[BOUNCE shop-talk@Autox.Team.Net: Message too long (>10000 chars)]
Date: Mon, 7 Apr 1997 22:33:51 -0700
From: cak@dimebank.com (Chris Kantarjiev)
Subject: Re: metal brake line replacement
Can anyone enlighten us on the use of bubble versus double flares?
At long last, here is my promised reply. Ray's Bugeye uses Girling
bubble flares, not ISO bubble flares...
---
The following text is a summary from many sources, most notably
messages from the wheeltowheel and (late lamented) hotrod lists,
authored by Scott Griffith and Dave Williamson. A fair bit of data
about the particulars of English braking systems comes from Dave Bean
Engineering's "English Ford Racers Catalog", a catalog that is
valuable for anyone racing any sort of car built in England.
I write this article somewhat from the viewpoint of English cars, and
in particular English racing cars, because that's what I know
best. I'm not intimately familiar with what Detroit is putting on its
cars these days, but I have good sources. I have never touched a car
with metric/ISO brakes. So the details on those two will be a little
fuzzier - but I have tried hard not to make anything up.
[Sadly, we live in litigious times, and I feel I must protect myself.
This article is a sincere effort to impart a basic understanding of the
complex arena of brake plumbing. If, while attempting to apply any of
the ideas, procedures or suggestions herein, you should experience a
brake system failure, it will be as a result of your own conscious
decision. I dislaim responsibility for your actions - and your
accident.]
English cars are almost always plumbed in 3/16" tubing, which utilizes
standard 3/8-24 UNF thread swivel nuts. The exception is the piping
used around tandem master cylinder differential valves, which
sometimes use 7/16-20 UNF swivel nuts in certain circuilts to prevent
incorrect routing or cross plumbing. The other exceptions are certain
clutch hoses which require the high-flow capacity afforded by the 1/4"
line (hence the 7/16-20 UNF thread).
Brake lines should always be fabricated from Bundy tubing having a
wall thickness of 0.028" minimum. Brake line pressures can and do
exceed 1000 psi. On no account should copper, aluminum, or commercial
fuel line be used, despite the fact that they are easily available and
appear to be easy to work with. Pur copper tubing work-hardens and
becomes very, very brittle- especially at flare fittings. Under the
cyclical loadings seen from brake applications, and in the presence of
moisture (and possibly high concentrations of chlorides, if you live
in a road salt area), they will almost certainly fail. They will tend
to crack and fail right at the flare, generally with little or no
warning. (Copper also catalyzes the auto-oxidation of fuel, the
mechanism of sludge formation. Copper should never be in contact with
fuel.) The British have developed what they call "copper brake pipe",
which seems to be a seamless tube made of a copper alloy that bends
easily, does not rust, and is in reliable use for "classic cars". I
don't have experience with this stuff, so I won't say good or bad
about it - I *do* know that it is different from pure copper. It is
almost certainly not as reliable as honest-to-gosh Bundy tubing, and
thus shouldn't be used on a race car.
Bundy brake pipes are available in various lengths at most parts
stores, though they almost always have the wrong swivel nuts (no
matter which one you need!). If you're doing a lot of this work, you
can buy the Bundy tubing in 20 foot coils from Aircraft Spruce,
Pegaus, or your local circle track store for about $8.
The swivel nuts from the old lines can sometimes be re-used if they
are in good shape. But chances are that you're reading this because
you want to upgrade your fittings, rather than just replacing what's
there. Most auto parts stores can supply the standard fittings that
are on current production cars; various British parts suppliers can
sell you new Girling parts, and aircraft and race suppliers will
happily take your money for AN fittings. Play it safe - if you're in
doubt, buy new. While you're at it, you might follow the lead of many
folks doing race car prep and only use steel fittings, even though
aluminum is available.
There are four (more or less) flaring styles in common use for brake
systems. British cars have a bubble flare (aka Girling flare) backed
up with a male swivel nut or a 45 degree double flare backed up with a
female swivel nut. Metric cars have ISO bubble flares, where the
pipes and threads are metric sizes rather than inch. Detroit iron has
a 45 degree double flare backed up by a male threaded nut. Most
(non-British) race cars are plumbed with AN (aka JIC) type single
flares - a 37 degree single flare with a backup sleeve and inch
threaded swivel nut (some people make a double flare here, which is
useless overkill and may lead to failure; see below). Lastly, some
brake fittings use tapered pipe fittings.
Take the easy one first: tapered pipe fittings are not really a
positive seal under adverse conditions. They may do the job for a
street car, but they certainly have no place on a race car.
The bubble flare is used with a male swivel nut, and seals at the
bottom of a drilled and tapped hole, with a nice angled bottom. While
it can usually be resealed, it has a limited lifetime - there's no
good way to get back the deformation that was crushed out for the
first seal, short of remaking the flare from scratch.
Making such a flare is easy: if you are in possession of a standard 45
degree double flaring tool, the bubble flare is what results after the
first half of the operation. Simply stop there, and you have the
bubble flare which will seat nicely at the bottom of the hole. If you
continue, inverting the form tool and finishing the job, you *then*
have the more familiar double flare used by Girling and the US
automotive industry.
The SAE 45 degree double flare usually has a male-threaded tube nut
that bears directly on the OD of the flared tube- so you need a double
flare to help control galling that can result in stress cracking right
at the flare. In short, you need "give" there. Problem is, the
deformation that results is kind of irreversible, so the next cycle or
two will result in your having to use astronomical torques to keep the
flare from weeping. Worse is trying to use a single flare in an SAE
flare nut and seat, and worse still if the seat is brass- the flared
tube is sqaushed from both sides, even as it is deformed by the nut
galling on it. The brass seat deforms and work hardens. It may seal
_once_, with a ton of torque and some luck. It's not recommended
practice - it's not even a good idea.
Racers (and aircraft, which is where the system originated as the
"Army-Navy" or AN standard in WWII) use the single 37 degree flare.
The AN single flare is still a concave flare, but its 37deg angle
seals by stretching, not squashing. The tube is supported by a
separate sleeve that the female-threaded tube nut bears upon. This
isolates the flare from the torques imparted by the nut. So rather
than trying to get a seal despite the presence of rotating torques and
the resulting galling, you press the flare between precisely-machined
(steel!) seat and precisely-machined support sleeve. The sealing area
under compression is at least double that of the SAE flare. An
additional bonus is that the OD of the nut is a lot larger than the
3/8" of an SAE nut, which means you won't kill as many trying to get
the proper sealing torque. (Even so, you should always use a proper
flare nut wrench on *any* tube nut.)
The SAE stuff was designed to go together once on the assembly line,
and then be "immortal", as defined by Detroit. It's pretty good at it,
too! The AN stuff is designed for field serviceability, long fatigue
life, and a level of bulletproofness the SAE never considered. A
further, Very Strong, recommendation is that single flared AN fittings
are the only thing that Carroll Smith will suffer to put on his race
cars.
The reason for harping on repeatability and multiple mate-demate
cycles is that, to the best of my knowledge, I have never once put
something on the car, and had it _stay_ put on. I always forgot
something, or broke something during the season, or needed to swap out
something because it was at the end of its service life. While the
double flare has that nice "squish" feeling as you tighten the flare
nut the first time, the AN fitting has the same torque requirement for
the second mating cycle as the first. That's where the reliablity
across multiple cycles comes from.
*Never* use compression fittings for brake plumbing. I have a further
rule that I use, which is to never use anything but steel fittings.
I know racers that have gone so far as to purge all the SAE double
flare fittings out of the plumbing of their track cars, except
possibly the fittings at the master cylinder. The rationale is
two-fold: first, the above-mentioned repeatability, and second, the
fact that the AN parts are universally available at every race shop in
every backwater in the country.
I have not gone quite so far on my British cars, which are plumbed
with Girling flares. It is a happy coincidence (at least, it seems
like a coindcidence, perhaps it is by design) that the female Girling
flare hardware will mate comfortably with a male AN-3 fitting; I have
thus replaced the fancy fittings on the rubber hoses with the
appropriate AN-3 male-to-male bulkhead fittings, and the hoses
themselves with -3 female fittings on Teflon -3 line. My master
cylinders all have machined flat surfaces, so the outlets are easy to
adapt to -3 hoses as well (-4 for the clutch, typically).
A few words about Teflon lines: they may not have a place on your
street car. For starters, the Teflon lines you make up at home don't
have DOT approval, because they haven't been through the DOT approval
process and you're not a certified manufacturer. Even the Teflon lines
you can buy pre-built are probably not DOT approved (though there are
a few coming onto the market) for the same certification reasons. The
main reason for this lack of approval is that manufacturers of racing
gear don't care to spend the money - they're building the hoses for
racers. The next reason is that the DOT expected lifetime for such
brake hoses is five years, and the prudent racer will replace his or
her brake hoses every other season, just to be sure. (This is a good
reason to make your own - then you can reuse the fittings and just buy
new olives and hose.) There are some concerns about using Teflon line
in cold environments, though Aircraft Spruce is willing to sell you
lines custom made from Aeroquip Teflon hose and rate them to -40 degF,
so I'm not so sure that's a concern. It may well be that the intended
application for those lines is not subject to as much repeated
deflection as the flex line snaking through your front suspension out
to the caliper.
There is one other kind of sealing involved in braking systems,
usually at caliper or cylinder entry and often when converting over
from one kind of flaring system to another: a washer face at the top
of a hole. This requires a crush washer of soft copper, aluminum, or
in extreme cases, a Stat-O-Seal, which is a combination of O-ring and
aluminum crush washer. To make this an effective seal, a precision
machined surface is required around the drilling, exactly
perpendicular to the axis of the threaded hole. If this is not there,
you *must* use a bottom sealing fitting. Conversely, if the hole does
not have a nicely formed drill point form at its bottom of the fitting
will not reach the bottom, you must use a sealing washer (and, of
course, provide the machined washer face for it to seal against).
A neat trick I learned a while back is that copper crush washers can
be reused - *IF* you anneal them. Places the washer in question on a
brick and play your propane torch across it until it glows orange. Let
it cool. It's now as soft as it was originally and will re-crush and
seal perfectly.
Good flaring tools are easy to come by. Get the catalog from:
Name: Aircraft Spruce and Specialty Company
Address: 201 W. Truslow Avenue
P.O. Box 424
Fullerton, CA 92632
Phone: (800) 824-1930 order hotline
(714) 870-7551
(714) 870-7315 customer service
(714) 871-7289 fax
They'll charge you $5 for the catalog, spend almost that much shipping
it, and include a gift certificate for $5 on your first order. Well
worth the trouble. This company specializes in selling stuff to
aircraft homebuilders- which makes it an exellent source for wierd
tools, small quantities of raw materials (ever try to buy just 2 feet
of 4130 chromoly tubing before?), GOOD hardware (meaning NAS and AN
mil/aero-spec fasteners) and other nifty arcane thinguses. If you are
in a part of the country that doesn't have places that sell to us
crazed enthusiast-types, you need this catalog.
They have the Rol-Aire Imperial, which is the flaring tool of choice
for 37deg AN single flares, and several SAE and ISO flaring tools as
well. I have several tube benders, but my favorite came from Eastwood,
another source of interesting tools for the home mechanic.
Name: Eastwood Company
Address: 580 Lancaster Avenue
Box 296
Malvern, PA 19355
Phone: (800) 345-1178
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