<VV> Fuel Leak

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 
like that.
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.

-----Original Message----- 
From: roboman91324--- via VirtualVairs
Sent: Wednesday, September 26, 2018 9:52 PM
To: virtualvairs at corvair.org ; judynrandy at comcast.net
Subject:  Fuel Leak

Randy,

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 
with gasoline.

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.  :-)

Doc

In a message dated 9/26/2018 9:00:11 AM Pacific Standard Time, 
virtualvairs-request at corvair.org writes:

Message: 1
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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