[JayJay]

Gumby - Brentwood Bob may be on to something. There were two "versions" (in a broad sense, not counting the Z1-Z2-Z3 castings) of the Zenith lower. The early lowers (I'm not sure how "early", but I kind of gather the switch was made in 1928) used a screw-in brass seat at the bottom of the GAV chamber. The later had the seat machined into the casting. Really the only ways to tell them apart are that the "early" had a threaded portion that allowed the jet to screw in, and the surface you see looking down the hole with the jet removed is flat. The machined in seat is obviously shaped to match the jet, and the surface just outside the machined jet is sloped. The hole in the version that takes the brass jet is also larger than the cast orifice, but that's not that obvious unless you have them side by side.

I just refurbished six Zeniths (in varying states of crudiness and rustiness), one had the screw-in seat and five had the cast-in seat. More, it was difficult to tell which was which on the used bodies until I had soaked in Chemtool and then carefully bead blasted down the hole.

Take a good look at what you have. The GAV needle should not need machining to fit. If you have that much gap it's sounding like you need the brass GAV seat but don't have it. It's also possible that somewhere along the 90-year history someone took a grinder and re-ground the seat, I've heard of that but not seen it.

BTW, one of the manuals I read says to remove the ring on the GAV housing. I've never seen a housing without that ring.

I live not too far from you (lower peninsula), I'd be happy to take a look or maybe show you examples of both.

JayJay