My references show that AN816-6D is for 3/8 tube, with 1/4 inch pipe thread.
If you want a fitting for 3/8 tube, with 3/8 inch pipe thread, you would use AN816-6-6D.
Or, if you want one for 3/8 tube, with 1/8 inch pipe thread, you would use AN816-2-6D.
I believe that "4-way" actually refers to a valve with 4 ports. In this case, you will find that there are 3 side ports which would be "in" ports, each of which can be connected one at a time to the fourth port which is on the bottom of the valve assembly. There is also an off position. This is not what we have.
A "3-way" valve has three ports, positioned at 90 degree intervals on the sides of the valve. In our case, the two ports which are 180 degrees opposed are the inlets, and the third port is the outlet. In our early C170 valves, the plug is cross-drilled thru, with a third hole drilled into the cross-drill, so that the plug can be positioned with all 3 ports connected (Both). The other 3 valve rotation positions will be Left tank connected to the outlet, Off (with both tanks connected together but not connected to the outlet), and Right tank connected to the outlet.
Unfortunately, the Eaton 6749 valve does not have this plug configuration. It has two holes drilled at 90 degrees which connect, but neither hole goes thru. So, this valve has Left, Off, Right, Off. As George stated, this valve can be changed to the C170 configuration by adding the cross-drill to the plug. There also will be some cutting required on the stem, to interface with the actuating shaft. This is certainly possible to accomplish, but none of Eaton's catalog valves is a drop-in replacement.
I was forced into looking into some of these details a few years ago, due to problems with my fuel valve.
I expect that I will have to deal with it again.
I believe that there is an STC for use of an Allen fuel valve in 140's, and this looks like a very high quality valve (though certainly not inexpensive).
I suspect that there are just not enough C170's with the early valve arrangement to be worth going to the trouble for anyone to get approval of an STC. Perhaps one of us will be forced into it someday.
