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Thread: tap / drill bit size help please

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    Pit Crew 5tox's Avatar

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    tap / drill bit size help please

    I cant get my head around this at all despite google.
    i have to tap a thread into some ally, the tap i need to use is 3/8-19bsp so what size drill bit do i need to drill the hole?

    Thanks Steve

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    Re: tap / drill bit size help please

    Quote Originally Posted by 5tox View Post
    I cant get my head around this at all despite google.
    i have to tap a thread into some ally, the tap i need to use is 3/8-19bsp so what size drill bit do i need to drill the hole?

    Thanks Steve

    https://www.newmantools.com/taps/bsp.htm

    It's a big drill bit... It won't go in a normal 1/2 inch drill chuck. But a reduced size shank would of course...


    I must admit I'm not sure where the 3/8 part comes into it.. as 3/8" is only about 10mm
    Last edited by rallyrob; 23-01-2019 at 20:12.

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    Re: tap / drill bit size help please

    to be honest it just blew my mind, every time i searched for the drill size for a 3/8 tap it was coming up with drill bits that were way too small, not sure what tpi or any of the other stuff means :-)

    Thanks for the link, its really helpful, looks like i need a 15mm drill bit then

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    Re: tap / drill bit size help please

    TPI= tooth per inch, can apply to a saw blade or thread pitch

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    Re: tap / drill bit size help please

    ahhh i see so that refers to the thread pitch

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    Re: tap / drill bit size help please

    Still working on the old imperial system helps...

    A hack saw blade is a good example of tpi

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    Re: tap / drill bit size help please

    Quote Originally Posted by 5tox View Post
    to be honest it just blew my mind, every time i searched for the drill size for a 3/8 tap it was coming up with drill bits that were way too small, not sure what tpi or any of the other stuff means :-)

    Thanks for the link, its really helpful, looks like i need a 15mm drill bit then
    yes as its alloy 15mm is ideal, search for "blacksmith drill" bit as they have reduced shank for 10 or 13 mm chucks

    with regard to the tpi , if theres more than one listed then the higher the number the finer the thread.
    Last edited by alladdin; 23-01-2019 at 21:34.

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    Re: tap / drill bit size help please

    thanks guys appreciated, i'm hoping soon to upload pics of a custom duratec water rail i have designed and am making from aluminium

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    Re: tap / drill bit size help please

    You need a 5/16" drill bit for a 3/8" tap. 7.5mm will work just as well. You certainly do not want 15mm!
    A 3/8" tap is 9.5mm, so obviously you need a hole that is smaller, which is then tapped to create a threaded hole.
    Last edited by MemphisTwin; 23-01-2019 at 23:59.

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    Re: tap / drill bit size help please

    No he doesn't mean a standard 3/8 imperial hole/thread size . Its 3/8 BSP ( british standard pipe ) which are pipe fittings. From memory and a long distant apprenticeship you measure the o/d of the threaded fitting first. Then minus 1/4 of an inch and this gives you the size ie in this case 3/8 BSP. I think the 19 is the number of threads per inch, but who came up with the system and why I have no idea but was many years ago.

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    Re: tap / drill bit size help please

    Quote Originally Posted by Wigan RSte View Post
    No he doesn't mean a standard 3/8 imperial hole/thread size . Its 3/8 BSP ( british standard pipe ) which are pipe fittings. From memory and a long distant apprenticeship you measure the o/d of the threaded fitting first. Then minus 1/4 of an inch and this gives you the size ie in this case 3/8 BSP. I think the 19 is the number of threads per inch, but who came up with the system and why I have no idea but was many years ago.
    Apologies. I missed the BSP bit!

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    Re: tap / drill bit size help please

    Found this it may help.
    British Standard Pipe
    Parallel or Taper
    BSP or BSPT
    Nominal Size
    (inches) T P I Major Diameter
    (inches) Tapping Drill Size Tapping Drill Size
    BSP BSPT

    1/16 28 0.304 6.6 mm
    Letter drill G
    1/8 28 0.383 8.8 mm 8.4mm
    11/32 inch 21/64"
    1/4 19 0.518 11.8 mm 11.2mm
    29/64" 7/16"
    3/8 19 0.656 15.25 mm 14.75mm
    19/32" 37/64"
    1/2 14 0.825 19.00mm 18.25mm

    3/4" 23/32"
    5/8 14 0.902 21 mm
    53/64"
    3/4 14 1.041 24.5 mm 23.75mm
    31/32" 15/16"
    7/8 14 1.189 28.25 mm
    1-7/64"
    1 inch 11 1.309 30.75 mm 30mm
    1-13/64" 1-11/64"
    1-1/4" 11 1.650 39.50 mm
    38.5mm
    1-35/64" 1-33/64"
    1-1/2" 11 1.882 45.5 mm
    44.5mm
    1-25/32" 1-3/4"
    1-3/4" 11 2.116 51.00 mm

    2"
    2" 11 2.347 57 mm
    56mm
    2-1/4" 2-3/16"
    Back to Top of Page

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    Re: tap / drill bit size help please

    Sorry that didn’t turn out like it is on the page which makes easy reading, see if a screen shot makes it easier. Couldn’t do it pic to big! Don’t know how to downsize.

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    Re: tap / drill bit size help please

    Get yourself a Zeus book
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    Re: tap / drill bit size help please

    Quote Originally Posted by 5tox View Post
    to be honest it just blew my mind, every time i searched for the drill size for a 3/8 tap it was coming up with drill bits that were way too small, not sure what tpi or any of the other stuff means :-)

    Thanks for the link, its really helpful, looks like i need a 15mm drill bit then
    When you googled for the drill size did you include the term 'BSP'. All BSP sizes are physically bigger that the size you might expect. It can be tricky if you don't already know this. I've been caught out before. IIRC BSP stands for 'British Standard Pipe', the term '3/8' refers to the internal bore of a gas or fluid pipe. They call it 3/8", but the outside diameter of that pipe would be closer to 5/8". A 15mm drill bit with reduced shank is what I used when I had to tap a 3/8BSP in an alloy block recently.

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    tap / drill bit size help please

    Thank you for all of the useful replies, much appreciated

    Sent from my SM-A520F using Tapatalk

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    Re: tap / drill bit size help please

    It's 15.2mm drill bit you need and use soluble oil or paraffin for lubricant
    Last edited by davemk1; 24-01-2019 at 21:14.

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    Re: tap / drill bit size help please

    rather than confuse issues , 15.2 is closer to ideal but in alloy 15mm will be fine as will wd40

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    Re: tap / drill bit size help please

    Quote Originally Posted by alladdin View Post
    rather than confuse issues , 15.2 is closer to ideal but in alloy 15mm will be fine as will wd40
    Agreed

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