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  #1  
Old 09-15-2021, 03:28 PM
rileyspoon rileyspoon is offline
 
Join Date: Oct 2020
Location: WINSTON SALEM
Posts: 29
Default Bubble, bubble means there is trouble

Did my fuel system pressure test today. Out of what 15 or so connections? I had two that didn't leak. Man I held my breath when cranking on those flare nuts to get them to seal. Followed the instructions but it still took a lot of work getting everything tight enough so that it would hold at 40psi. Took this picture after I started disconnecting the lines so I could tighten the "T" fitting.
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Last edited by rileyspoon : 09-15-2021 at 03:30 PM. Reason: Corrections
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  #2  
Old 09-15-2021, 03:49 PM
fr0gpil0t fr0gpil0t is offline
 
Join Date: Apr 2013
Location: Colorado Springs
Posts: 72
Default

I didn't think you were meant to put sealant on the threads of flare fittings ? But I'm a new builder.

Last edited by fr0gpil0t : 09-15-2021 at 07:59 PM.
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  #3  
Old 09-15-2021, 05:49 PM
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rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,595
Default

Quote:
Originally Posted by fr0gpil0t View Post
I didn't think you were meant to out sealant on the threads of flare fittings ? But I'm a new builder.
Heís correct. Sealant should only be applied to the NPT portion of fittings and nothing on the side that gets a B nut for aluminum line or hose end.
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Van's Aircraft Engineering Prototype Shop Manager
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RV-6A (aka "Junkyard Special ")
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  #4  
Old 09-15-2021, 06:32 PM
Untainted123 Untainted123 is offline
 
Join Date: Jan 2021
Location: Azle, TX
Posts: 42
Default Big No

Quote:
Originally Posted by fr0gpil0t View Post
I didn't think you were meant to out sealant on the threads of flare fittings ? But I'm a new builder.
An RV was lost due to a plug of sealant in the fuel line. Use proper lube on the NPT side and nothing on the flared side of any kind, the flare seating against the flared nipple side is what seals it, and the Al is compressed slightly. If you have used RTV or any kind of sealant that hardens or can come off in a blob, I would audit all connections and clean it all and flush several times before reassembling.
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  #5  
Old 09-15-2021, 06:47 PM
Osprey2gp4 Osprey2gp4 is offline
 
Join Date: Mar 2016
Location: Inver Grove Heights, MN
Posts: 8
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It appears from your picture that the flare on your tube is way undersize. Maybe Iím wrong, because pictures are sometimes distorted. The flare should be roughly the same size as the flare on the fitting that it mates to. The flare on your tubing looks much smaller than the fitting. Maybe your tubing slipped during the flaring process. Or you didnít have enough tube sticking out past the flaring tool die.

I would take a close look at your flares. As others have said, donít put anything on a flared fitting. The fitting seals by metal to metal contact on the flare surface. A properly made flare will hold in excess of 2000 psi with no sealant on the surfaces.

Best regards,

Bill Wuorinen
RV-7A N237W
Osprey 2 amphibian N137W
A&P I/A
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  #6  
Old 09-15-2021, 08:09 PM
Captain Avgas Captain Avgas is offline
 
Join Date: Oct 2005
Location: Melbourne, Australia
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Quote:
Originally Posted by Osprey2gp4 View Post
It appears from your picture that the flare on your tube is way undersize.
Yep, way undersized...not within spec.
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  #7  
Old 09-15-2021, 08:39 PM
6ato14a 6ato14a is offline
 
Join Date: Jan 2016
Location: longmont, co
Posts: 182
Default Flare size

What are you looking at to tell the flare is undersiza?
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  #8  
Old 09-15-2021, 08:42 PM
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rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by Osprey2gp4 View Post
It appears from your picture that the flare on your tube is way undersize. Maybe Iím wrong, because pictures are sometimes distorted. The flare should be roughly the same size as the flare on the fitting that it mates to. The flare on your tubing looks much smaller than the fitting. Maybe your tubing slipped during the flaring process. Or you didnít have enough tube sticking out past the flaring tool die.

I would take a close look at your flares. As others have said, donít put anything on a flared fitting. The fitting seals by metal to metal contact on the flare surface. A properly made flare will hold in excess of 2000 psi with no sealant on the surfaces.
Good catchÖ. I agree ( I didnít zoom in on the photo when I first looked). It is likely the cause of your numerous leaks

A good guide for a proper flare is that the flare outer diameter should closely match the OD of the sleeve.
__________________
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.

Scott McDaniels
Hubbard, Oregon
Van's Aircraft Engineering Prototype Shop Manager
FAA/DAR
RV-6A (aka "Junkyard Special ")
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  #9  
Old 09-15-2021, 08:53 PM
sparkcrafter sparkcrafter is offline
 
Join Date: Jun 2014
Location: Battle Ground, WA
Posts: 51
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I found this diagram to be helpful in determining the proper flare size. Per the chart, the outside diameter for this flare should measure 0.46Ē-0.49Ē for 3/8Ē tubing.
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  #10  
Old 09-15-2021, 09:46 PM
RFSchaller RFSchaller is offline
 
Join Date: Oct 2010
Location: Phoenix, AZ
Posts: 3,145
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Stupid question: are you using an Aviation 37-degree flare? The automotive 45-degree flare will not seal.
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