What is the I.D. of a 3/8" compression fitting?

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by Probedude, May 17, 2009.

  1. Probedude

    Probedude New Member

    Messages:
    137
    Location:
    CA
    Trying to figure out a sizing problem with a 3/8" compression faucet flex line and a 3/8" compression angle stop. Right now they are a tight press fit with each other instead of a drop in fit.

    Question:
    - what's the O.D. of the tube that fits into a 3/8" compression fitting on an angle stop?

    - what the I.D. of the bore for a 3/8" compression fitting on an angle stop?

    TIA,
    Dave
  2. kingsotall

    kingsotall Plunger/TurdPuncher

    What are you "dropping" in¿ A flex supply should screw right on. A 3/8" soft copper would slide into the outlet of the angle stop then the ferrule and nut would create the connection.
  3. Probedude

    Probedude New Member

    Messages:
    137
    Location:
    CA
    It's a braided flex line but the middle brass 'tube' inside the end which should be a slip fit into the angle stop connection is more like a interference fit.

    The connection gets tight but if you tighten a bit more, the torque levels off then gets tight again. The first 'tight' point I believe is the middle brass tube contacting the opening of the angle stop. The 2nd 'tight' point is when the rubber gasket makes contact with the angle stop.

    All 4 of them, differing lengths flexes and separately bought angle stops, did the same thing so it must be their design (Brasscraft/Masco).

    Dave
  4. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Compression fittings are based on tube size. 3/8 tubing, in most common usage, is ACT type tube, and the OD is EXACTLY 3/8". I don't have a spec on the ferrule, but obviously it is just right to fit smoothly over the tube.

    There is 3/8" type L tube, which is 1/2" OD, but in common usage in things like faucet supplies and dishwasher supplies, the tube commonly referred to as 3/8 is the ACR, where the nominal, or stated, size is the OD.

    In any event, when referring to any type of compression fitting or ferrule, the stated number refers to the OD of the tube it is used with.
  5. Probedude

    Probedude New Member

    Messages:
    137
    Location:
    CA
    Thanks Jimbo.
    Looks like BrassCraft is making the inner brass portion of the flex hose oversize. There's ribbing on it that is pushing the O.D. to 0.395". The inner step is 0.375" (exactly 3/8").

    What I'm finding is that when I connect the flex on the 3/8" compression end, the nut starts to get tight and you think you're done. However if you tighten some more, the torque remains the same but the nut can be tightened another ~2 turns and then the final torque is reached I believe when the rubber gasket finally makes contact.

    I've contacted Brasscraft asking if this is intentional.

    Here's a picture of what I'm talking about.
    [​IMG]
  6. Terry

    Terry Administrator Staff Member

    Messages:
    15,002
    Location:
    Bothell, Washington
    The rubber gaskets at the ends make the seal.

    Just snug them up and you are good to go.
  7. 99k

    99k Radon Contractor and Water Treatment

    Messages:
    460
    Location:
    Fairfield Co.,Connecticut
    I agree to just snug them up - hand tight plus 1/2 turn. Overtorque will damage the rubber seal.
  8. Probedude

    Probedude New Member

    Messages:
    137
    Location:
    CA
    Of course it does, I wasn't questioning that. The tightening I am doing is not extreme. I've always had an good sense of correct torque on fasteners having working on mechanical things all the time.

    It does appear however that the brass center does not fit into the compression fitting on the valve body and that makes contact first before the gasket makes contact, hence the torque on the nut going up, then leveling off, then going up when the gasket contacts. The initial torque makes you feel like it's done being tight.

    I just verified the interference fit by taking apart one of the connections that has been together and working fine for the past couple of days.

    - I had to 'pull' the flex out of the angle stop as the brass center was now wedged into the fitting.
    - the gasket appears fine and is at the same depth it is in the picture above of a new flex.
    - the top 'rib' of the center brass tube has broken off and is now a thin ring of brass shaving. (the ribbing sheared off, but the brass tube is the same overall length/protrusion.)



    I wonder why Brasscraft would machine that center brass piece that way. All 6 flexes I have have the same machining and dimensions.

    Edit: just measured the "turns to tightness"
    - 2 turns the torque abruptly changes - this is when the center brass contacts the valve fitting opening.
    - another 1-3/4 turns and the gasket is reached as this final 'tightness' is mushy like a gasket is being compressed.

    Again, not paying attention, the first tightness one might think the joint is now tight and done with.
    Last edited: May 18, 2009
  9. Terry

    Terry Administrator Staff Member

    Messages:
    15,002
    Location:
    Bothell, Washington
    I don't use brasscraft.
    Maybe that's why I don't see that.

    I have my supplier get what I like.
    He brings them in special for me.
  10. Probedude

    Probedude New Member

    Messages:
    137
    Location:
    CA
    An engineer from Brasscraft called me today.
    His answer? It's done that way intentionally and yet it is an interference fit. He says their 3/8" compression flexes show this the most.

    Weird.

    Dave
  11. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    I measured up the inside of a fitting and get about 0.380, so that is a snug fit but 5 thousanths clearance is not "press fit".
  12. kingsotall

    kingsotall Plunger/TurdPuncher

    Man, this is nano-plumbing... :eek:
  13. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,631
    Location:
    Cave Creek, Arizona
    supply

    Do like plumbers do and stop using BrassCraft supply tubes.
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