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Newbie Question on tube flaring


Well Known Member
Hi all,
I have been flaring my 3/8 fuel lines...and I thought I was done. But....

The short story:
..how many many turns of a Rolo-flare should it take to get a 'good' flare on a 3/8 aluminium fuel tube?

The long story:

The "instructions" on my Rolo-flare tool tell me to put the tube against the flare stop in the tool, then rotate the burnisher "until there is resistance" Hunh? How 'much' flare is 'enough' flare?????? and how much is 'enough resistance'????

Well, I looked at the Aircraft practices guide, and it's only guidance was a cut-away of the flare which shows the flare edge just proud of the collar fitting. OK...copy that. When I did it this way, the inside of the tube is burnished, and the outside of the tube is only slightly distorted, and the tube is just proud of the collar (about 1/128") This took 1 and 1/2 turns of the burnisher. Tighten the AN fitting over the collar till its hand tight, then wrench turn it a flat and a half. Hmm...that seems to have very little torque resistance. Torque it another flat...no change. You guessed it, the tube had formed itself back to straight and slipped through the collar.

OK. Try again, only this time, I turned the burnisher 2 1/2 turns...(the 'until there is resistance' advice doesnt work for me...I have nothing to compare to.

The flare is now about 1/16 proud of the collar, I can turn the fitting tight, then wrench it 1 -1/2 flats....torque is going up as I think it should.

But I AM NOT SURE. I want to be. See question above.....

Thanks for your help,
When you tighten the flaring tool, there is definitely an increased resistance when the flare bottoms out. There should be no question.
Also, smear a small amount of Bearing grease on the cone. This will prevent chattering and the lip will be smooth as glass. If the formed edge feels rough, it is a bad flare. I just finished going through this with my -8.
You seem to be confused because there is resistance from just turning the burnishing tool against the tubing when the flare is beginning to be formed, and you are wondering if this initial resistance or torque is telling you to stop.

Do this experiment: take a piece of scrap tubing, and flare it in the tool until the burnishing cone completely squashes the flare, ruining it. Do not go so far that the burnishing cone is in danger of contacting the mandrel, however. Did you notice how the "Prevailing" torque from just doing the initial flare increased markedly when the flare became squished against the mandrel? That increase in torque is where you want to stop. I think this is more reliable than counting turns on the tool.
Just remembered a couple of things about flaring.

1. if you turn to full stop, the flare will be too large to fit inside the fitting.

2. I watched the formation of the flare while turning. All of a sudden, you know when the flare is right. IIRC, it is 5-6 half turns to a perfect flare.
Be very carefull!

Hello Chris and others

There where lost a few RV's because of bad flarings within the first few hours of flying! In switzerland we lost the first RV-9 due to that! Be very carefull here.

It is a shame how the supplyer of the flaring tool describe how to use it. Also Vans should write a page about how to do it right!

In an accident report they refere to Norm SAEJ533 below I copy in that what I found on different sources. Resurch for yoursefl or ask your tecnical conselor!

Also is the material 3XXX not so strong, a "thinned out" flare will crack and separate from the tube under stress (due to not propperly adjusted, bended, under vibration, and so on, tube). 5052 is stronger but more expensive, especially to send around, they get sended straight and not coild.

Also when you tighten it the first time, let the material "settle" down, open the connection and tighten again. There is also a table with values for the tourque that is needet to tighten a nut or you can count the flares. Here is a link to Vans page with original information: http://www.vansaircraft.com/pdf/Torque_Spec_Aluminum_Fittings.pdf

I hope it helps, and I hope also a person who is in the "flare" bussines join in and give his comment and advices.

Regards, Dominik

· Ensure that the end of the tube is cut square within ±1 degree and remove the burrs and sharp edges.
· Wipe the ID and the OD of the deburred Tube end properly.
· Insert the nut and sleeve on to the tube as shown below

Flare the Tube with a special flaring tool to 37 degree, dimensions of Flared Tube according to SAEJ533 standard as shown below.

Screw the nut on the body until finger tight. Further tighten the nut with a wrench ¼'' turn in order to make a leakproof metal to metal sealing connection.


Dimensions are for reference only, and are subject to change without notice.
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Thanks for all the input.

I have done a few more experimental flares using the advice from:

1: Advice from Greg: Add lubricant before flaring
2: Advice from Dominik ...Make sure the end is clean and smooth
3: Advice from Mel...Turn until there is a definite change in resistance
4: Advice from SAEJ533 via Dominik: Measure the flare - between .46-.49

How about that...it was about 4 and a bit full turns to an increase in resistance, the first time it measured out at .505, so I went a bit too far. It looks far bigger than I 'thought' it should be....I guess thats why we have standards. When you dry assemble the pieces with the collar and the fitting and no nut, you can see how the male taper of the fitting is completely covered by the flare.

A local RVer has also offered to drop by to check out my technique.

Thanks everyone, I'l get it right yet!

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