This drill and tap reference provides factory-authenticated machinist specifications for cutting internal threads and drilling bolt clearance holes in steel, cast iron, brass, and aluminum. Designed for automotive fabricators and engine builders, each blueprint card pairs exact 75% thread engagement number and letter tap drills with their closest fractional shop bit equivalents, 50% thread drills for hard alloys, and standard hex wrench sizes.
SAE Coarse (UNC) - Numbered & Fractional
| Thread Size | Exact Tap Drill (75%) | Closest Fractional Bit | Clearance (Close Fit) | Clearance (Free Fit) |
|---|---|---|---|---|
| #4 - 40 | #43 (0.0890") | 3/32" (0.0938") | #32 (0.116") | 1/8" (0.125") |
| #6 - 32 | #36 (0.1065") | 7/64" (0.1094") | #27 (0.144") | 5/32" (0.156") |
| #8 - 32 | #29 (0.1360") | 9/64" (0.1406") | 11/64" (0.172") | 3/16" (0.188") |
| #10 - 24 | #25 (0.1495") | 5/32" (0.1562") | 13/64" (0.203") | 7/32" (0.219") |
| 1/4" - 20 | #7 (0.2010") | 13/64" (0.2031") | 17/64" (0.266") | 9/32" (0.281") |
| 5/16" - 18 | Letter F (0.2570") | 17/64" (0.2656") | 21/64" (0.328") | 11/32" (0.344") |
| 3/8" - 16 | 5/16" (0.3125") | 5/16" (Exact) | 25/64" (0.391") | 13/32" (0.406") |
| 7/16" - 14 | Letter U (0.3680") | 3/8" (0.3750" / 65%) | 29/64" (0.453") | 15/32" (0.469") |
| 1/2" - 13 | 27/64" (0.4219") | 27/64" (Exact) | 33/64" (0.516") | 17/32" (0.531") |
| 9/16" - 12 | 31/64" (0.4844") | 31/64" (Exact) | 37/64" (0.578") | 19/32" (0.594") |
| 5/8" - 11 | 17/32" (0.5312") | 17/32" (Exact) | 41/64" (0.641") | 21/32" (0.656") |
| 3/4" - 10 | 21/32" (0.6562") | 21/32" (Exact) | 49/64" (0.766") | 25/32" (0.781") |
SAE Fine (UNF) - Automotive & AN Standard
| Thread Size | Exact Tap Drill (75%) | Closest Fractional Bit | Clearance (Close Fit) | Clearance (Free Fit) |
|---|---|---|---|---|
| #6 - 40 | #33 (0.1130") | 7/64" (0.1094") | 9/64" (0.141") | 5/32" (0.156") |
| #8 - 36 | #29 (0.1360") | 9/64" (0.1406") | 11/64" (0.172") | 3/16" (0.188") |
| #10 - 32 | #21 (0.1590") | 5/32" (0.1562") | 13/64" (0.203") | 7/32" (0.219") |
| 1/4" - 28 | #3 (0.2130") | 7/32" (0.2188") | 17/64" (0.266") | 9/32" (0.281") |
| 5/16" - 24 | Letter I (0.2720") | 9/32" (0.2812" / 65%) | 21/64" (0.328") | 11/32" (0.344") |
| 3/8" - 24 -3 AN | Letter Q (0.3320") | 21/64" (0.3281") | 25/64" (0.391") | 13/32" (0.406") |
| 7/16" - 20 -4 AN | 25/64" (0.3906") | 25/64" (Exact) | 29/64" (0.453") | 15/32" (0.469") |
| 1/2" - 20 -5 AN | 29/64" (0.4531") | 29/64" (Exact) | 33/64" (0.516") | 17/32" (0.531") |
| 9/16" - 18 -6 AN | 33/64" (0.5156") | 33/64" (Exact) | 37/64" (0.578") | 19/32" (0.594") |
| 5/8" - 18 | 37/64" (0.5781") | 37/64" (Exact) | 41/64" (0.641") | 21/32" (0.656") |
| 3/4" - 16 -8 AN | 11/16" (0.6875") | 11/16" (Exact) | 49/64" (0.766") | 25/32" (0.781") |
Metric Coarse, Fine & JIS Automotive (ISO)
| Thread Size | Exact Metric Tap Drill | Closest Fractional / # Bit | Clearance (Close Fit) | Clearance (Free Fit) |
|---|---|---|---|---|
| M3 x 0.5 | 2.5 mm (0.0984") | #39 or 3/32" (0.0938") | 3.2 mm (1/8") | 3.4 mm (9/64") |
| M4 x 0.7 | 3.3 mm (0.1299") | #30 or 1/8" (0.1250") | 4.3 mm (11/64") | 4.5 mm (3/16") |
| M5 x 0.8 | 4.2 mm (0.1654") | #19 or 11/64" (0.1719") | 5.3 mm (7/32") | 5.5 mm (7/32") |
| M6 x 1.0 | 5.0 mm (0.1969") | #9 or 13/64" (0.2031") | 6.4 mm (1/4") | 6.6 mm (17/64") |
| M7 x 1.0 VW / Euro | 6.0 mm (0.2362") | 15/64" (0.2344" / Letter B) | 7.4 mm (19/64") | 7.6 mm (5/16") |
| M8 x 1.25 Coarse | 6.8 mm (0.2677") | 17/64" (0.2656" / Letter H) | 8.4 mm (21/64") | 9.0 mm (23/64") |
| M8 x 1.0 Fine | 7.0 mm (0.2756") | 9/32" (0.2812" / Letter J) | 8.4 mm (21/64") | 9.0 mm (23/64") |
| M10 x 1.5 Coarse | 8.5 mm (0.3346") | 21/64" (0.3281" / Letter Q) | 10.5 mm (13/32") | 11.0 mm (7/16") |
| M10 x 1.25 JIS Fine | 8.8 mm (0.3465") | 11/32" (0.3438") | 10.5 mm (13/32") | 11.0 mm (7/16") |
| M10 x 1.0 Brake / VW | 9.0 mm (0.3543") | 23/64" (0.3594" / Letter S) | 10.5 mm (13/32") | 11.0 mm (7/16") |
| M12 x 1.75 Coarse | 10.2 mm (0.4016") | 13/32" (0.4062" / Letter Y) | 13.0 mm (33/64") | 13.5 mm (17/32") |
| M12 x 1.5 Euro Fine | 10.5 mm (0.4134") | 27/64" (0.4219" / Letter Z) | 13.0 mm (33/64") | 13.5 mm (17/32") |
| M12 x 1.25 JIS Fine | 10.8 mm (0.4252") | 27/64" (0.4219") | 13.0 mm (33/64") | 13.5 mm (17/32") |
| M14 x 1.5 Lug / Sump | 12.5 mm (0.4921") | 1/2" (0.5000" / 31/64") | 15.0 mm (19/32") | 15.5 mm (39/64") |
| M14 x 1.25 Spark Plug | 12.8 mm (0.5039") | 1/2" (0.5000") | 15.0 mm (19/32") | 15.5 mm (39/64") |
| M18 x 1.5 O2 Bung | 16.5 mm (0.6496") | 21/32" (0.6562" / 41/64") | 19.0 mm (3/4") | 19.5 mm (49/64") |
Tapered Pipe Threads (NPT & BSPT) - Sensors & Fluid Ports
| Pipe Thread Size | Tap Drill (Direct Drill) | Closest Fractional Bit | Tap Drill (With Taper Reamer) | Common Automotive Application |
|---|---|---|---|---|
| 1/16" - 27 NPT | Letter D (0.2460") | 1/4" (0.2500") | Letter C (0.2420") | Carburetor Galley Plugs, Oil Galley Plugs |
| 1/8" - 27 NPT | Letter R (0.3390") | 11/32" (0.3438") | 21/64" (0.3281") | Oil Temp/Pressure Senders, Boost/Vac Ports |
| 1/8" - 28 BSPT UK/JDM | 8.6 mm (0.3386") | 11/32" (0.3438") | 8.4 mm (0.3307") | Japanese & British Oil Pressure Switches |
| 1/4" - 18 NPT | 7/16" (0.4375") | 7/16" (Exact) | 27/64" (0.4219") | Fuel Regulator Blocks, Block Coolant Drains |
| 3/8" - 18 NPT | 37/64" (0.5781") | 37/64" (Exact) | 9/16" (0.5625") | Intake Manifold Temp Senders, Fuel Pumps |
| 1/2" - 14 NPT | 45/64" (0.7031") | 45/64" (Exact) | 11/16" (0.6875") | Heater Hose Nipples, Oil Sump Returns |
| 3/4" - 14 NPT | 59/64" (0.9219") | 59/64" (Exact) | 29/32" (0.9062") | Water Pump Bypass Ports, High-Flow Filters |
Vintage British Standard (BSF & BSW - 55 Deg Whitworth)
| Thread Size | Exact Tap Drill (Metric / #) | Closest Fractional Bit | Clearance (Close Fit) | Clearance (Free Fit) |
|---|---|---|---|---|
| 3/16" - 32 BSF | 3.8 mm (#25 / 0.1495") | 5/32" (0.1562") | 13/64" (0.203") | 7/32" (0.219") |
| 1/4" - 26 BSF | 5.1 mm (#8 / 0.1990") | 13/64" (0.2031") | 17/64" (0.266") | 9/32" (0.281") |
| 5/16" - 22 BSF | 6.6 mm (Letter G / 0.2610") | 17/64" (0.2656") | 21/64" (0.328") | 11/32" (0.344") |
| 3/8" - 20 BSF | 8.1 mm (Letter O / 0.3160") | 5/16" (0.3125") | 25/64" (0.391") | 13/32" (0.406") |
| 7/16" - 18 BSF | 9.5 mm (Letter V / 0.3770") | 3/8" (0.3750") | 29/64" (0.453") | 15/32" (0.469") |
| 1/2" - 16 BSF | 10.9 mm (0.4291") | 27/64" (0.4219") | 33/64" (0.516") | 17/32" (0.531") |
| 1/4" - 20 BSW Coarse | 5.1 mm (#7 / 0.2010") | 13/64" (0.2031") | 17/64" (0.266") | 9/32" (0.281") |
| 5/16" - 18 BSW Coarse | 6.5 mm (Letter F / 0.2570") | 1/4" or 17/64" (0.2656") | 21/64" (0.328") | 11/32" (0.344") |
| 3/8" - 16 BSW Coarse | 7.9 mm (5/16" / 0.3125") | 5/16" (Exact) | 25/64" (0.391") | 13/32" (0.406") |
For more custom fabrication calculators and vintage engine specs, head back to the main Frank's Workshop Hub.