- Threading Tap to Drill Bit Chart -

0-80 NF ----------------- 3/64"
1-64 NC ------------------- #53
1-72 NF ------------------- #53
2-56 NC ------------------- #50
2-64 NF ------------------- #50
3-48 NC ------------------- #47
3-56 NF ------------------- #46
4-40 NC ---------- #43 or 3/32"
4-48 NF ------------------- #42
5-40 NC ------------------- #38
5-44 NF ------------------- #37
6-32 NC ---------- #35 or 7/64"
6-40 NF ------------------- #33
8-32 NC --- #29(.136") or 9/64"
8-36 NF --- #29(.136") or 9/64"
10-24 NC -- #25(.150") or 5/32"
10-32 NF - #21(.157") or 11/64"
12-24 NC --------- 16 or 11/64"
12-28 NF ---------- 15 or 3/16"

1/4-20 NC --- #7(.201") or 13/64"
1/4-28 NF ---- #3(.213") or 7/32"
5/16-18 NC - letter "F" or 17/64"
5/16-24 NF -- letter "I" or 9/32"
3/8-16 NC ----------------- 5/16"
3/8-24 NF ---------------- 21/64"
7/16-14 NC ---------------- 3/8"
7/16-20 NF --------------- 25/64"
1/2-13 NC ---------------- 27/64"
1/2-20 NF ---------------- 29/64"
9/16-12 NC --------------- 31/32"
9/16-18 NF --------------- 33/64"
5/8-11 NC ---------------- 17/32"
5/8-18 NF ---------------- 37/64"
3/4-10 NC ---------------- 21/32"
3/4-16 NF ---------------- 11/16"
7/8-9 NC ----------------- 49/64"
7/8-14 NF ---------------- 13/16"
1-8 NC -------------------- 7/8"
1-14 NF ------------------ 15/16"

- National Pipe Taper (NPT) Tap Chart -

Tap Size -------- Drill Bit Size

Tap Size -------- Drill Bit Size

1/8" NPT --- R or 11/32"
1/4" NPT --------- 7/16"
3/8" NPT -------- 37/64"
1/2" NPT -------- 23/32"

3/4" NPT -------- 59/64"
1" NPT --------- 1-5/32"
1-1/4" NPT ------ 1-1/2"
1-1/2" NPT ---- 1-47/64"

14mm x 1.25 pitch (spark plug) use 15/32" or 12mm drill bit.