Side by side
A wrong fitting that will not thread on is an inconvenience. A wrong fitting that threads on and holds pressure on the bench is a callback, and sometimes worse. These pages separate the two: what genuinely interchanges, what assembles and will still leak, and what simply will not go together.
The same 37 degree flare and the same thread at every dash size. AN is the tighter aerospace specification of the same joint, not a different connection.
The check that settles it: You cannot separate them with a caliper or a pitch gauge - the dimensions are the same. Read the part marking and the finish. AN and MS part numbers are stamped on the fitting, and aerospace AN parts are normally anodised to a colour code, where the industrial part is plain steel or plated.
The single most common mis-identification in the trade. Six of the eight shared sizes have exactly the same thread and a different cone angle, so they screw together and the joint weeps.
The check that settles it: Put a seat gauge in the cone, or hold the two side by side. A 45 degree cone is visibly blunter and shorter than a 37 degree cone. Alone it is very hard to judge; against a gauge it takes a second. If you have no gauge, count the threads at 3/4 in diameter: 1 1/16-12 is JIC and 1 1/16-14 is SAE 45.
Two tapered pipe threads that look identical and are five degrees apart on the flank. At 1/2 and 3/4 they share a thread count and sit within half a millimetre on diameter, so they start on each other.
The check that settles it: Gauge the thread form, not the diameter. A 60 degree blade rocks on a 55 degree thread and a 55 degree blade rocks on a 60 degree thread; the one that seats flat is the answer. If you only have a rule, count threads: 18 per inch at about 13.5 mm is NPT 1/4, and 19 per inch at about 13.1 mm is R 1/4.
One is tapered and seals on the thread; the other is parallel and physically cannot. Mixing them is the most common cause of a port that will not stop weeping.
The check that settles it: Measure across the crests at both ends of the male thread. Same reading at both ends means parallel, so BSPP; smaller at the entering end means tapered, so NPT. Then check the flank angle with a pitch gauge to confirm 60 against 55 degrees. A BSPP male also carries a flat shoulder or a chamfer behind the thread for its seal washer; an NPT male has nothing there.
Both seal on an O-ring and neither seals on the thread, which is where the confusion starts. The threads are a size apart at every dash, so they will not assemble - but the two ends often appear on the same fitting.
The check that settles it: Look at the nose. ORFS is dead flat with a shallow circular groove machined into the face. ORB has no groove in the face at all - its O-ring sits on a plain shoulder between the thread and the hex, and on a used fitting the ring is usually gone and only the shoulder is left.
Seven threads carry one tube size in the light series and a different one in the heavy series. The thread tells you nothing about which fitting you are holding.
The check that settles it: Measure the TUBE, not the thread. The tube outside diameter and the thread together identify the fitting; the thread alone does not. Heavy series bodies are also visibly thicker across the hex flats for the same thread, and most makers stamp the series letter on the nut.
Why this happens
Most of these families were standardised separately, in different countries, for different fluids, and several of them landed on the same Unified or Whitworth threads by arithmetic rather than by agreement. A -08 JIC, a -08 SAE O-ring boss and a half-inch tube SAE 45 degree flare all run 3/4-16 UNF. What differs is where the joint seals - a 37 degree cone, an O-ring on a shoulder, a 45 degree cone - and none of those three sealing arrangements works against either of the others.
Every size page on this site therefore lists what its size can be confused with, worked out from the published diameters rather than from a hand-written list: any fitting in another standard whose male thread sits within 0.9 mm of it, which is wider than a careful caliper reading but well inside what a worn thread, a plated thread and a hurried measurement produce together.
Where these pages say a combination will assemble and then leak, that is a safety claim and it is only made where a published source supports it or where the site's own tables show an identical thread with a different seat. Where no source supports it, the page says nothing rather than guessing.