After three tries to get the correct AN fitting for the fuel return line, I finally got it right. I just needed a 3/8" bulkhead fitting right?
1. Ordered an AN816-6D pipe to nipple transition for 3/8" tube. The 6D is for 3/8" tube. - WRONG. It's 3/8" tube, but also has a measured 3/8" pipe thread and I need 1/2" to go into the fuel return bung.
2. Ok, I've got this figured out. I need an AN816-6-8D - which is a 3/8" nipple (the -6) transition to a 1/2" pipe (the -8D). This fitting was surprisingly hard to find and had to get it from Skygeek. WRONG AGAIN. The 1/2" end was way too big, like 3/4" or more. What am I missing? NPT pipe thread is what I was missing. What measures approximately 1/2" is actually 3/8" NPT. So then why wasn't the original AN816-6D correct? 3/8" nipple and 3/8" pipe, right? No - standard transitions apparently are downsized on the pipe side. So, an AN816-6D is really the same as an AN816-6-4D. I guess everyone knows that.
3. So finally figured out that I need an AN816-6-6D and spruce carries those. Correct part finally arrived today.