OAK tube diameter

I am going to install an OAK on my freedom insert and was wondering what the most common diameter of the pipe is

Usually 3 to 4 inches, depends on the stove. It's usually discussed in the owners manual.

My 2 CF heritage and my 2.85 CF BK require a 3" tube but the smaller stoves from some other brands require a 4". If the run is relatively short, I would not hesitate to use a 3" tube on any stove.

Might depend somewhat on how well your chimney drafts, since that's the force that has to pull air into the stove, e.g short chimney = larger tube. It's an hypothesis, anyhow.

I have a buddy down the road with a nice fabrication shop, so I am going to bring the air in the rear of the stove and make a block off plate for the front.The air inlet tunnel goes from front to back.I want to try and eliminate drafts in this place.There is a lot of room between the insert and the fireplace rear.Will take photos of the "procedure"

It's going to be around a 4 ft run so may go with 4 inch

I'm not familiar with your particular insert, but on mine the inlet at the front provides both combustion air and the air that gets pushed through the air jacket, heated and out into the room.

I think you will find there is three passages the one in the center is the air control for the fire and the 2 outside are the heater-fan passages ,the center does not connect with the two outside passages

No, mine is built somewhat differently, and it would not be so simple to isolate the combustion air on my unit. It may be feasible for you; I just wanted to toss out that concern in case you hadn't considered it.

Jon,thanks for the concern,I picked the stove a couple of days ago and have been poking around it to see what improvements I can make before it goes in.The change should be pretty straight forward.I think I am going to fabricate an offset for the pipe outlet so I can bring the stove out 3 more inches for a cooking surface and to get more convection in a power outage.
I was thinking of the combustion air and the heating air mixing...fan on high and you have a forge LOL

Bigger is always better but a 4ft run is fine with 3".

I think I will work out the area of the front air intake and find tube diameter that is close