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Is there anyone out there who has leak tested a 320 fuel tank with the
extra bay?  I have put on a test with 28" water column pressure ( about 1
psi) and I have also pulled a vacuum of 3"mercury.  ( 1.5psi differential
to atmospheric) I have been unable to hold either the vacuum or the
pressure overnight.  I lose about an inch of water pressure every 5
minutes. I have been all over every seam, aileron connection bellcrank
bracket etc etc etc with soapy water not a bubble to be seen or evidence of
the suds being pulled into the tank.  I have used a listening tube but not
a hiss to be heard.

Does anyone have any ideas as to how I can locate this leak?  I would be
very grateful to hear of other people's experiences.

Incidentally when I glued on the bottom skin I used a jig to clamp the skin
down on every glue line so I am reasonably certain that it is glued as I
had squeeze out at every point.  I also applied two thick coats of the
Jeffco tank sealer from Lancair prior to close up.

I am loath to put on more than 1psi given the pressure that even that
modest amount exerts on the total skin. I suppose that I could go with more
vacuum with less risk but I have no experience to draw upon.

All suggestions will be welcomed.

Regards

Ian Crowe.