M13x.8 tap

[avbill]

if you take the different between the two measurements you get 12.2 mm for drill for the tap. or 0.4805".

What would thefts drill be for the tap?

12mm = .4724
12.2 = .4805
31/64 = .4824
12.5 = .4921

Going smaller seems a better idea Am I correct on that assumption?

thanks

 

[Brooks803]

I use a 31/64

 

[jj9ball]

I agree with the 31/64 that Brooks said. I would add that the best way to know is to use 31/64 and if the threads are stiff or they bind a little then go just a little bigger. 12.3mm would also probably work. Depending on the tolerances on your drill bits it really is about the same difference. If all else fails you can use a small boring bar if you have a metal lathe. The main thing is that you keep most of the .8mm thread.... say .6 or so. If you get much smaller than that you may have a cap the slides on and falls off.

 

[Joey-Nieves]

Maybe this will help I use a 75% thread Guhring, Inc. - Tap-Drill
I find that if the threads are to tight the pen "feels" wrong.
http://www.guhring.com/Tech/TapDrill/default.aspx

 

[Brooks803]

The flip side to which drill bit is also what dia to turn your tenon to cut your male threads.

I turn to 12.7 - 12.8 and use a 31/64 drill bit. Works great for me.