<VV> Fuel Leak
Hugo at aruncoaches.co.uk
Thu Sep 27 05:19:26 EDT 2018
An interesting hypothesis, but I'm going to disagree with most of what you
say - apart from the fact that if it is proving so difficult to get a seal
when there's only a couple of pounds pressure, there is clearly something
fundamentally wrong. It just should not be that critical.
A bit of background - I am a coach operator based in the UK. we have a thing
called 'wheel loss syndrome', whereby the left-hand rear wheels tend to fall
off buses. At least that's what the politicians call it - I call it simply
bad engineering practice.
Some years ago we switched to the European system of having right-hand
threads on all wheel fastenings. British vehicles used to have left-hand
thread lug nuts on the left side of the vehicle (as did quality cars also).
This is to counter the effects of 'precession', which will tend to make them
unscrew otherwise. A bicycle pedal will have left-hand threads on the left
pedal for the same reason, which seems counter-intuitive at first, till you
understand the principle of precession.
Anyway, back to bus wheels falling off - the coach and bus industry as a
whole is paranoid about this 'syndrome'. But I am not. The main reason that
wheels keep falling off is because people adopt the procedure you have
outlined - they fit everything dry. Let me try to explain - a bolt or
set-screw is like a spring. When you tighten the nut, it will stretch, and
clamp everything together like a strong spring. On old British Triumph
motorcycles, they didn't give a torque setting for the big-end bolts (conrod
bolts) - you just used to tighten them till they stretched 1/8" or something
On the back wheel of a bus, with twin wheels, you have a total of six mating
faces. Any dirt, rust or paint on these faces when they are assembled will
inevitably wear off in use, leaving the whole assembly loose. If the studs
are not stretched sufficiently, the whole lot will fall to bits. So the
first thing I do is clean all mating faces. Then I grease them lightly. I
never put two metal surfaces together dry. Then I use copper grease
(anti-seize grease) on the studs and lug nuts. That is to minimise the
friction on the threads, and ensure that the stud is properly stretched and
applying sufficient clamping force. Many if not most operators will fit the
lug nuts dry. The problem with that is that a large part of the tightening
torque is absorbed in overcoming the friction of the thread, and the stud
doesn't stretch sufficiently to clamp everything together. The nuts are
tightened till they are stiff rather than tight. These are the people who
have to check their wheel nuts with a torque wrench every day and fit little
plastic indicators so they can see when they are coming loose. These are the
people who end up in traffic court when their wheels have fallen off, and
everybody scratches their heads because "all the proper procedures have been
followed". I never check mine from one year to the next, I never use a
torque wrench, and they never come loose. The only remaining mystery is why
American trucks & buses don't seem to suffer from this problem.
I realise this diatribe is of limited value when discussing fuel lines.
Power-steering lines will hold a couple of thousand PSI without leaking, so
as I said earlier, it should not be necessary to adopt all sorts of exotic
practices to stop gasoline leaking out at a couple of PSI. I reckon you are
missing something somewhere.
From: roboman91324--- via VirtualVairs
Sent: Wednesday, September 26, 2018 9:52 PM
To: virtualvairs at corvair.org ; judynrandy at comcast.net
Subject: Fuel Leak
Great advise overall. I would like to expand on your statement.
When Engineers design threaded fastening systems, they almost always assume
dry thread-to-thread contact. This dry condition results in the intended
and predictable amount of friction between the male and female threads as
you tighten. As you tighten the fastener, the force between the threads'
contact faces increase and they deform in the process. The increased
compression increases the frictional force until the design torque is
reached. The result is that there is enough holding force to hold things
where they should be while ensuring there is enough friction that the
threads will not back out. Again this assumes dry contact between the
threads. (The use of thread sealers, etc. is another issue.)
When there is a lubricant between the threads, two potentially bad things
happen due to reduced friction. The first bad thing is that you have
increased potential to strip the threads. You imply this with your
cautionary note. Because of reduced thread-to-thread friction, you will
compress/stretch/deform the threads significantly more before you reach the
specified torque. This could strip the threads. That is the case with
bolts and such. In this particular case, the threads will probably survive
but you will likely crush the flared end of the tube resulting in fractures.
The second bad thing is that with the reduced friction and the "finger
tight" torque, the connection has increased potential to loosen and leak.
Keep in mind that the fitting will be experiencing repeated heat/cool
cycles. A fractured flare or loosened fitting are very bad when dealing
Your suggestions to inspect mated surfaces and even prepping them are good.
However, I would like to modify your grease suggestion. Put grease ONLY on
the inside of the flared tube OR on the male side. The threads must remain
clean and dry. ALL of the sealing is done on the face of the flare.
Threads are not intended to do any sealing in this application. This way,
you can tighten the fitting normally. Never use thread sealer or tape. It
is too easy to get that stuff into the fuel system. The tiny bit of grease
in the fuel is soluble in gas and will quickly dissipate.
If you suspect you have a bad connection, the best solution is to cut off
the flare and put a new flare on it. I think you can borrow a tool from
your FLAPS if they have one. Better yet, bring your tube with you and
perform the operation in the store. Make sure the cut is square to the axis
of the tube and the face is clear of burrs from the cut. Oh, and make sure
the female fitting is on the tube before you flare it. Don't ask how I know
about this. :-)
In a message dated 9/26/2018 9:00:11 AM Pacific Standard Time,
virtualvairs-request at corvair.org writes:
Date: Tue, 25 Sep 2018 22:42:51 -0400 (EDT)
From: JUDY HOOK <judynrandy at comcast.net>
To: virtualvairs at corvair.org
Subject: <VV> fuel leak
Message-ID: <485071148.3836.1537929771441 at connect.xfinity.com>
Content-Type: text/plain; charset=UTF-8
Try this wonderful trick. Disassemble the fuel lines and physically inspect
each sealing surface. Check for things like micro-cracks and deep scratch
marks. Also, a small piece of dirt or debris stuck to the fittings will
cause a leak, too.
Make sure they "mate" up properly. You might even try lightly sanding and
polishing each sealing surface area with a piece of fine grit sandpaper to
insure smoothness and a good sealing surface area. Now for the kicker -
Lightly coat the sealing surface, tube, and fitting with some axle grease. I
use Amsoil's synthetic grease. Then, assemble everything just finger tight.
When you go to tighten things, just tighten them slightly. With the grease
on them, it's super easy to over tighten them. I usually use the "just snug,
then a little more" formula. Remember, you can always tighten it a smidge
more, if needs be.
Hope this helps,
Randy (Cap'n) Hook
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