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
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.
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
TPI= tooth per inch, can apply to a saw blade or thread pitch
ahhh i see so that refers to the thread pitch
Still working on the old imperial system helps...
A hack saw blade is a good example of tpi
Last edited by alladdin; 23-01-2019 at 21:34.
thanks guys appreciated, i'm hoping soon to upload pics of a custom duratec water rail i have designed and am making from aluminium
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.
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.
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"
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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.
Get yourself a Zeus book
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I have one in my pocket all the time but then I’m a machinest
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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.
Thank you for all of the useful replies, much appreciated
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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.
rather than confuse issues , 15.2 is closer to ideal but in alloy 15mm will be fine as will wd40
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