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I almost had a disaster installing a capacitance fuel probe in my header
tank.  This post is to tell you to avoid the problem, and hopefully to find
out how to do it right.  (I have to install two more...)

The basic task was to cut a hole in the tank big enough for the body of the
probe to fit thru, then glue on an aluminum backing plate (the one with the
five holes) onto the inside wall of the tank.  

I used a 2-5/8" hole saw to cut the hole.  It worked great.  The probe body
(one of those Skysports probes) fit quite nicely into the hole.  Not snug,
but not loose and sloppy, either.  I realized that there was very little
margin for the backing plate to be off-center.  What better centering device
than the probe itself?  So I covered the probe with release tape ("Flash
Tape" from Aircraft Spruce is what I use and it normally works well) and
bolted the probe to the plate.  I did a trial fit of the probe by inserting
this assembly into the hole from the inside, snaking the wires thru the hole
first, then the probe body, then the backing plate -- which of course, did
not fit thru the hole.

Then I mixed up some epoxy-flox and applied it to the backing plate.  I
brushed a light coat of pure epoxy (no flox) onto the
cleaned-then-scuffed-then-cleaned inner tank wall to ensure that there would
be no leak paths between the backing plate and the tank wall.  I reinserted
the probe-and-plate until the plate fit up against the tank wall and the
excess flox squished out.  I then hung a 5 pound weight from the probe body,
attached via the safety wire holes in the bolts holding the probe to the
plate.  I scraped off the squished out excess flox from the tank inner
surface with a tongue depressor and applied peel-ply over the residue.

So far, so good.

Imagine my surprise when, a few hours later, I tried to pull the probe off of
the plate and failed!  Yes, I had removed the bolts.  But there was enough
epoxy gluing the flash tape to the backing plate and holes edges that it was
EXTREMELY hard to remove.  I finally managed to pry the probe off with my
fingers, but it was a very near thing.  I don't want to do this again on my
wing tank fuel probes.

Any ideas?

Oh, yes.  The next day I put 3-BID over the plate to complete the job.

- Rob Wolf
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