Yeah, angle and hardness of material, as well as how strong any cast parts are. I think harness and angle were my problem on one fitting, it dug a line in the angled dealing surface and wouldn't seal. Replaced with aero quip.

 

Believe it or not, the anodize finish plays a factor in how hard the material is. Ideally you want a Type III finish (hardcoat) which would provide the most thickness and best wear resistance.
Imagine if you take the fitting on and off 10 times the finish is what will wear down create gaps and leak.

I do see what you're saying. If for some reason the sealing surface isn't exactly 37* or is slightly out of round, whatever, it will also leak.

 

Yes, it sucks to have a hydraulic system spew fluid at 35K feet. I've attached a decent FAA document about fittings and lines. Not hard reading and has a decent amount of technical information.

https://dl.dropboxusercontent.com/u/...83-30_Ch07.pdf

 

Remember, you have 2 surfaces that needs to seal. The flare side and the fitting side. Both need to be within the tolerance to seal at the recommended torque.

 

Totally agree.

 

Yes. And that car blew up.

I'm sure it was because i used Ebay fittings on my breathers.