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Form tap & STI Helicoil drill chart

Thread-forming tap drills at 65% thread and STI insert tap drills for every UN and ISO size.

Reading your screen…
Unified inch · ASME B1.1
#10-32UNF
0.190" / 4.83 mm major diameter
Tap drill · 75%
#21
0.159" · 4.04 mm
76.4% thread
Hardware-store bit
5/32"
0.1563"
83.1% thread
Wrench / hex key
3/8"nut5/32"Allen

Drills & threading

Major diameter
0.190"4.83 mm · nearest 3/16"
Pitch
32 TPI UNF0.0313" · 0.794 mm
Tap drill 75%
#21 · 76.4%0.1590" · 4.04 mm
Hardware-store bit
5/32" · 83.1%0.1563" from a 1/64" set
Form / roll tap 65%
#160.1770" · 4.5 mm
STI / Helicoil
#7#10-32 STI tap · 0.2010"
Clearance · close / free
#9 / #70.1960" / 0.2010"
Counterbore
3/8"0.375" · 9.52 mm

Wrenches, keys & torque

Hex head
5/16"head 0.120" · 3.05 mm tall
Nut
3/8"0.125" · 3.17 mm thick · flange OD N/A
Socket cap key
5/32"head ⌀ 0.312" · 7.92 mm
Button / flat · set
1/8" · 3/32"
Flat washer SAE
0.219" / 0.5"No USS washer at this size
SAE Gr 2 · Gr 5
31 in-lb · N/A
SAE Grade 8
N/ASAE J429 grades start at 1/4"
18-8 stainless
37 in-lbGalling risk: use anti-seize on stainless and tighten slowly.

Dry steel (K=0.20, ~75% proof load). Reduce by 20–25% if lubricated.

Full chart 70 rows

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Form / roll tap & STI Helicoil drill chart

70 rows
Form / roll tap & STI Helicoil drill chart · ScrewBoltChart
screwboltchart.com · inch-first · tap drills 75% (soft metals) · torque dry, K=0.20 · verify against the governing standard
3/64" (80.7%)#54 · 0.0550" · 1.4 mm#0-80 STI#52 · 0.0635"
#53 (66.5%)#51 · 0.0670" · 1.7 mm#1-64 STI5/64" · 0.0781"
#53 (74.8%)#51 · 0.0670" · 1.7 mm#1-72 STI5/64" · 0.0781"
#50 (69%)5/64" · 0.0781" · 1.98 mm#2-56 STI#42 · 0.0935"
#50 (78.8%)#47 · 0.0785" · 1.99 mm#2-64 STI#43 · 0.0890"
#47 (75.7%)#43 · 0.0890" · 2.26 mm#3-48 STI#36 · 0.1065"
#45 (73.3%)#43 · 0.0890" · 2.26 mm#3-56 STI#37 · 0.1040"
#43 (70.8%)#38 · 0.1015" · 2.58 mm#4-40 STI#31 · 0.1200"
#42 (68.4%)#37 · 0.1040" · 2.64 mm#4-48 STI#31 · 0.1200"
#38 (72.4%)#33 · 0.1130" · 2.87 mm#5-40 STI#29 · 0.1360"
#37 (71.1%)#32 · 0.1160" · 2.95 mm#5-44 STI#29 · 0.1360"
#36 (77.6%)1/8" · 0.1250" · 3.18 mm#6-32 STI#26 · 0.1470"
#33 (77%)#30 · 0.1285" · 3.26 mm#6-40 STI#26 · 0.1470"
#29 (69%)#25 · 0.1495" · 3.8 mm#8-32 STI#17 · 0.1730"
#29 (77.6%)#24 · 0.1520" · 3.86 mm#8-36 STI#17 · 0.1730"
#25 (74.8%)11/64" · 0.1719" · 4.37 mm#10-24 STI13/64" · 0.2031"
#21 (76.4%)#16 · 0.1770" · 4.5 mm#10-32 STI#7 · 0.2010"
#16 (72.1%)#8 · 0.1990" · 5.05 mm#12-24 STI#1 · 0.2280"
#14 (73.3%)#7 · 0.2010" · 5.11 mm#12-28 STI#1 · 0.2280"
#7 (75.4%)#1 · 0.2280" · 5.79 mm1/4"-20 STIH · 0.2660"
#3 (79.8%)15/64" · 0.2344" · 5.95 mm1/4"-28 STIG · 0.2610"
F (76.9%)L · 0.2900" · 7.37 mm5/16"-18 STIQ · 0.3320"
I (74.8%)M · 0.2950" · 7.49 mm5/16"-24 STI21/64" · 0.3281"
5/16" (77%)S · 0.3480" · 8.84 mm3/8"-16 STIX · 0.3970"
Q (79.4%)T · 0.3580" · 9.09 mm3/8"-24 STI25/64" · 0.3906"
U (74.9%)13/32" · 0.4063" · 10.32 mm7/16"-14 STI29/64" · 0.4531"
25/64" (72.2%)Z · 0.4130" · 10.49 mm7/16"-20 STI29/64" · 0.4531"
27/64" (78.2%)15/32" · 0.4688" · 11.91 mm1/2"-13 STI17/32" · 0.5313"
29/64" (72.2%)31/64" · 0.4844" · 12.3 mm1/2"-20 STI33/64" · 0.5156"
31/64" (72.1%)17/32" · 0.5313" · 13.49 mm9/16"-12 STI19/32" · 0.5938"
33/64" (65%)17/32" · 0.5313" · 13.49 mm9/16"-18 STI37/64" · 0.5781"
17/32" (79.3%)37/64" · 0.5781" · 14.68 mm5/8"-11 STI21/32" · 0.6563"
37/64" (65%)19/32" · 0.5938" · 15.08 mm5/8"-18 STI41/64" · 0.6406"
21/32" (72.1%)45/64" · 0.7031" · 17.86 mm3/4"-10 STI25/32" · 0.7813"
11/16" (77%)23/32" · 0.7188" · 18.26 mm3/4"-16 STI49/64" · 0.7656"
49/64" (75.8%)53/64" · 0.8281" · 21.03 mm7/8"-9 STI29/32" · 0.9063"
13/16" (67.4%)27/32" · 0.8438" · 21.43 mm7/8"-14 STI57/64" · 0.8906"
7/8" (77%)15/16" · 0.9375" · 23.81 mm1"-8 STI1-3/64" · 1.0469"
59/64" (72.1%)31/32" · 0.9688" · 24.61 mm1"-12 STI1-1/32" · 1.0313"
1.25 mm1.45 mmM1.6x0.35 STI1.7 mm
1.6 mm1.8 mmM2x0.4 STI2.1 mm
2.05 mm2.3 mmM2.5x0.45 STI2.65 mm
2.5 mm2.8 mmM3x0.5 STI3.2 mm
2.9 mm3.25 mmM3.5x0.6 STI3.7 mm
3.3 mm3.7 mmM4x0.7 STI4.2 mm
3.5 mm3.8 mmM4x0.5 STI4.15 mm
4.2 mm4.65 mmM5x0.8 STI5.2 mm
4.5 mm4.8 mmM5x0.5 STI5.15 mm
5 mm5.55 mmM6x1.0 STI6.3 mm
5.25 mm5.65 mmM6x0.75 STI6.25 mm
6.8 mm7.45 mmM8x1.25 STI8.4 mm
7 mm7.55 mmM8x1.0 STI8.3 mm
8.5 mm9.35 mmM10x1.5 STI10.5 mm
8.8 mm9.45 mmM10x1.25 STI10.4 mm
9 mm9.55 mmM10x1.0 STI10.3 mm
10.2 mm11.25 mmM12x1.75 STI12.5 mm
10.5 mm11.35 mmM12x1.5 STI12.5 mm
10.8 mm11.45 mmM12x1.25 STI12.4 mm
12 mm13.1 mmM14x2.0 STI14.5 mm
12.5 mm13.35 mmM14x1.5 STI14.5 mm
14 mm15.1 mmM16x2.0 STI16.5 mm
14.5 mm15.35 mmM16x1.5 STI16.5 mm
15.5 mm16.9 mmM18x2.5 STI18.75 mm
16.5 mm17.35 mmM18x1.5 STI18.5 mm
17.5 mm18.9 mmM20x2.5 STI20.75 mm
18.5 mm19.35 mmM20x1.5 STI20.5 mm
19.5 mm20.9 mmM22x2.5 STI22.75 mm
20.5 mm21.35 mmM22x1.5 STI22.5 mm
21 mm22.65 mmM24x3.0 STI25 mm
22 mm23.1 mmM24x2.0 STI24.5 mm

05 / Field guide

Form it,
don’t cut it.

Thread-forming (roll) taps push the material into shape instead of cutting it away. They make no chips and produce stronger threads in ductile metals, but they need a larger hole than a cutting tap.

Why the hole is larger

A forming tap displaces metal upward into the thread crests, so the hole has to start bigger or the material has nowhere to go. Aim for about 65% thread. Use them in aluminium, copper, brass and low-carbon steel; avoid cast iron and brittle plastics, which crack instead of flowing.

hole = major diameter − 0.0068 × %thread ÷ TPI

STI taps for Helicoil inserts

A wire thread insert repairs stripped threads or adds wear resistance in soft metals. The hole is tapped with an oversized STI tap so that, once the insert is wound in, it presents the original thread size. The STI drill is larger than the screw itself.

STI drill ≈ major diameter + 0.325 × pitch

Practical notes

Form taps need more torque, so use plenty of lubricant and a tap handle with good leverage. A 1/4"-20 forming tap uses a #1 drill (0.228"); the cutting tap uses #7. Mixing them up either breaks the form tap or leaves a shallow thread.

Forming or cutting: choosing

Choose forming taps for production in aluminium, brass and mild steel, for blind holes where chips are a problem, and wherever thread strength matters. Choose cutting taps for cast iron, hard or brittle materials, very small sizes where torque would break a forming tap, and repair work where you are cleaning up an existing hole. Never run a forming tap into a hole drilled for a cutting tap: there is not enough room for the displaced metal and the tap will seize.

Questions

Answered plainly.

01What drill size for a 1/4-20 forming tap?+

A #1 drill (0.228") for about 65% thread. The cutting-tap drill (#7) is too small for a forming tap.

02What drill size for a 1/4-20 Helicoil?+

The 1/4"-20 STI tap uses an H drill (0.266"). Wind the insert in after tapping and the hole takes a standard 1/4"-20 screw.

03Are forming taps stronger?+

In ductile metals, yes. The grain flows around the thread instead of being cut, and the surface work-hardens. Strength gains of 20% or more are common in aluminium.

04Can I form-tap stainless?+

Austenitic stainless can be form-tapped, but it needs high torque, good lubrication and a coated tap. Small sizes are prone to breakage, so many shops cut threads in stainless instead.

05What is the M6 Helicoil drill?+

A 6.3 mm drill for an M6 × 1 STI tap, per the insert manufacturer tables.

06Do forming taps work in blind holes?+

Yes, and they are ideal there because there are no chips to clear. Drill a little deeper than the thread to allow for the tap’s lead.

07What drill size for a 10-32 forming tap?+

A #16 drill (0.177") for about 65% thread. The cutting-tap drill for 10-32 is #21.

08Why use a Helicoil instead of re-tapping larger?+

An insert restores the original screw size, so the mating part doesn’t change. It also gives a stronger, wear-resistant steel thread in soft aluminium or magnesium.

09What is the 8-32 STI drill?+

A #17 drill (0.173") for the 8-32 STI tap, followed by an 8-32 wire insert.

10Do forming taps need special lubricant?+

A good tapping oil or forming paste matters more with forming taps than with cutting taps, because all the work is done by friction and pressure. Water-soluble coolant is not enough for hand tapping.

11What drill size for an M6 forming tap?+

About 5.55 mm for M6 × 1.0 at 65% thread. The cutting-tap drill is 5.0 mm, which would leave no room for the displaced metal and seize a forming tap.