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My MFL flare fitting keeps leaking. What is wrong?
An SAE flare seals where the coned face meets the fitting, so a leaking flare is almost always a face problem, not a thread problem.
Understand what is sealing
MFL and FFL fittings are 1/4 inch SAE 45 degree flares defined by SAE J512. The seal is metal-to-metal, or face-to-face, where the 45 degree cone of one part presses against the mating cone of the other. The thread only supplies clamping force.
Because the thread is not the seal, anything you do to the thread does not stop a leak, and some thread treatments make it worse.
The tape trap
The most common mistake is wrapping PTFE tape on a flare thread. On a tapered pipe thread like NPT the tape helps, but on a flare it does nothing useful and can pack the thread enough to hold the two cones slightly apart. Take the tape off, clean the thread, and let the faces meet.
The same logic applies to parallel BSP joints that seal on a washer: the fix is a good washer, not tape.
Check the faces and the torque
Look at both cone faces under good light. A scratch, a burr or a fleck of debris across the face gives gas a path out. Wipe them clean, and if a face is scored, replace the part. Tighten the nut firmly but do not crank it, because over-torquing can deform the flare and, on a MFL to FFL joint, cut a fresh leak path.
A dab of keg-safe lubricant on the threads (not the face) can help the nut pull the faces together evenly.
Confirm the match
Make sure you are actually mating a male flare to a female flare of the same 1/4 inch SAE size, not a flare to a barb or a flare to a BSP thread. Cross-standard joints will never seal. If the parts are correct and clean and the faces are good, a hand-firm nut should hold gas without a hiss.
Get the right fitting
Open the KegFitFinder toolMatch male and female flaresFind 1/4 MFL FFL flare fitting on eBayaffiliateRelated
Sources: SAE J512 flare standard