'37 Carter W-1 automatic choke to manifold fitting?

[jdshott]

Happy New Year Flathead Fans:

 

I'm in the process of replacing lines to/from carburetor and fuel pump.  There is a 1/4" tube that, I think, goes from the exhaust manifold to the automatic choke of the Carter W-1.  What I took off appeared to be a "conventional" brass ferrule and nut compression fitting.  It seems as if that may not be the correct fitting for this connection to the cast zinc automatic choke housing.

 

I think that a normal nut for a 1/4" compression fitting has 7/16-24 threads ... but even the nut alone doesn't seem like it wants to thread onto the cast zinc choke housing.

 

What is the proper fitting for the 1/4" line that connects to the W-1 automatic choke?  Are there any special tricks to starting a nut onto a cast zinc thread?  It sure seems as if it would be easy to damage.

Thanks for your consideration,

 

John