tap & drill question

[CliffS]

Since we use the metric system overhere the next question:

Like to tap and drill some threads in my intake to fit some fittings. The fitting size is 3/8 NPT, what drill and tap size should I use?

Thx.

[hotrdblder]

npt is npt (national pipe thread, i think) it is its own thread, drill size will be stated on the 3/8 npt tap.more then ikely you can find adapters to make it american or metric rather then pipe thread, what exactely are you tapping?
http://www.newmantools.com/tapdrill.htm

[rolltide]

9/16 drill size for 3/8 NPT (National Pipe Taper). The tap is simply a 3/8 NPT tap.

[toxicz28]

If we can get metric here, can't you get sae there???

[CliffS]

I like to run two extra hoses from the back of the heads to the water cross-over. SAE will be hard to get overhere (special order) but I just wanted to be sure to get the right tap. My buddy in St. Louis went to 3 different hardware store places and gave him a 5/8 NF tap for these Aeroquip fittings. The threads on the tap and fitting measured the same but in a try out the fitting wouldn't go in at all!

So outcome is to get a 3/8 NPT tap and a 9/16 drill?

[Karch]

Cliff, are you running hose from eheads heater outlet to the back of the intake crossover?

I was thinking of that...maybe -12 line or possibly hardline.

[Bad Bowtie]

Cliff,
Are you looking to do something like this. I did this setup on a recent build and it works good. I used 1/2 aluminum tubing made by moroso and fit tube flare / tube nut for connections to the AN fittings.
BB

[CliffS]

Karch, BB

That is exactly what I'm planning to do. I've got all hardware; braided line, couplers and fittings, Only thing is to drill and tao. A 3/8 NPT tap does not work, way too small for the 3/8 NPT coupler/fittings.

[CliffS]

Cliff,
Are you looking to do something like this. I did this setup on a recent build and it works good. I used 1/2 aluminum tubing made by moroso and fit tube flare / tube nut for connections to the AN fittings.
BB

Btw,

engine looking good, any specs on it?

[Karch]

I would think you'd want at least 1/2" NPT on -12, but you can use 3/8" NPT on 1/2" tubing.

For the NPT tap to work, you need to drill it using the correct size drill, than the tricky part.

The tap will not, and must not, go all the way in to the hole. You are cutting a tapered thread, and as such, the more you twist the tap in, the larger the hole actually gets. You are best to go some with the tap, back it out, and try your fitting.

I recall that you want at least a few full threads showing above the surface when it's tight. Maybe as little as two.

Again, it is a tapered fit, which is of utmost importance to keep in mind.

BTW, BB...I too plan to use 1/2" aluminum or stainless (probably aluminum due to galvanic compatibilites) as it's less expensive and hides better. Though I will probably cut off the water crossover and tap straight into the crossover.

[Bad Bowtie]

Cliff,
I just checked my SAE Pipe thread taps and each tap says which size drill to use.

• 3/8 NPT tap uses a 37/64 drill bit which is equal to a 14.65mm drill bit.
• 1/2 NPT tap uses a 23/32 drill bit which equals to a 18mm drill bit

I think the AN fittings I used were 1/2 NPT by -10 AN size

Just like Karch said: "The tap will not, and must not, go all the way in to the hole" very important as this is what help the sealing of the fitting.

The pontiac was a 467ci.. 400 block w/ 455 crank w/ported e-heads, Comp Cams solid lifter cam, motor was full of torque, very fun to drive.
Hope this helps
BB

[CliffS]

Ok Thanks guys, I will try the 1/2 tap and 23/32 drill.