what size drill bit for 1/8 npt tap?

[Spooled-up]

I'm going to do the boost gauge install tomorrow and I finally found a 1/8 npt tap. What size drill bit do I use for this tap? The guy who sold it to me had no idea, and I can't seem to find a quick answer. Any help would be appreciated.



Thanks.

 

[RJOL]

5/16" for 1/8" NPT. Some have used 11/32"...

 

[jodog]

WHY DRILL

IF you look hard enough,you should find a pluged hole!!!!!

 

[TSielski]

Jodog is right. Should be a 1/8 NPT threaded hole with a pipe plug in it on the intake manifold. Was on my 95.

If you must drill, an "R" (letter size) drill is more commonly used for a 1/8 NPT tap. 5/16 will work, but will be tight.

 

[illflem]

I second why drill?

The easiest way to install a boost gauge on a 24V Dodge is to first acquire a 3/4" NPT to 1/8" NPT reducer fitting. Next locate the 3/4" NPT pipe plug in the intake manifold. The best one of these to use is located on the back of the engine between the fuel filter and the firewall. The 3/4" NPT plug has a 1/2" square in the center of it and can be removed easily with a 1/2" drive ratchet and extension. Replace the 3/4" NPT factory plug with the reducer plug and the fittings that come with the boost gauge tubing kit. This will allow the tubing to be run from the intake manifold to the boost gauge. Boost readings will be extremely close to tapping the manifold.



For future reference the drill bit size is written on the side of most all taps.

 

[RADdodge]

I would add that if you did have to drill and were reluctant to do so, Piers has some special intake manifold bolts that have a fitting for a boost gauge. Not expensive.

 

[tcourse]

Just did my exhaust. 11/32 is correct.

 

[Miklos]

I'm sitting in front of a drill chart that I have on the wall behind my 'puter and it says to use a "Q" drill (. 332) if you just drill, without reaming.



CNC programmer/ machinist for an aerospace company

 

[Spooled-up]

Thanks for all the help guys. I'll post some pics of my gauge install when I'm done.