If you mean the actual idle tube/jet that screws into the bowl side of the nozzle bar, it's a special 1/4-32 thread.
If you mean the brass fitting (air bleed) in the middle of the nozzle bar it's also a speacial thread 3/8-32
Main jets are also 1/4-32, and there are taps on EBAY for 1/4-32.
Unsuccessful finding replacement main jet for alloy, Model A updraft carb by Simmons. I've attached a photo. Holley 94 jet looked similar, but wrong thread size; needs to be 3/8-24.
Unsuccessful finding replacement main jet for alloy, Model A updraft carb by Simmons. I've attached a photo. Holley 94 jet looked similar, but wrong thread size; needs to be 3/8-24.