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Discussion Starter · #1 ·
Mine is a 1957 Giulietta Spider Normale. I have no confidence in the original metal brake lines due to decades of storage. The string-starter information in an old post (by GTD : brake fittings, brake nuts - review Sep 5, 2012) was excellent but left me with a question due to my ignorance.

The original lines used a bubble flare, which, being a single-use type, is less than desirable. The old post/string mentioned using Fedhill parts for replacing the male nuts, and possibly using new 3/16" (4.75mm) copper-nickel tubing. Sounds great, but must we continue using the bubble flare ?

The Fedhill nuts will work with the bubble flare, but will also work with the far more desirable double-flare. But will the component which the male nut fits into accept and work properly with the double flare and the new nut ? Or is the shape of the mating surface within the component unable to seal with the double-flare ? *

* The answer is NO ! The "female" shape of the tapered mating surface within the component IS unable to seal with the double-flare which needs a "male" tapered seat.

While I was writing and researching this, I got excellent assistance form Michael at Centerline who sent me a picture of the two types. I found better pictures and they are attached here. So it is obvious that a double flared end cannot be used in a component expecting a bubble flare.

Now the question I will pose is an engineering one. It looks to me that if you had a small, steel, machined "adapter" which fit inside the threaded hole, and had tapered male cones on both sides, it would change the component's female taper into a male taper to match the double-flare tubing end. It changes the double-flare end into a "male" end which appears would seal under the nut's pressure against the "female" seat of the component.

There must be qualified engineers out there to vote on this. My intuition about it tells me that the adapter needs to made with a good steel. Since the (1/4"+) OD and 1/8" ID are pretty much set, it seems that the thickness needs to be enough more than just the tapered surfaces (a few thousands ?) to withstand the compressive-crushing forces that tightening the connection will impose on it.

It appears that the double-flare's seat angle of 42 degrees would work well - it matches the design of a double-flare seat, and the slightly higher angle seems that it would seat well into and against the bubble seat's 30 degree angle. If this presents a deformation-pressure problem, then one side could be a matching 30 or 35 degrees.

Any of us desiring to change out brake components is affected by this issue. It would certainly be nice if we could disassemble and reassemble our components for servicing without having to cut and flare or totally replace the hard lines.

Insanity or OK ?

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I have installed Fed Hill's "Cunifer" lines and re-made new bubble flares on a couple of cars, and had no issues.

That said, someone must have already produced bubble-to-double adaptors.
 

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I use 3/16" steel line from NAPA with a bubble flare- usually both ends.
Cut off 1 end. Install old fittings from your line.
Re flare double flare on the cut off end.
I have done many new lines on lots of Alfas this way.
Never an issue. Including race cars.
 

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Discussion Starter · #6 ·
I use 3/16" steel line from NAPA with a bubble flare- usually both ends.
Cut off 1 end. Install old fittings from your line.
Re flare double flare on the cut off end.
I have done many new lines on lots of Alfas this way.
Never an issue. Including race cars.
Do I understand correctly - You install the double-flared end using the original bubble-end nut into the unmodified bubble socket and it works ?
 
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