Drill bit for tapping 1/8" NPT?

[RobG]

Potentially stupid question, but what size drill bit do I use to drill the hole that I want to tap with a 1/8" NPT tap?

Thanks!

Rob

 

[Archive]

RobG, my handy reference chart shows you should use a 11/32 drill for 1/8 pipe thread.

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Rob Hanson
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[Andy Perreault]

You need an "R" drill bit. The decimal equivalent of an "R" is . 3390, slightly smaller than an 11/32 which is . 3438.
Andy

 

[RobG]

So does this mean an 11/32 won't do? Looks like the difference is only five thousandths...

Rob

 

[Wayne M.]

Rob, if you can get to a hardware/ tool store and get an R drill I would do it. If you're not the critical type, dont sweat it, the tap will just go in a bit more, and so will the fitting. I know a machinist who gets by with 11/32 in fact we were just debating about this the other day.

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'92 W250,3. 54,auto,reg cab, raven hi top cap, tach,boost,transtemp,pyro. 16cm housing.

 

[Steve M]

I bought my tap and drill bit as a set in the same package. It came with a 7/16th" bit. I drilled and tapped my elbow with it for my pyrometer and it worked fine. 7/16th" did seem alittle tight on the tap though.

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98 2500 CC SB 4X2 12v 5spd 4. 10, -silencer, AFC+20, Edelbrock IAS, VDO gauges, Reese 15k Classic
97 Dodge Stratus 150HP
92 Harley Fatboy, Dynojet fuel management, Supertrapp 2-1
00 Jayco 25' 5th wheel, single slide

 

['956Wheel]

7/16 is the tap drill size for 1/4" NPT.

11/32 is fine for 1/8" NPT. Note: this is a tapered thread, (to seal), when tapping check thread depth with the fitting to be used, be sure to leave "room" for tightening to seal. DO NOT TAP TOO DEEP. Also, anti seize is a great tapping lube and will catch most chips.

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9535hundred

 

[illflem]

Steve,7/16 is the correct bit for 1/4 NPT. For 1/8 NPT I've seen taps that say use 11/32,others say use R. I personally have found it works just as well and is easier to tap when using the larger 11/32 bit.

 

[RobG]

Just for the record, I'm not drilling and tapping my manifold; been there done that months ago. Instead, I'm drilling and tapping the in-cab control for my air springs.

For some reason, Firestone chose to use hose barbs to connect the air hose to the in-cab control in several spots, and these things leak like mad. I replaced the barb tees with push-on tees, but the input to the guage itself uses barbs too, so I'm drilling into those fittings and tapping them to add push-ons, and will braze the barbed ends shut.

I've been unable to disasseble the control to try to simply replace them with something else, so the drilling/tapping step is what is necessary.

Rob