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Drilling holes for barbs?

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Jan 18, 2003
Ok, well, I'm in the process of creating a few waterblocks and I want to try and go for a "true 1/2" ID system". In order to do this, I'm going to be using barbs for 5/8" ID tubing. In this way, the barbs will be 5/8" OD and 1/2" ID, thus creating no flow restriction as barbs for 1/2" ID tubing do. My question here is this...

If I'm going to be using these barbs, which are NPT thread, what size hole do I need to drill, and what kind of tap do I need to use to cut the threads? The barbs I plan on using will be bought from mcmaster.com

Item #: 5463K474
Nylon Single-Barbed Tube Fitting Barbed X NPT Male For 5/8" Tube ID, 1/2" NPT

Can anyone help me out here?


Oct 30, 2002
Lancaster, PA
Ah, I had been trying to figure this out earlier too, though I didn't get around to asking on the forums here (which in retrospect probably would have saved me a bunch of time).

Anyhow... what you need are 5/8" by 3/8"NPT barbs (yeah sounds weird, I know... but 3/8"NPT is 1/2" ID). For the holes, you need a 9/16" bit (about $14-15 at Home Depot/Lowes). The tap is a 3/8" NPT tap.

If you use the 5/8" tube ID 1/2" NPT I believe you need 3/4" holes. And your tap would correspond to the barb's threads.


Jan 11, 2003
Yes, use 3/8 NPT unless you want really huge holes. The recommended drill size is actually 37/64" for a 3/8 NPT tap. If you're already buying from McMaster, you can pick one of those up at the same time.

If, however, you choose 1/2 NPT, you will need a 23/32" drillbit. Again, all of this, including the taps, will be available from McMaster.