GAV valve seat

M

Mad Mac

Senior Member
Shining a torch down my Zenith 1 GAV assembly hole I saw no sign of a brass valve seat, just a small hole. I assume it must be a threaded hole, so I would like to chase the thread with a small tap - but what is the correct size?

I didn't find an answer using the search function, or from any of the excellent internet articles on carb restoration.
 
I wouldn't drill or thread anything yet till someone who knows gives you an answer. I think I read somewhere that early carbs just had the seat cast into the carb....no brass insert added. Let's see what the experts say.
 
The brass GAV seat was used from beginning of production through February of 1930. After that it was part of the casting. Mad Mac, your is probably a later carburetor with the seat as part of the casting. See the restoration guidelines (2011) pages 1-12 & 13.
 
I have just found one published reference:

"The Gas Adjusting Valve housing's outside threads are 1/8" 27NPT (Pipe). Use Caution not to cut threads deeper, as it is a pipe thread".

Does that mean the hole I can see is 1/8" NPT pipe thread? Or maybe its just the hole in an old dirty brass valve? I have measured the depth of the hole and it bottoms out at 30 mm (1.18") - would the hole be deeper if there was no brass valve?
 
Your casting is as Bill Cilker describes. The small hole would need to be redrilled and tapped for the Adjustment needle seat for the 1928 casting . The parts houses sell the jet tap, 5 mm by .75 pitch USA.
You can use what you have as is, just clean it up. The gav seats in that little hole.
 
A friend of mine (now 91 years old) operated a Model A only restoration shop for many many years (we call him the Zenith Guru), his opinion is that the later cast in seat is preferable. I’m just sayin, some may disagree.
 
Thanks Guys,
Now I have a new (but related) problem: my new secondary well sits too high - see pic. Shouldn't it be flush with the flat surface of the bottom casting? I cannot see anything to obstruct it even when I shine a penlight down there.
 

Attachments

  • Secondary well too high.jpg
    Secondary well too high.jpg
    29.6 KB · Views: 32
Should be below the flat surface. Try the jet in place in the top half. look in the port for the comp jet. Should be 1/4 inch to 3/8 inch of the bottom of the secondary well showing. Use a drill bit by hand to clean out the casting.
 
Back to the original question ref the GAV seat... since the GAV is only to meter the flow, not to shut -off tight, the condition of the "seat" is really not critical. My $0.02 worth
Joe B
 
Thanks again everyone for your helpful comments.

Just to round this thread off: When I discovered there was brass in the bottom of the secondary well, my 85-year-old neighbor removed it using an "easy-out", without damaging the thread. For future reference: once the secondary well is cleared out of old brass bits, the new secondary well is screwed in about 10mm below the flat surface of the casting. It has to be down that low for the idle jet to fit when the two halves of the carburetor are bolted together.

Re the GAV adjusting needle hole: It was the later type with a hole in the casting to receive the adjusting needle (no brass valve). I followed Werner's suggestion and gently cleaned out the hole with fine valve-grinding paste, then washed it with WD40 and blew it out with compressed air.
All seems OK now.
 
You must log in or register to reply here.
Back
Top