Technical NPT thread, how to measure?

Quick one, trying to find out what the thread size is on my rochester 2g's. I've conatcted a few 'experts', but there doesnt seem to be a 'standard'. I want to run banjo fittings, the carbs came with solid fuel lines and tube nuts, The nuts measure 1/2" across the thread, but I need a thread 'pitch' to get the right banjo fittings. I checked for 'thread gauges' like you would use on a regular thread, but there doesnt seem to be anything, not at least on the world oracle 'google'. Any help appreciated

 

i'm not a 2G expert , but i think they use a 5/16 inverted flare fitting on the fuel line. i believe they use 1/2-20 nuts

 

Are you sure you are looking for NPT (National Pipe Taper)?

 

There is more than one thread size for the 2gs

 

Rochester 2G Carburetor Inlet Fittings

• 5/8" x 18

• 7/8" x 20

https://www.carburetor-parts.com/ht...comRochester-2G-2GC-2GV-Technical_ep_288.html

 

i know nothing about rochesters, but a banjo fitting is not using pipe thread. its straight common thread for the size of the bolt you are using. usually, fine thread, but not always.

 

Banjo fittings have a straight thread, not NPT. I believe the 2G inlet is 1/4" NPT, and typical installations use inverted flare with an adapter. You'll need an adapter for something like 3/8" - 24 to 1/4" NPT, or whatever size your banjo fitting is.

 

Mine measures 1/2-20 thread on 5/16 flare tubing, Probably different 2G sizes out there, but "tube nuts" are not NPT generally , big difference between tube sizes and pipe sizes.

 

Do you have the gauge set from tap and die set?

 

With photo 5/16 line has 1/2-20 nuts

 

2GC carb inlets are inverted flare not NPT. Should be 3/8 inverted flare

 

I think it is 3/8 line my memory is fuzzy sometimes so maybe it is 5/16. For some reason I think it is the same as a Q-jet which is 3/8

 

Yes there are.

2g outboard carbs & the center 2gc on 55-57 Olds tri-power were NPT threaded front inlet with no integral filter in the top cover. 348 Chevy tri-power in 58 was NPT, but then went to side inlet with inverted flare in the integral filter "nut" for 59-up.

I am somewhat sure the 55 Olds single 2 barrel (2gc) engine was also NPT threaded inlet. 56 Olds had a non-2gc model single 2 barrel.

look in this pic, you can see a NPT elbow into the carb top that accepts a flared line.

 

That's the nice thing about standards, there are so many to choose from.

 

2 jets have 2 size threads (the inverted flair is on the filter nut into the carb). The one with the filter nut (front feed large base and side feed small base) which is a straight thread (with a gasket) AND the front feed/side feed small base that is pipe thread (used on factory tri power and older V8s).