View Full Version : Which drill bit for a 3/8"x16 tap?

Tony Ennis
12-31-2010, 05:12 PM
I'm drilling cast iron. This web page (http://www.stanford.edu/~jwodin/holes.html) says I need a .3320" (size Q) drill. However, my Machinerys Handbook says a 5/16" (.3125) hole is called for (assuming my threads are American Standard Form...) My tap is an H3 tap if it matters.

What's a newb to do?

(curiously, MH says a Q is appropriate for 3x8"x24 TPI threads.)

12-31-2010, 05:16 PM
I'd use the 5/16" drill and not worry about it.

12-31-2010, 05:23 PM
akk the charts I have seen say 5/16 thats what I use.

12-31-2010, 06:02 PM
Yup, 5/16


Paul Alciatore
12-31-2010, 06:05 PM
My tap drill calculator gives a value of 0.3141" diameter for a 3/8-16, 75% thread. This is close to the 5/16" (0.3125") drill size and 5/16" is probably as close as you are going to get.

The Q (0.332") drill would give you a thread that is only about 52% of the full thread form. This is a bit too small for my liking. For one thing, your actual hole would probably be several thousanths larger than the drill's size so the 52% would be reduced even further.

CI is relatively easy to tap. I would go with the 5/16" size.

12-31-2010, 06:18 PM
Going from memory, I use a letter O, .316", for 3/8-16 threads.

12-31-2010, 06:21 PM
My favorite tap drill chart lists drill sizes from .307" to .325" for a 3/8--16 thread. The length of thread engagement is the determining factor in drill size.

The longer the thread engagement the larger the drill.

12-31-2010, 06:26 PM
Use 8mm x 1/25.4 = 0.315" ~ 0.3125" = 5/16"

I keep both inch drills in 1/64" (0.0156") increments and metric drills in 0.1mm (0.004") increments for just this reason.


12-31-2010, 06:37 PM
I use an awsome thread chart that lists 50% and 75% drill size, in both fractional/number/letter/whatever is closest form, AND in decimal form, So I can actualy use my calipers to pick out a 'close enough' bit, And see just how close a perticular bit is.

Usally, In aluminum you want 75%, And steel/etc 50%, though I usally aim for somewhere around 60%~70% in steel.. because It makes me feel like im cheating and getting a better thread, but not stressing the tap too much with a 75% depth.


12-31-2010, 07:55 PM
Write This on the wall in ur shop for quick reference

TD = D - 1/N

and get a chart off the net...just a tip with stainless go 60 to as low as 50%


PS The book that Oldtriffie has a pic of is excellent for the price!!

01-01-2011, 12:25 AM
Motorworks formula in easier to remember words:

Tap Drill Size = Major Diameter - Pitch

Pitch is the distance from a point on one thread to the corresponding point on an adjacent thread, so for inch threads:

Pitch = 1/Threads Per Inch

That formula will give you roughly 75% thread engagement when using a cutting tap (any tap with flutes). If you're using a forming tap you need to use a larger tap drill, otherwise the tap will bind because of the displaced metal.

The formula for forming taps is Tap Drill Size = Major Diameter - (Pitch/2)

Just in case you don't know how the thread designation breaks down, for 3/8-16, the 3/8 is the nominal major diameter and 16 is the TPI, so the pitch would be 1/16 or .0625". Using the cutting tap formula we get:

.375 - .0625 = .3125 for the tap drill

For metric threads, the designation is slightly different. In an M6 x 1.0 thread, 6mm is the major diameter and 1mm is the pitch. Both cutting and forming tap formulas are the same for inch and metric threads.

Tony Ennis
01-01-2011, 12:38 AM
Thanks all. The 5/16ths drill worked fine.