NPT pipe plug size versus hole size...?

You should be able to tap that hole for 1/16" pipe.

Here's a link to a page with Tap Drill sizes that include 1/16". The size you mention looks perfect already.

http://academic.evergreen.edu/projects/biophysics/technotes/fabric/pipe.htm

The tricky part might be finding a tap. I have a very extensive assortment of threading tools (including a set purchased from Ferrari in the 60's) but no 1/16" pipe.

I need to get one.

 

DAVID ---

Why do you want to use NPT plugs over straight-threaded plugs? You can get the same degree of sealing performance with straight threads if you use a proper thread sealant.

The reason I question the NPT approach is this ---

I assume the manifold is a cast Aluminum piece?

When tapping an internal NPT thread in soft / brittle materials, it is preferred to drill a tapered pilot hole before tapping. This is very difficult to do accurately by hand --- milling equipment and depth monitoring methods are generally used to accomplish this.

Attempting to force a tapered NPT plug tap into a straight-walled pilot hole can crack the piece --- if it is sufficiently thin and / or composed of weaker materials (e.g., cast Al).

BE CAREFUL

 

In more than 30 years of drilling and tapping holes in all types of materials, I've never seen a tapered tap drill bit. A decent chamfer usually gets the tap started. Use a good lubricant and be careful and you're good to go.

I've broken a few taps (usually in stainless or hard steel) over the years but I doubt there's much chance of breakage in an aluminum manifold.

A straight-threaded plug like you recommend would need an o-ring or metal sealing washer against a precisely machined surface to seal properly. I don't think this was what David was looking for.

 

look harder

http://www.mcmaster.com/#tapered-end-mills/=2xj96r

 

Since he's talking about an intake manifold, I'm assuming the goal is only to seal air at low pressure (positive and negative atmospheric)?

Take a look at any intake manifold --- nearly every piece of plumbing coming off it is attached with a straight thread fitting (if it uses threads) and a sealant.

Yes, using an NPT plug is the "better way" to go --- it's just not always required.

 

Tapered end-mills. Not really a drill bit and overkill for this application. controlling the drilling depth is pretty critical. Go a little deep and you're oversize.

I'll stick with conventional tap-drills.

Since you sourced McMaster Carr, here's one of their famous diagrams showing the difference between tapered and straight pipe threads. Straight requires a seal (and a flat surface perpendicular to the hole to seal against).

http://www.mcmaster.com/ctlg/DisplCtlgPage.aspx?ReqTyp=CATALOG&CtlgPgNbr=2&RelatedCtlgPgs=3,2&CtlgEdition=115&ScreenWidth=1280&McMMainWidth=1062&sesnextrep=807257435329948

Without a proper seal the plug is merely a bolt in a threaded hole. When tightened, there is a clear path for leakage around the "back" side of the thread. Using some type of sealant might prevent leakage in a low pressure application but I've seen it fail (oil sump studs are a typical example).

I've looked at "any intake manifold" (I've got a few from different cars) typically they have straight thread fittings that use a sealing washer (typically copper or fiber or rubber o-ring) no sealant required, or they use NPT or BSPT tapered pipe fittings with "dope" or teflon tape on the threads.

David was looking for drill size information and I think he's got what he needs. It doesn't sound like a big engineering project, just a few holes need filling.