Dripping fuel drain valve

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drak130
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Dripping fuel drain valve

Post by drak130 »

Hey Y'all,

I have a dripping fuel drain valve. Part number is MDI F-391-18. Went ahead and ordered all four.

I can't find any specs though. It looks like you just put a socket on it and unscrew it, plug the hole with your thumb and then put in the new one. Surely it can't be that easy?

Also, can't find any torque recommendations. The Curtis equivalent recommends 80-85 in-lbs. Curtis also recommends installing with thread sealant, either Permatex 80632 or Loctite 592. On some other site someone recommended EZ Turn.

Any help appreciated

Cheers
Tim

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montana maule
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Re: Dripping fuel drain valve

Post by montana maule »

Tie a rag around your arm above the elbow to stop the AV gas from running all the way down to your arm pit. Wait for a fairly warm day. Mine seen to like to malfunction at the most inopportune times. Try replacing at -20F some time!

VA Maule
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Re: Dripping fuel drain valve

Post by VA Maule »

The original equipment drains MDI F-391-18 is not serviceable other than to replace. I swapped out all 4 of mine with SAF-Air CAV-110 which are easy to clean the grit out of or replace the little “O”ring Aircraft Spruce carries them too. Learned the value of serviceability several years ago when the airport’s fuel pump hose was breaking down and deteriorating on the inside and the little gritty flecks of old rubber hose was getting under the “O”ring seat of the drains . It was quite a mystery what was going on for a while until a group of us figured out what was happening.

Most any pipe thread sealant is satisfactory Teflon based paste is best. Torque is simply tight enough not to leek. Every time you reinstall the valve you’ll stretch the female fitting in the fuel tank, so just tighten enough not to leek.

Handy technique secret is to tie a towel/ rag around both of your wrist to soak up the fuel dribble that’s gonna run down your arm and saturate your shirt and beyond, have the new valve pipe doped and ready to go in your other hand
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Andy Young
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Re: Dripping fuel drain valve

Post by Andy Young »

Sounds like you’re doing the ones at the tank, correct?

If you’re talking about the two belly drains, it’s a bit more complicated. If you don’t get under the floor to put a wrench on the fitting that the belly drain threads into, you risk twisting the fuel line and breaking it off. No way to stop the flow of fuel then, until the tank is dry.

For any of the drains, I prefer to drain the tanks through those belly fittings before removing the drains. Takes a bit longer, but less messy, easier to get them properly sealed, and less risk of spill/fire.

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Re: Dripping fuel drain valve

Post by Kirk »

Andy, I used to know but forgot. Which of the belly drains is which? Forward is mains aft is aux?

Kirk

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montana maule
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Re: Dripping fuel drain valve

Post by montana maule »

Front will drain left main and back will drain right main.

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drak130
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Re: Dripping fuel drain valve

Post by drak130 »

Yup, it is the wing drains. Thanks for all the advice.

I already ordered the replacements, but next time the Saf Air looks like a better option.

Tim

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