Short answer: NPT is a tapered pipe thread that seals on the threads themselves; inverted flare (SAE 45°) seals on a formed cone at the tube end, with the threads just providing clamping force. They solve the same basic problem, joining a fitting to a port, in two mechanically different ways, and knowing which one you’re looking at changes how you install it.
NPT: the thread does the sealing
National Pipe Thread is tapered, the diameter narrows slightly along the thread’s length, so as it’s tightened, the threads themselves wedge together and form the seal. That’s why NPT fittings typically get thread sealant or tape rather than relying on a separate washer or flare: the seal is the thread engagement itself, and dry-fitting an NPT connection without sealant is asking for a weep. You’ll see 1/8-27 NPT most often on this side of brake hardware, bridging to a generic pipe port on a proportioning valve, distribution block, or similar fitting.
Inverted flare: the tube shape does the sealing
SAE inverted flare fittings are straight-threaded, not tapered, and the threads exist purely to pull the flared tube end tight against a matching cone in the fitting body. The seal happens at that metal-to-metal flare contact, not at the threads, which is why a flared brake fitting doesn’t get thread sealant, and why a bad or damaged flare will leak even with the nut torqued down hard. This is the standard on the vast majority of brake caliper and hose-end connections.
Telling them apart at a glance
NPT threads taper visibly if you look down the length of the fitting, the diameter at the base is noticeably larger than at the tip. Inverted flare threads run straight, parallel, the same diameter end to end, with the actual sealing cone visible at the tube end rather than in the threads.
Where they meet
When a system needs to connect an NPT port to an SAE-flared line, an adaptor bridges the two, NPT on one end, inverted flare on the other, rather than trying to force one standard’s fitting into the other’s port.
What we stock
BrakeQuip Products carries NPT-to-SAE adaptors including BQ48 (1/8-27 NPT to 3/8-24), along with the broader Thread Adaptors category for other metric and SAE combinations.