In aluminum you design tapped holes for thread shear, not for bolt strength, because the threads let go long before the fastener does. This calculator gives you the internal thread shear area, the actual pull-out load at your depth, and the minimum engagement that makes the bolt the weak link, then tells you whether a coil insert is the cheaper answer.
Last updated July 2026
The design factor covers thread form error, tap wear, hole perpendicularity and the nut dilation effect that the ideal shear model ignores. 1.5 is the working default. Selecting frequent disassembly adds 0.25 on top of whatever you enter.
White threads are the engaged length carrying load. The dashed line is the recommended minimum engagement with the design factor applied. Anything below it is material you are not using.
| Insert Length | Inches | Hole Depth | Pull-Out (lb) | vs Bolt |
|---|---|---|---|---|
| 0.5D | 0.125 | 0.300 | 3,196 | 0.84x |
| 1D | 0.250 | 0.425 | 6,391 | 1.67x |
| 1.5D | 0.375 | 0.550 | 9,587 | 2.51x |
| 2D | 0.500 | 0.675 | 12,782 | 3.35x |
| 2.5D | 0.625 | 0.800 | 15,978 | 4.18x |
| 3D | 0.750 | 0.925 | 19,173 | 5.02x |
Countersink the top of the STI hole 0.030 in deep at 82 degrees so the tang breaks clean and the first coil sits below the face, or specify a tangless insert and skip it. Blind hole depths include 0.050 in of chip clearance below the last full thread. STI tap drill values for the common UNC sizes match published coil insert charts. Everything else is nominal plus one pitch snapped up to the next standard drill, so check it against your insert supplier before you drill.
Tap it directly. 0.360 in of engagement in 6061-T6 / T651 fits inside 0.500 in of material with a design factor of 1.50 applied.
Alloy shear strengths are published typical values for the temper. Verify against the mill certification for anything structural.
Strength per inch and minimum engagement are worked for a 1/4-20 UNC with an SAE Grade 5 bolt (3,819 lb ultimate) at a design factor of 1.5, with a floor of one diameter. This is the single table that explains why the 1.5D rule of thumb works in 6061 and fails in cast plate.
| Alloy and Temper | Ultimate Tensile (psi) | Tensile Yield (psi) | Ultimate Shear (psi) | Brinell | 1/4-20 Strength per Inch (lb) | Min Engagement (in) | As Multiple of D |
|---|---|---|---|---|---|---|---|
| 6061-T6 / T651 | 45,000 | 40,000 | 30,000 | 95 | 16,169 | 0.36 | 1.44D |
| 6061-T4 / T451 | 35,000 | 21,000 | 24,000 | 65 | 12,935 | 0.45 | 1.80D |
| 7075-T651 | 83,000 | 73,000 | 48,000 | 150 | 25,871 | 0.25 | 1.00D |
| 7075-T7351 | 73,000 | 63,000 | 44,000 | 135 | 23,715 | 0.25 | 1.00D |
| 7050-T7451 | 76,000 | 66,000 | 44,000 | 140 | 23,715 | 0.25 | 1.00D |
| 2024-T351 | 68,000 | 47,000 | 41,000 | 120 | 22,098 | 0.26 | 1.04D |
| 5083-H321 | 46,000 | 33,000 | 27,000 | 82 | 14,552 | 0.40 | 1.60D |
| 5052-H32 | 33,000 | 28,000 | 20,000 | 60 | 10,780 | 0.54 | 2.16D |
| 5000 series cast tool and jig plate | 23,000 | 12,000 | 15,000 | 65 | 8,085 | 0.71 | 2.84D |
Cast tool and jig plate values are typical for the product class, not a specification minimum. A fixture plate tapped as if it were 6061 will strip, because it needs about 2.8 diameters where 6061-T6 needs 1.4.
Ds min is the class 2A minimum major diameter of the external thread. En max is the class 2B maximum pitch diameter of the internal thread. Pull-out per inch is the shear area per inch multiplied by 30,000 psi for 6061-T6, so divide by 30,000 and multiply by your alloy shear strength to move it to another material.
| Size | D (in) | TPI | Ds min (in) | En max (in) | Stress Area (sq in) | Shear Area per Inch (sq in/in) | Pull-Out per Inch, 6061-T6 (lb) | STI Tap Drill |
|---|---|---|---|---|---|---|---|---|
| 6-32 UNC | 0.1380 | 32 | 0.1312 | 0.1218 | 0.0091 | 0.2777 | 8,330 | 0.1695 (#18) |
| 8-32 UNC | 0.1640 | 32 | 0.1571 | 0.1475 | 0.0140 | 0.3343 | 10,029 | 0.1960 (#9) |
| 10-24 UNC | 0.1900 | 24 | 0.1818 | 0.1672 | 0.0175 | 0.4011 | 12,033 | 0.2340 (A) |
| 10-32 UNF | 0.1900 | 32 | 0.1831 | 0.1697 | 0.0200 | 0.4300 | 12,901 | 0.2210 (#2) |
| 1/4-20 UNC | 0.2500 | 20 | 0.2408 | 0.2224 | 0.0318 | 0.5390 | 16,169 | 0.2950 (M) |
| 1/4-28 UNF | 0.2500 | 28 | 0.2419 | 0.2311 | 0.0364 | 0.5127 | 15,380 | 0.2900 (L) |
| 5/16-18 UNC | 0.3125 | 18 | 0.3026 | 0.2817 | 0.0524 | 0.6818 | 20,454 | 0.3680 (U) |
| 3/8-16 UNC | 0.3750 | 16 | 0.3643 | 0.3401 | 0.0775 | 0.8281 | 24,843 | 0.4375 (7/16) |
| 7/16-14 UNC | 0.4375 | 14 | 0.4258 | 0.3985 | 0.1063 | 0.9640 | 28,921 | 0.5156 (33/64) |
| 1/2-13 UNC | 0.5000 | 13 | 0.4876 | 0.4565 | 0.1419 | 1.1235 | 33,705 | 0.5781 (37/64) |
| 5/8-11 UNC | 0.6250 | 11 | 0.6113 | 0.5732 | 0.2260 | 1.4249 | 42,747 | 0.7188 (23/32) |
| 3/4-10 UNC | 0.7500 | 10 | 0.7353 | 0.6927 | 0.3345 | 1.7232 | 51,695 | 0.8594 (55/64) |
STI tap drills for 6-32, 8-32, 10-24, 1/4-20, 5/16-18, 3/8-16 and 1/2-13 match published coil insert charts. The rest are nominal plus one pitch snapped up to the next standard drill, so confirm against your insert supplier before drilling.
Unified inch threads: At = 0.7854 x (D - 0.9743 / n) squared, with D in inches and n in threads per inch. Metric: At = 0.7854 x (D - 0.9382 x p) squared, with D and p in millimetres, then divide by 645.16 to get square inches.
F_bolt = At x UTS_bolt. F_proof = At x PROOF_bolt. Recommended clamp load = 0.75 x F_proof. A 1/4-20 Grade 5 gives At = 0.0318 sq in and F_bolt = 3,819 lb.
AS_per_in = pi x n x Ds_min x [ 1 / (2 x n) + 0.57735 x (Ds_min - En_max) ], in square inches per inch of engagement. The shear area at your chosen depth is AS = AS_per_in x Le. For metric threads, convert D, p, Ds_min and En_max to inches first and use n = 25.4 / p as the effective threads per inch.
P = AS x TAU_ULT, where TAU_ULT is the published ultimate shear strength of the base alloy. This calculator uses tabulated shear values rather than the 0.6 times UTS shortcut, because that shortcut overstates 6061 and understates 7075.
Le_theoretical = F_bolt / (AS_per_in x TAU_ULT) is the depth at which the bolt and the threads fail at the same load. Le_rec = max(1.0 x D, Le_theoretical x DESIGN_FACTOR), rounded up to the nearest 0.010 inch. The default design factor of 1.5 covers thread form error, tap wear, hole perpendicularity and the nut dilation effect that the ideal shear model ignores. Frequent disassembly adds 0.25 on top.
STI tap drill diameter D_sti = D + p, snapped up to the nearest standard fractional, letter or number drill. The STI major diameter formed in the parent material is D_sti_major = D + 1.2 x p. Recompute the shear area with Ds_min replaced by D_sti_major and En_max replaced by (D_sti_major - 0.6495 x p), then P_insert = AS_per_in_sti x Le_insert x TAU_ULT. Hole depth = insert length + 0.5 x D for the tap lead, plus 0.050 inch of chip clearance in a blind hole. Countersink 0.030 inch deep at 82 degrees for the tang break, or specify a tangless insert.
Need the stock to cut these holes into, get an instant quote on DFARS compliant 6061, 7075 and 7050 plate cut to your size, with the mill cert that tells you which shear strength row above actually applies to your part.
About 1.5 times the fastener diameter in 6061-T6 with a Grade 5 or class 8.8 bolt, and about 2 times diameter with a Grade 8, class 12.9 or socket head cap screw. In 7075-T651 you can drop to roughly 1 to 1.25 diameters because its shear strength is 48 ksi against 30 ksi for 6061. Below those depths the aluminum threads shear out before the fastener reaches its rated load, which is the failure mode you are designing against.
Multiply the internal thread shear area by the ultimate shear strength of the alloy. The shear area per inch of engagement is pi times n times Ds_min times the quantity 1 divided by 2n plus 0.57735 times the difference between Ds_min and En_max. Multiply that by your engagement length and by 30,000 psi for 6061-T6 or 48,000 psi for 7075-T651. A 1/4-20 at 0.375 inch of engagement in 6061-T6 comes out around 6,060 pounds.
When the available depth is less than the calculated minimum engagement, when the joint will be taken apart repeatedly, or when a high strength fastener would otherwise strip the parent threads. A coil insert carries load at the larger STI diameter, so a 1.5 diameter insert typically pulls out at roughly the same or higher load than 2 diameters of directly tapped 6061. It also gives you a repairable joint, which is worth more than the strength on service parts.
For a 1/4-20 in 6061-T6 with a Grade 5 bolt, about 7 to 8 full threads, which is 0.36 inch or 1.4 diameters. For a 10-32 in the same material, about 10 to 11 threads. Counting threads is less reliable than counting diameters, because a fine pitch gives you more threads in the same depth without proportionally more shear area. Design to a multiple of diameter, then check the pull-out load.
The STI tap drill is approximately the nominal fastener diameter plus one thread pitch, then rounded up to the nearest standard drill. A 1/4-20 needs 0.295 inch, which is a letter M drill. A 10-24 needs 0.234 inch, which is a letter A. A 3/8-16 needs 7/16 inch, and a 1/2-13 needs 37/64 inch. Drill the hole at least the insert length plus half a diameter deep so the STI tap lead does not bottom out.
Yes, by roughly 60 percent. 7075-T651 has an ultimate shear strength around 48,000 psi against 30,000 psi for 6061-T6, so the same tapped hole holds about 1.6 times the pull-out load, and you need about a third less engagement depth for the same fastener. The catch is that 7075 is not weldable, is more susceptible to stress corrosion cracking in the T651 temper, and costs roughly twice as much per pound.