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Technical NPT thread, how to measure?

Discussion in 'The Hokey Ass Message Board' started by neilswheels, Feb 15, 2019.

  1. neilswheels
    Joined: Aug 26, 2006
    Posts: 611

    neilswheels
    Member
    from England

    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
     
    chryslerfan55 likes this.
  2. 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
     
  3. woodiewagon46
    Joined: Mar 14, 2013
    Posts: 1,392

    woodiewagon46
    Member
    from New York

    Are you sure you are looking for NPT (National Pipe Taper)?
     
  4. saltflats
    Joined: Aug 14, 2007
    Posts: 9,440

    saltflats
    Member
    from Missouri

    There is more than one thread size for the 2gs
     
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  5. mgtstumpy
    Joined: Jul 20, 2006
    Posts: 7,560

    mgtstumpy
    Member

  6. mgtstumpy
    Joined: Jul 20, 2006
    Posts: 7,560

    mgtstumpy
    Member

  7. rusty valley
    Joined: Oct 25, 2014
    Posts: 704

    rusty valley
    Member

    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.
     
    rockable, VANDENPLAS and Blues4U like this.
  8. Blues4U
    Joined: Oct 1, 2015
    Posts: 3,720

    Blues4U
    Member
    from So Cal

    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.
     
  9. Heavy Old Steel
    Joined: Feb 1, 2019
    Posts: 50

    Heavy Old Steel
    Member
    from Virginia

    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.
     
    deathrowdave and rockable like this.
  10. Chappy444
    Joined: Jan 27, 2012
    Posts: 818

    Chappy444
    ALLIANCE MEMBER
    1. Maryland HAMBers

    Do you have the gauge set from tap and die set?
     
  11. Heavy Old Steel
    Joined: Feb 1, 2019
    Posts: 50

    Heavy Old Steel
    Member
    from Virginia

    A96B9F5A-3C42-4355-A73D-9E7F00AB9F9D.jpeg
    With photo 5/16 line has 1/2-20 nuts
     
    rockable likes this.
  12. OLDSMAN
    Joined: Jul 20, 2006
    Posts: 1,820

    OLDSMAN
    Member

    2GC carb inlets are inverted flare not NPT. Should be 3/8 inverted flare
     
  13. OLDSMAN
    Joined: Jul 20, 2006
    Posts: 1,820

    OLDSMAN
    Member

    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
     
  14. F&J
    Joined: Apr 5, 2007
    Posts: 13,177

    F&J
    ALLIANCE MEMBER

    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.
    j2.jpg
     
  15. Truck64
    Joined: Oct 18, 2015
    Posts: 3,466

    Truck64
    Member
    from Ioway

    That's the nice thing about standards, there are so many to choose from.
     
    Rich S. likes this.
  16. 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).

    [​IMG]
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    [​IMG]
    [​IMG]
     
    mgtstumpy likes this.

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