drill size for 1/8 NPT tap?

[The Cobbler]

I have to drill & tap a 1/8" NPT thread in to cast aluminum. It has to seal well and once the plug is installed I need to grind it down flush . I'm repairing an air sander where a pressed in plug used to drill an air port internally blew out & is no where to be found. online I have found 3 drill sizes to use, 21/64 ( .3281) Q (.3320) and R ( .3390) the current hols is 5/16, not that that matters. I'm confused as to which drill is best for perfect threads?

 

[Walkers]

It is not really going to matter in aluminum. Just use the smallest drill, and tapping lube and it will be fine.

 

[nadogail]

21/64 or 11/32, the 21/64 is for tapered threads and the 11/32 for straight threads.

 

[paulsomlo]

https://www.carbidedepot.com/formulas-tap-pipe.htm

If you're not going to follow up drilling with a tapered reamer, the chart says use a "Q".
With NPT, I think you need to thread a little, try the plug, thread a little, try the plug again, until you get the fit you want.

 

[RoninB4]

Inserts, plugs, and fittings can be a bit different so follow paulsomio's advice and tap a little at a time, trying the plug for fitment as you go. Once you can get the plug just started it doesn't take much more tapping to go too far so go 1/4-1/2 revolution at that point for fitment. Tapping into cast aluminum should cut like butter so drill size isn't critical but be careful with alignment and do use some sort of cutting/tapping fluid as aluminum likes to micro-weld itself to taps. For aluminum, or any of the "white" metals I use Tap Magic, kerosene, WD-40, or even very soapy water if nothing else is at hand.

 

[Chrisb62]

For most applications 4 complete revolutions with the tap is "correct" but if wanting a certain fit do as others have described.

Edit to add depending on a short or long projection tap.

 

[rlitman]

https://www.carbidedepot.com/formulas-tap-pipe.htm

If you're not going to follow up drilling with a tapered reamer, the chart says use a "Q".
With NPT, I think you need to thread a little, try the plug, thread a little, try the plug again, until you get the fit you want.

The expectation is that the leading threads will be shallow, so overdrill a little for them to seal it best in the middle of the taper and make it easier to tap.

You could also drill to 21/64, start the tap to get some threads engaged, and when it's feeling tight, drill the outer half to Q, if you don't have a tapered reamer.

 

[The Cobbler]

thanks for the tips everyone

 

[Cruzan80]

The first question I would asking myself is what is the runout on the drill you are using?

 

[Mr.N]

I bought a AvE Tap Chart t-shirt. https://ave.creator-spring.com/listing/Metric_US?product=2

 

[Monza Harry]

I have had cheap air tools that the inlet ****** would not start, I ended up having to shorten the tap to make it tap deep enough/fit. I would stuff some TP/snot rags in the hole to prevent chip infiltration. Grease as a "tapping fluid" helps for taking the chips out (stick to the tap +/-) rather than just TP for protection. Have you considered a fine thread set screw? Tap with a plug tap for an interference fit along with some Loctite, 577(I think) https://www.henkel-adhesives.com/ca/en/product/thread-sealants/loctite_5771.html
or Loctite bearing retainer.

Retaining Compounds

Secure bearings, bushings, and cylindrical parts into housings or onto shafts

www.henkel-adhesives.com

I would use a tap drill of a 0.302" dia. which is a letter "N" that would be 100% thread depth. Harry

 

[rlitman]

...Grease as a "tapping fluid" helps for taking the chips out...

Yes, grease works. My favorite tapping fluid is Fluid Film, which can be as snotty as grease and sticks to the chips.

 

[The Cobbler]

finally got around to this project today. ended up using the Q bit . greased it up, drilled the hole. greased up the tap, ran it in. tested the fit of the plug, ran the tap deeper a gain. tested the fit again, was satisfied with the fit.
cleaned everything up , put it all back together. a bit of pipe dope on the plug, wound it in. no leaks , all is good.
for reference it's the brass plug just to the right of the trigger
tks to everyone that chimed in

 

[signcrafter]

Yes, grease works. My favorite tapping fluid is Fluid Film, which can be as snotty as grease and sticks to the chips.

I never thought about using fluid film for tapping but now that you say it I may try it.

 

[The Cobbler]

actually I stuffed some cotton twine in to the port to block any stray chips. pulled it out with tweezers after I was done . sprayed everything with brake clean letting it run out by gravity . dowsed it with wd-40 .