fuel resistant. Neither is all aircraft O-rings such as the AN 6227 or AN 6230 types, or the MS 28778 types, all of which are most commonly found and intended for hydraulic oil use. Then there is the size issue. I.D. and O.D. must be correct or the valve will not seat completely or will seat such that the O-ring can be driven off the seat. Spruce does actually offer O-rings for the SAF-AIR brand of valves, and even gives the correct type and size (MS 29513-006 -tip, and MS 29513-12 -seat) but the freight will still add to the price well beyond the value of the O-rings, and you'll still be using an old valve with possibly corroded seats and weakened springs that can fail due to corrosion.)

[Bruce Fenstermacher]

Having just gone through this my advice would be to replace the valves with Saf-Air valves. If then at some point the o-ring starts leaking you can evaluate the overall condition of the valve. If it still works smoothly and isn't pitted then just replace the o-ring other wise get a new valve.

[lowNslow]

I would say just for the hassle factor, it would be easier to just replace the whole valve. No need to drain the tank, just quickly replace the old valve with the new one. You'll maybe lose a cup of gas. Just do it when the fuel level is low to minimize the fuel pressure.

[cessna170bdriver]

The valves (Saf-Air) are (were?) cheap enough that I keep an extra one in a baggie the glove box, along with a few sets of loose o-rings. If a valve starts leaking, it's a simple matter to switch it out, and if the old valve is still in good shape otherwise, just replace the o-rings, and it becomes the spare.

If you're stuck out somewhere and don't have access to the AN, MS, FAA-blessed o-rings, and all you have is a NAPA store, I suspect buna-n would be the material of choice, since that is what is specified for most fuel and oil resistant MS and AN 0-rings. That said, I've never had a problem with the longevity of Viton® o-rings in the drain valves. Silicone is NOT a good choice due to it's low tear resistance.

For a quick tutorial on the different type of materials for o-rings, check out http://www.sisweb.com/vacuum/o-rings/oring.htm. This company sells all types, so it doesn't have anything to gain by pushing one over the other. They even sell o-rings made of gold and platinum.

Miles

PS: Reference Bruce's signature for the value of this post. All I know is what I've read and what I've experienced.

[n3437d]

To "one and all" I extend a might THANK YOU. I have decided to replace the valves. I read in the SRAM that aside from doing the "Dutch Boy finger in the Dike" trick and then realizing that the new valve is more than an arms length away, trying to scoot the new valve closer with the use of tip toe and then quickly bending down to retieve said valve while fuel flows freely I have decided to use the wooden dowel trick to quickly plug the hole.

My next thought is: "Am I allowed to do this?" "or does this procedure come under the "don't ask don't tell" guise?

In closing--who said the new dues structure is too high? The given advice would have cost many times that - Thanks

[cessna170bdriver]

My next thought is: "Am I allowed to do this?" "or does this procedure come under the "don't ask don't tell" guise?

I've always been told that the most common answer to multiple choice questions is "B".

Miles

[Bruce Fenstermacher]

If you are sure you have the correct valve ie the threads are the same then there is no need for the dowel.

First as someone mentioned it would be slightly better if the tanks where nearly empty. Specially if they would end up draining on the ground.

Next you should be wearing a short sleeve shirt. Take a rag and tie it around both your arms between your wrist and elbow . That way the fuel that runs down your arms won't make it to your arm pits and beyond.

Loosen the old valve till it's nearly ready to come out. It will only drip a little.

When you are ready remove the old valve with one hand and quickly replace the with the new one your ALREADY holding in your other hand. You should have a bucket near by in case something goes wrong. If you need to, put the old valve back in.

I've done this several times now and it's really no big deal IF you think about it first.

Are you allowed to do this? The correct answer would be yes under the supervision of an A&P who is going to make a log book entry when done.

[n3437d]

OK, Just spent a 1/2 hr going through my McCurtain parts CD. I cannot find anywhere where they evenmention the drain valve, let alone show a picture of it. Anyone want to by the CD cheap?
Again thanks to your support I now know what to do. But what if the fuel soaked rag that I have tied around my arrm catches on fire? Do I run around the field flapping my arms and doing my burning marshmallow routine?

[zero.one.victor]

Ditto the rag advice: take off your wristwatch & wrap a rag around each wrist in the wristwatch area. Otherwise suffer gassy armpit disease.
I bought all new quick-drains. After installation, I re-Oringed the old ones (which were pretty coroded in the tip area) and carry a couple for easy changeout if I come up with a leaker away from home. My ragwing (with the extra tank) has two valves which are SafAir p/n CAV-110, and two which are CAV-170. Don't recall which goes where. Your old valves should have the part number on them somewhere visible, but it may not be legible anymore.
I just fixed a leak at the gascolator valve- the Oring was fine, it turned out to be some gook keeping the Oring from seating properly. Installed one of my spares temporarily whilst I de-gooked the good one. Then reversed the process. So you might not need a new valve.
My newest Chief catalog shows the SafAir 110 at $8.75, and the 170 at $10.75. Cheap insurance. Flush mounted MDI valves are more, Curtis is about the same. But Curtis apparently doesn't have an equivalent to SafAir's CAV-170 (7/16"-20).

Eric

[GAHorn]

If the fuel tank is original and the threads haven't been re-tapped....the correct PN is the SAF AIR CAV-170 which has 7/16-20 threads for wing tank fuel drains.
If you've never replaced one before, don't forget to install the base O-ring onto the valve before installing it or it'll leak. Keep a fire extinguisher handy, preferably with a buddy nearby.

[Roesbery]

Fellow flying medivac stops in the WX office when he comes here, and tells the story of having a fuel leak in his C-185. Said it was a hot (80-90 degree) day at his cabin NW of Anchorage when he started to fix the leak. The ground was wet with fuel and he dropped a wrench, which apparently created a spark hitting a rock. Said within five minutes the plane was almost totally consumed, as in nothing left but a few small pices of metal, and a dark spot on the ground where it had been. So use extreem caution replaceing the drains, especially in hot temperatures.