The flare joint done right — cut, deburr, correct projection, torque — and a reference to the brass fitting designation system stocked in the catalogue: U1 half-unions, U2 unions, UR2/UR3 reducers, US4/US5 swivels, N5/NS4 nuts, P2/P3 plugs, E1/E2 elbows and T2 tees.
An SAE 45° flare seals metal-to-metal: the tube's flared cone clamped between fitting nose and nut. Its reliability is entirely workmanship:
Flares that weep after thermal cycling were almost always under-projected or scored. Remake rather than muscle them: brass threads gall long before a bad cone seals.
| Prefix | Family | Stocked examples |
|---|---|---|
| U1 | Half-union, male flare × male NPT | U1-6B 3/8"×1/4", U1-10E 5/8"×3/4" |
| U2 | Union, flare both ends | U2-8 1/2", U2-12 3/4" |
| UR2 | Reducing union, male × male | UR2-86 1/2"×3/8", UR2-108 |
| UR3 | Reducing adapter, female × male | UR3-46, UR3-108 |
| US4 / US5 | Swivel nuts / swivel-to-solder adapters | US4-06, US5-88 |
| N5 / NS4 / NRS4 | Blanking nuts / forged nuts / reducing nuts | N5-6, NS4-6, NRS4-0064 |
| P2 / P3 | Flare plugs / NPT plugs | P2-4, P3-B |
| E1 / E2 | 90° elbows flare×NPT / flare×flare | E1-6B, E2-6 |
| T2 | Equal flare tees | T2-6, T2-8 |
| NFT5 / FAS | Flare caps / flare-solder adapters | NFT5-6 cap, FAS-4 |
The size digits are sixteenths of an inch (−4 = 1/4", −6 = 3/8", −8 = 1/2", −10 = 5/8", −12 = 3/4"), with letter suffixes for the NPT end on U1/E1 (A = 1/8" … E = 3/4"). NPT threads seal with an approved sealant on the taper (never on flare cones); flare ends seal dry.
A van stocked with U2 unions in 1/4–5/8", UR3 step adapters, NS4 nuts, P2 plugs and N5 blanking nuts closes out the overwhelming majority of flare-side service: gauge ports capped (caps with seals), disconnected lines blanked (N5 includes gasketless versions — note the listing), adapters bridging the odd 5/16" appliance connection (UR2-54). Service valves with flare ports and piercing valves for retrofit access live alongside — see the access-port practice in the purge guide.