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Aircraft applications frequently use straight thread O-ring ports (SAE J514 and
MS16142). These ports are characterized by a spot face with a countersink (about
15 degrees) and a UNF-2B or a UN-2B thread (depending on size) just like in your
tap and die set. The O-rings used can be ordered by their industrial size (Parker)
in the format 3-9xx where xx is the fitting size, so -4 would be 3-904 and -12
would be 3-912, from an hydraulic supply house. While these O-rings are close in
size to the "standard" industrial sizes, they are slightly different.  The
preferred material for the O-ring is Viton in a 70 durometer hardness but 90
durometer will work.

For the fittings, they are available again from any hydraulic supply house.
Specify a "straight thread O-ring port to male JIC 37 degree" in the dash sizes
required. Avoid 45 and 90 degree fittings as they are much more expensive and are
significantly more restrictive to flow. As a rule, never use a 90 degree fitting
on the inlet of any pump as it can promote cavitation.

One trick is to substitute a male flare union for a port to flare adapter. You
will need to cut off the conical tip down to the first thread  on one end of the
union (if you are uncertain about this, call your rabbi), add an O-ring to this
end and insert it into the port. Voila! Instant adapter and a "tip" for your
trouble.

Use quality steel fittings on anything that shakes a lot and carries flammable
liquids. Aluminum is more likely to fatigue and break. I was once forced to retire
a race car in the 22nd hour of a 24 hour endurance race because a $3 aluminum oil
fitting cracked. At least there I could pull into the pits.

"Quality is the result of a keen memory for pain".

Regards
Brent Regan