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by Mike Mavrigian
photos by author
If hard lines are preferred to flexible hose, but you want to connect using AN 37-degree seating/sealing, you’ll use tube nuts and tube flares on metal tubing. The hard line (steel, aluminum or stainless steel) must be flared, using a 37-degree flaring tool. A tube sleeve slips on immediately behind the flare. This tube sleeve features a concave seat that mates to the back side of the tube flare. Behind the tube sleeve is a tube nut. This draws the tube sleeve and the flare onto the tube to the male 37-degree AN fitting or adapter.
When making tube assemblies, first slip the tube nut over the unflared tube, followed by the tube sleeve. The tube nut’s threaded port must face the end of the tube that will be flared. The tube sleeve’s smaller O.D. shoulder faces the tube nut, with the larger O.D. sleeve shoulder facing the end of the tube that will be flared.
Once the tube nut and sleeve are in position on the tube, then the tube is flared. Don’t forget to install the tube nut and sleeve onto the tube before flaring (this is a common mistake that many people make).
As far as sizing is concerned, the AN dash size refers to the outside diameter of the tubing.
Hard tubing, equipped with the appropriate size tube nut and sleeve, will allow tubing connection to -3, -4, -6, -8, -10, -12 or -16 male AN fittings/adapters. This hard tubing plumbing may be used for any plumbing application where the flexibility of hose is not required. Examples are plumbing fuel and nitrous oxide from the engine-mounted distribution block to the nitrous oxide injectors; or plumbing a fuel line from an engine-mounted fuel pump to the carburetor. If any movement is anticipated between the two points, use a flexible hose plumbing system instead. For example, never run a hard line from a chassis-mounted location to the engine as engine rock will require plumbing flex.
In some cases, hard tubes are chosen for appearance, where a street rodder has decided that clean hard line plumbing will best complete the visual theme of the build. If the application calls for hard-line plumbing, the use of -AN tube nut/sleeve assemblies provide a secure and leak-proof connection, as opposed to connecting a hard line to a nipple fitting with a short piece of rubber hose and a hose clamp. It also looks much more professional
Hard-line plumbing isn’t relegated to small diameter tubes only. Notice the curved -12 tube that serves as a water feed at the cylinder head and the 90-degree scavenge tube at the front of the rocker cover.
Tube nuts are used in conjunction with tube sleeves on metal tubing that features a 37 degree flare. This allows a secure sealing connection to -AN male fittings/adapters.
(Illustration courtesy Russell Performance 18-194)
Granted, this example has nothing to do with plumbing fluid, but for this street rod engine, I decided to run the spark plug wires inside 3/8-inch aluminum tubing, feeding them through the base of the air cleaner housing using tube nuts and sleeves, with the nuts attaching to a series of -6 to 1/4-inch NPT adapters.
Tags: -AN TUBE NUTS AND SLEEVES, HARD LINE PLUMBING, PLUMBING
