Recently while I was reinstalling my exhaust I happened to notice my rear most vent hose (180 degree section) has collapsed. So After reading most of the prior posts I could locate, I have two basic questions:

-what is the inside diameter of the upper rear vent tube on the tank?
-is the inside diameter of the hose that connects to that tube “critical”?
help is appreciated.

I believe it is roughly 5/8" and I wouldn't think ID is critical

You probably need a hose bender.

 

2 hours ago, Patcon said:

I believe it is roughly 5/8" and I wouldn't think ID is critical

Hmmm, I thought the O.D. Of the metal tube is 5/8”?  Wouldn’t that make the inside say 5/16” or 1/2”?  Hate to drop the tank just get a micrometer on the inside when someone has a tank out already. 
 

Assuming the I.D. Is 5/8” then if the vent line is reduced to say 1/2” would the restriction cause an issue when filling the tank or would it just require a slower flow at the gas pump?

10 minutes ago, DatsunZGuy said:

You probably need a hose bender.

 

Have some on order, but looking more specific measurements. I spent a half day reading the past posts and other options for this issue. Anybody have a gas tank sitting on the floor and could get me accurate measurements. 
 

8 hours ago, 7tooZ said:

Assuming the I.D. Is 5/8” then if the vent line is reduced to say 1/2” would the restriction cause an issue when filling the tank or would it just require a slower flow at the gas pump?

As I see it Don, most of the air that is displaced when filling the tank comes out of the filler hose, not so much from the vent hose. This is more for when you have the cap on and the engine is pulling fuel. That way you don't get vapor lock in the tank, It has to breathe. 

I have a tank out, and can measure the barb but it will be later this afternoon, that is if no one else can offer the spec before I get home from work.

Thanks David.  I appreciate your help. 

My tank is out, here is a pic with the measurements. I have the original hose and it measured 17mm ID (.67"). Hope this helps.

7 minutes ago, CanTechZ said:

My tank is out, here is a pic with the measurements. I have the original hose and it measured 17mm ID (.67"). Hope this helps.

@CanTechZ

Perfect that was exactly what I was looking for. Well done.

That pretty much answers my second question about the critical size of the interior diameter of that vent hose.

Carl Beck stated:

“The Gas Tank has to be vented to allow air to escape as liquid gasoline is pumped in, otherwise an air pocket would form at the top of the tank and prevent the tank from filling completely. Note that the Filler Neck enters the tank well below the top of the tank. Air also has to be drawn into the tank as the fuel is pumped out to prevent a vacuum from forming in the tank.”

So my thinking is that (approx) a 1/2” maybe the minimum size I.D. to allow the flow of fuel into the tank without shutting off the pump. The vacuum “forming in the tank”part I assume would require even less I.D.

thanks everyone  

 

 

 

It looks to me that 5/8" ID hose would be the size to try, I don't think that 1/2" would fit on the barb with out some sort of adapter. Even 5/8" will be a tough fit.

Look like you have your answer Don. 
 

Yes back on the road. New clutch, bearing, turned flywheel, and new main seal. Replaced the vent hose, cleaned up the exhaust mounts and installed the ceramic coated exhaust down tube.