Need to know how to measure AN (army/navy) fittings to determine what size they are. I need a new set of front brake line and there either 3AN or 4AN. Is the hole of the fitting,outter thread measured, or what?
It's the OD of the tube that fits through the nut and sleeve, in 16ths of an inch. It'd be very rare for a brake system at the wheel to be anything but -3. If you've got a 3/16 brake tube (typical) and the tube nut takes a 7/16 wrench then it's -3. In some rare cases at the MC it might be -4
Most drums brakes i have seen take a 1/4" line, or -4AN; most disc brakes i have seen take 3/16ths or -3AN. Wheel cylinders need volume to operate so the larger line. You can use -4 for either - i have seen some fords with -3 rear drum brakes but i wouldn't copy it.
Curious, I can't think of a single drum brake I've ever worked on that had 1/4in hard lines at the wheel, but admittedly my exposure to prewar stuff is thin. I just (as in this morning) wrenched the hoses off a set of '37 Chevy Huck drums that will be going on the pseudo-Brooklands-roadster project, and they had 3/16in SAE tube nut seats in 'em. All the '50s and '60s Fords I've ever wrenched on had 3/16 hard lines. The only place I've ever seen 1/4in hard lines was from master cylinder to junction block on GM product, and I think the only reason GM did that was so that some assembly-line worker didn't figure out a way to screw the wrong line into the wrong port. I'm prepared to be educated to the contrary, but that's what I've seen.
A caliper will be more than accurate enough. The tube sizes are (for what you care about) in 1/16in steps. In the modern world 3/16in tube is sometimes called 4.75mm but it's the same stuff. Some late production vehicles have an epoxy anti-corrosion coating on it but if it's less than 1/4in OD then the tube inside is 3/16in.
It's a 3AN . The smallest we use on our aircraft is 4AN which takes a 9/16 wrench and that is giant for an automotive brake system.
I bought a huge selection of #4 AN fittings at a government auction about 20 years ago. Thete are hundreds of unions, nuts and sleeves, bulkhead 90 degree fittings as well as bulkhead tees. I have plumbed several dozen brake systems in #4 and it works awesome.
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