I’m planning to install the Viking 195T for my 750 Super Duty.

Instead of rubber hose, I intend to use braided stainless steel, conductive-teflon hose for my fuel lines. The main feed line from the wing tank is AN6. I understand I need a vent line from the header tank back to the wing tank, and I want to minimize the size hole that I have to drill through the rear spar for that vent line.

What is the minimum recommended size for the vent line?

Also, I haven’t been able to find any information regarding the best location for the vent line to penetrate the rear spar. What are others doing?

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Looking at the fuel system video from Viking, Jan used the same hose for both fuel and vent, though he made no comment as to size requirement. Unfortunately the wing routing isn't shown. You should submit these questions to them. Jan and Alissa don't check the online forums very often, but they're pretty good at answering emails.

Thanks for pointing this out Mike, and thank you for your email - just so everyone below may know as well:

Universally we use 5/16th everywhere - the smallest vent line acceptable is 1/4. In order to minimize the hole size in the rear spar you could use a pass through fitting in the spar. Or, if you are using a rubber hose for the vent line, a brass mender part. 
-Alissa

I had gas pouring out my vent (Zenith 601XLB), so I plugged the vent and added a check valve ball vent to my fuel cap. It's .2" at the opening, but after the check ball it gets smaller. Amazon has these Motorcycle Gas Cap two way breather. I modified one cap in case it didn't work, but so far it's working great, and I'm not loosing a ton of fuel.

I am planning on the same engine in the same airframe.

My choice for fuel and vent lines is 3/8" aluminum versatubing (https://www.aircraftspruce.com/catalog/mepages/3003versatube.php?cl...).  Its cheap enough but a little difficult to work as you have to flare the ends which can be hard to do in some tight places.  Its also fussy getting your bends right.

The route for my vent line goes straight through rear rib #2 (its tricky drilling a hole in the right place in the rib) for a few inches, takes a 45 degree or so bend aft, goes through the second lightening hole of rear rib #3 and then straight to the aft spar.  This puts it intersecting the aft spar outboard of the strut attachment point.  I go through the aft spar with a 45 degree AN6 bulkhead fitting.

This is what I did on my right wing, which is now covered.  I'm currently working on this area on the left wing and will try to post a picture when I have something to show

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