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I was getting new tires for my F250 pickup the other day, and there was a Hot Rod Magazine laying on the waiting-area table (sorry, didn't get the date of publication, but I think it was relatively recent). They had an interesting article on AN fuel fittings and specifically looked at AN-6 fittings (straight and 90 degree) from various manufacturers such as Aeroquip, Earl's, Summit, etc. They had some engineers measure the fittings' various dimensions and grade them for "quality." They did NOT specifiy what their "quality" rating scale was based on.
Anyhow, I was pleased to see that the Earl's fittings (that's what I used in my 750!) were top-rated for "quality" over all the other brands in both the straight and 90 degree categories - 1 point higher than Aeroquip on a 5-point scale.
All AN6 fittings hover around 0.25" for the smallest i.d. in the fitting. I've seen some builders agonize over this as the hose is 3/8" and they worry about the smaller i.d. in the fitting restricting fuel flow, but apparently this is not a problem. For what it's worth, as I recall from the article, the Earl's fittings were a little over 0.26" in smallest i.d. and the Aeroquip were a little under 0.24" in i.d. That makes the Aeroquip fittings a minimum of 15% smaller in cross-sectional area. (actually, the difference is even greater, but I don't remember the exact i.d.'s). I'm no engineer, so I have no idea what difference that makes in flow rates!
The other take-home point of the article is that since the fittings do vary in dimensions in other areas (not just the i.d.'s), do NOT mix different brands of fittings.
Now you have yet some more factoids to bore all your friends with at the New Year's Eve party! LOL!
Here's the link to an article on the difference between "AN" fittings and "JIC" fittings:
They look the same, but one is 40% stronger than the other. And, according to the article, just because a fitting is marked "AN" doesn't mean it conforms to the standards.