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Old 09-17-2021, 08:44 PM
Captain Avgas Captain Avgas is offline
Join Date: Oct 2005
Location: Melbourne, Australia
Posts: 2,011

Originally Posted by rileyspoon View Post
Anyone care to explain why sealant on A is less likely to cause an issue than B? I understand that sealant on a flared fitting is probably a waste of time and not needed, but I'm not sure I see the need to disassemble all the fittings and remove the sealant from the flared side and put it all back together. Especially now that it is holding pressure at 50psi.

Just asking for a friend.....
Here's a few comments:

1. Firstly, you may be building in a knowledge vacuum. Are you a member of an EAA chapter. If not it might be a good idea to join one.

2. The flared fittings require no sealant and in fact it is very poor practice to put sealant on flared's dangerous. If you have leaks at the flared fittings it is because you are not doing the flares properly (which you are not). My recommendation is that you disassemble every flared fitting with sealant on it and clean them all thoroughly. I'd also clean out all of your fuel lines in case you already have any bits of sealant in them. At the same time I'd ensure that all of your flares are within spec.

3. The sealant you are applying to the flared fittings is probably not achieving anything. If the flare leaks then fuel can escape via the collar and migrate out of the rear of the B-nut.

4. Putting sealant on flared fittings is not just an initial problem but will be a problem later on if you have to disassemble the fitting (which you will have to do on many flared fittings). At that time it will be difficult to stop bits of sealant from getting into the fuel line.

5. The torque values for flared fittings are dry torque values. Putting sealant on the threads will result in higher stresses on the aluminium flare. The 3003 aluminium tube that Vans supplies is not aviation grade and is extremely soft. The flares are therefore susceptible to deformation that can ultimately result in failure (quite common on flared fitting that are disassembled and retorqued numerous times).

6.The way you are applying sealant onto your NPT fitting is also problematic. You're applying way too much and it should never be applied to the first 2 or 3 threads to ensure it does not get into the fuel system.
You’re only as good as your last landing
Bob Barrow
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