AN to UNF Fittings

They're built for three different thread families, and mixing them up is the fastest way to a leak. UNF is a straight thread that seals through an O-ring or a controlled flare — it does not seal on thread engagement alone. NPT is a tapered pipe thread that seals as the threads compress together, which is why pipe fittings tolerate sealant but AN/UNF connections generally don't need it. BSP splits into two versions: parallel BSPP, which seals with an O-ring similar to UNF, and tapered BSPT, which behaves like NPT. If you're not sure which one you have, measure the thread — straight threads keep the same diameter along their length, while tapered threads narrow toward the tip.

Three quick checks before you order:
Count the threads per inch (TPI) with a thread gauge or by measuring — UNF is the fine-pitch version, UNC is coarse and will have visibly fewer threads over the same length.
Look for a stamped callout on the part itself; OEM and aftermarket components are usually marked.
If the part is from an aerospace, military, or high-fatigue application, it may be UNJF rather than UNF — the rounded thread root looks similar at a glance but is a different spec, so don't force a fit if resistance feels off.

Use this as a quick reference, then double-check against your part with a thread gauge or caliper:
-3AN = 3/8-24 UNF (3/16" tube)
-4AN = 7/16-20 UNF (1/4" tube)
-6AN = 9/16-18 UNF (3/8" tube)
-8AN = 3/4-16 UNF (1/2" tube)
-10AN = 7/8-14 UNF (5/8" tube)
-12AN = 1-1/16-12 UNF (3/4" tube)
When two sizes look close, the thread pitch (the second number) is the tie-breaker — matching diameter alone isn't enough.

No. A straight UNF thread is designed to seal through an O-ring or a 37° flare face, not through tape or paste wrapped around the threads. Adding sealant to a UNF connection can actually get forced into the flow path or interfere with the O-ring seating correctly. Install it dry: seat the O-ring, thread the fitting on by hand until it's snug, then tighten to the torque your application calls for — overtightening is more likely to cause a leak here than undertightening, since it can pinch or extrude the O-ring.

A correctly matched AN to UNF fitting, installed with the O-ring properly seated and tightened to spec, is designed for fuel, oil, and coolant duty — that's the whole point of the 37° flare and controlled seal design. What actually causes failures in the field is almost always a mismatched thread pitch, a missing or damaged O-ring, or overtightening, not the fitting design itself. For fuel lines specifically, it's worth double-checking the fit before your first start-up and doing a visual leak check once the system is pressurized, simply because the consequences of a fuel leak are more serious than a coolant or oil weep.