drill bit size for taps

what size bit should i use for a 1/4" tap?

It should say on the tap.

My Machinists Ready Reference say to use a 7/32 drill bit for a 1/4 28 thread per inch bolt.

http://www.newmantools.com/tapdrill.htm


if its a 1/4-20 (corse) you need a #7 tap drill
if its 1/4-28 you need a #3

Originally posted by ERIC-B



My Machinists Ready Reference say to use a 7/32 drill bit for a 1/4 28 thread per inch bolt.

thanks dude

my snap on taps have it on the tap till i broke one haha.

lol yea mine have them on the taps too usually.. or i just run to the computer

Edited: 2/12/2006 7:04:06 PM by lyinlow

rule of thumb is you take the tap major diameter and subtract 1/number of threads per inch

so.....1/4-20 would be

.250-1/20 =.200 or #7

1/2-13.5-1/13=.423 or 27/64

http://www.americanfastener.com/techref/tapdrill.htm

drill and tap chart