Technical Selection Views: 26

Countersink diameter and pilot hole matching - selection methods: The relationship between the countersink and the pilot hole directly decides countersink quality and screw assembly. Part 1 - Pilot hole diameter: the pilot hole is the hole the screw passes through; it must be slightly larger than the screw thread major diameter. Common through-hole (clearance) diameters: 1) M3 - φ3.2-3.5mm (precision 3.2, general 3.5); 2) M4 - φ4.2-4.5mm; 3) M5 - φ5.2-5.5mm; 4) M6 - φ6.6-7mm; 5) M8 - φ8.5-9mm; 6) M10 - φ10.5-11mm; 7) M12 - φ12.5-13mm; 8) M16 - φ17-18mm; 9) M20 - φ21-22mm. Note: these are clearance holes (for the screw to pass), NOT tapping holes! Tapping holes are much smaller (M6 tap drill φ5mm). Part 2 - Countersink head diameter: the countersink max diameter must exceed the max countersink diameter = screw head diameter. Common 90-degree flat-head screw head diameters: 1) M3 - about φ5.5-6mm; 2) M4 - φ7-7.5mm; 3) M5 - φ8.5-9mm; 4) M6 - φ11-11.5mm; 5) M8 - φ15-16mm; 6) M10 - φ18-19mm; 7) M12 - φ22-23mm; 8) M16 - about φ30mm; 9) M20 - φ36mm. Selection principle for countersink diameter: 1) 2-5mm larger than the screw head - full seating with margin; 2) not too large - more force, weaker, pricier: M6 needs φ16, not φ30; 3) not too small - cannot reach the size; the head will not sink: M6 with φ10 countersink (max φ10, head φ11.5) will not seat. Recommended matches: 1) M3-M4 - φ10 or φ12; 2) M5-M6 - φ12 or φ16; 3) M8 - φ16 or φ20; 4) M10 - φ20 or φ25; 5) M12 - φ25 or φ30; 6) M16 - φ30 or φ35; 7) M20 - φ35 or φ40. Part 3 - Piloted spot facer: the pilot enters the pilot hole for centering; pilot diameter must match the hole. Selection: 1) pilot = pilot hole - 0.05~0.1mm - inserts easily with good centering; 2) e.g. φ6.6mm hole - φ6.5mm pilot; 3) φ9mm hole - φ8.8 or φ9mm pilot; 4) standard pilots: φ3, φ4, φ5, φ6, φ8, φ10, φ12, φ14, φ16 - matching common holes. Part 4 - Depth and pilot hole: countersink depth is set by screw head height; the head normally sits 0.1-0.3mm below. 90-degree: countersink dia = pilot dia + 2 x depth. E.g. pilot φ6.6, target countersink φ11.5 (M6 head): depth = (11.5-6.6)/2 = 2.45mm. Part 5 - Common problems: 1) pilot hole too small - screw cannot pass; enlarge; 2) pilot hole too large - screw wobbles, poor centering; smaller hole or washers; 3) countersink too small - head will not sink; larger countersink; 4) countersink too large - more force, weaker; right size; 5) pilot too thick - cannot insert; thinner pilot or larger hole; 6) pilot too thin - poor centering, wobble; thicker pilot. Part 6 - Selection advice: 1) confirm the screw - M size and standard (GB 90 vs ANSI 82); 2) look up the head diameter - the required max countersink diameter; 3) choose countersink diameter - 2-5mm larger than the head; 4) choose pilot hole - per clearance standards; 5) batches - piloted spot facers with matching pilots; 6) unsure - samples first, or ask us. Countersink-pilot matching is the foundation of countersinking - get it right and you get quality countersinks. We carry countersinks and piloted spot facers in every size - ask us.