Bill,
(Any relation to William Bligh?)
I am a licensed ship's engineer and have a few other credits that
apply here and I an going to answer quite a few of your questions in
line - I don't like this, but it will work best here.

Bill Bligh wrote:
Trying to assemble a diesel fuel polishing device from filters,
pipes ( nipples ), gauges, magnet, a pump, hoses and many
valves.

Loctite 242 used by one manufacturer, Loctite 55 sold
for the purpose by plumbing supply company. The usual
sodden wharf rat layabouts urge teflon tape - but every
article or book which mentions teflon tape and diesel
fuel warn against the stuff - use the engine as a mooring
after using teflon...
The problem is not the teflon as a material. It is good.
Problem with teflon tape is multifaceted. It does not do well with
steel fitting (often used in diesel fuel systems) - because die cut
threads often have burns that tear up the tape, it balls up in the joint
and makes a solid seal hard to get. If tape is wrapped on in the wrong
direction, it will possibly ball - again. If tape is wrapped over the
end of the male thread so it is exposed, pieces may get knocked loose
and carried downstream and cause all manner of grief (probably the cause
of the engine reference) as the working clearances in diesel fuel
systems are very tight. Tape may also end up exposed if the wrapped
portion extended through the fitting it is threaded into - same-same.

Guess I will be calling polisher manufacturers asking
what they use. And, possibly L55 locked nipple and fitting
in a jar of diesel for a week or two?
L55 is a thread of stuff you wrap around the male fitting (as I recall),
it can't be much different than teflon tape.

Would have been cheaper and a lot less painful to have
bought a small commercially built polisher. A lot cheaper!
But you will have one that works the way you want.

One mo marine looking ball valves such as the full size
1/4 NPT valves from "Apollo," are they OK for marine
use carrying diesel?
Apollo used to be a good manufacturer. You will have to get a material
list for the specific valve you are planning to buy. If it is bronze,
stainless steel and teflon - yeah, it's good.

Thanks,

Bligh
Good Luck Guy
Matt Colie
Lifelong Waterman and Licensed Mariner