Fuel line length - how to locate the last connection/flare?

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Saville

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Ok so I'm a newbie to the rigid fuel line business. So I'm sure everyone knows the answer to this except me:

I'm connecting a fuel selector to an existing line that runs to the engine. The line will have a 90 degree bend under the selector then runs forward and has to precisely meet the existing line.

I've flared one end and made the 90 degree bend under the selector, so now the tricky part (to me) is to mark, cut and flare the other end so that it meets the existing male flare fitting with no stress - neither pushing or pulling. At this point the tubing is overly long.

I can mark where the tip of the male fitting is on the new tube. But that's not where I should cut it.

I could take about 3" or so of some tubing, measure it's length precisely, then flare one end and measure it again. This will tell me how much longer the cut has to be to accomodate the flare. But there's more to it than that, yes?

Any hints as to how to accurately cut the other end so that it meets the fitting exactly would be appreciated.

Thanks
 

TFF

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I usually cut the line about 6” long and get it in and trim and trim until happy. I tend to make the line slightly long in case I have to remake a flair.
 

Saville

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I usually cut the line about 6” long and get it in and trim and trim until happy. I tend to make the line slightly long in case I have to remake a flair.

Pardon my ignorance but I don't see how that could work given that you have to flare the line at the new end:

If you're long by only a little bit, I don't see how you'd have enough to cut the tube, flare the tube and not be too short.
 

Saville

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Put a loop in the line.
I thought about that. But the total run is only about 6"-7" a loop seems like a huge flow modifier for such a short run. Everything I've read says to make the fuel run as bend-free as possible.

There has to be a way to do this - It must have been done millions of times over the decades
 

Toobuilder

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The difference in the length of the tube between the raw cut end and finished flare is a variable that is determined by your tooling, style and type of flare (ie. double or single). figure out that dimension (lets say dressing and flaring the end "consumes" 0.125 inch), then simply cut the raw end until its 0.125 long of the seating surface and flare away.
 

Saville

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The difference in the length of the tube between the raw cut end and finished flare is a variable that is determined by your tooling, style and type of flare (ie. double or single). figure out that dimension (lets say dressing and flaring the end "consumes" 0.125 inch), then simply cut the raw end until its 0.125 long of the seating surface and flare away.
Are there no other factors that you have to take into consideration?Only the tube consumed by the flare?
 

Dana

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Best to route it so you have at least a little wiggle room, swing the next bend slightly wide or closer.
 

don january

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It all boils down to if your "lucky" with your cut length. You don't want push or pull on your line but that's the trick. Make a couple test cuts using the tool you attend to use and look for static mark for as close measurement as you can get and x- them fingers.
 

TFF

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You have to take it in and out a half dozen times until you get the final dimensions. The 1/8” is about right for the flair. As long as you keep cutting long, it never is too short. If the line is a short strait shot, you do need a bend in it for stress relief.
 

TiPi

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Shape the tube in a slight S-bend or 1/2 S-bend, then you can adjust the final length and it will also reduce stresses from temperature variations. Straight runs between fixed ends are not advisable.
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Daleandee

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Best to route it so you have at least a little wiggle room, swing the next bend slightly wide or closer.
I have a short fuel line that needed to have a small offset for clearance. That helped with the sizing.

Cutting the length while considering how much you need for the flare is the method to use but if you are a little long with the finished line a small "S" turn or offset might make it fit the length correctly.
 

Pops

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Take a piece of tubing the size you will be using. Measure the length. Put a flare on the end. Now measure how much shorter the flare made the tubing.
When making the flare tubing allow for the reduced length on each end.
 

Toobuilder

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Are there no other factors that you have to take into consideration?Only the tube consumed by the flare?
For length, yes. But only you, the installer/designer, can determine if the two ends are fixed enough to enable a truly rigid connection between the two. If there is ANY movement (to include significant thermal expansion), then you will absolutely need to build in a "cripple" in the tube (see post #11). One rule of thumb I've used is that you want at least 90 degrees of bend between fixed points to remove column or tension loads.
 

gtae07

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One little rule of thumb I’ve used when cutting the second end is to mark the tube even with the point on the fitting where the conical flared portion meets the cylindrical portion. If I cut it there, clean up the end of the tube, install the B-nut and ferrule in the right order and facing the right direction (don’t laugh, I can’t tell you how many times I’ve goofed up lines because I forgot to install them, and had to remake the line), then finally flare, the line usually comes out really close to the right length. Note, this is using an Imperial flaring tool (single flare) and a chipless tube cutter. YMMV.

Final tweaking, even with stainless lines, can be done with careful hand pressure or very judicious use of the tube bending tool, as long as the line is of sufficient length. The goal is no preload on the line.

I recently bent a few tricky lines out of stainless for my engine (governor oil line and EFI fuel lines). The routing is definitely not simple, with the added complication of having to make the lines match up with available points for P-clamps. It took a few prototypes made of soft aluminum before I felt ready to bend the stainless. Really made me miss the tubing functions in Catia and the automated tube bender at work. But then, doing it manually is pretty satisfying and rewarding once you get that complicated, smoothly-bent line with no preload.
 
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Pops

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A very small part of the Millwright craft's practical test is taking a tube schematic and making up all the lines needed and installing them on a large test jig.
Takes about a day and a half to do all the practical test. Welding test, threaded pipe test, build a large hydraulic system from a schematic and assemble and test and trouble shoot, Line up electric motor shafts and couplers to specs , etc, etc, etc.
 

Dan Thomas

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For length, yes. But only you, the installer/designer, can determine if the two ends are fixed enough to enable a truly rigid connection between the two. If there is ANY movement (to include significant thermal expansion), then you will absolutely need to build in a "cripple" in the tube (see post #11). One rule of thumb I've used is that you want at least 90 degrees of bend between fixed points to remove column or tension loads.
That there. No loops that can trap water or debris, just a bit of zigzag to provide some flex for thermal expansion and contraction and that will also allow a bit of adjustment to get the flare seated. The original 90° bend from the fuel selector helps, too, but without any further bends in the horizontal section, that 90° bend will take it all and the vertical section will be forced back and forth until its flare cracks.

It's easy to overthink some of this stuff. AC43.13 has plenty on the subject.
 

Saville

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I want to thank everyone for their contribution.

So it seems that the "trick" to making tubing to fit between two fixed points is the fact that you never have straight lines - you always put a bend in the line for vibration and stress reasons.

And that bend allows you to adjust the position of one end for the final fit.

Thanks!
 
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