The drill size you use before tapping a hole determines how much material is left for the tap to cut. Too small a drill bit and the tap binds and breaks (or the threads tear). Too large and you have only weak threads holding nothing.
75% vs 50% thread engagement
75% thread engagement is the default — strong threads, but harder tapping, especially in tough materials like steel or stainless.
50% thread engagement is only ~5% weaker but ~30% easier to tap. Use it for:
- Hard materials (304/316 stainless, hardened steel)
- Thin sections where breakage risk is high
- Through-holes where strength is shared with other fasteners
Don't use 50% for safety-critical fasteners — head bolts, main caps, rod bolts. Always 75%+ there.
Common engine taps
- 1/4-20 (UNC): 75% → #7 drill (0.201") · 50% → #8 (0.199")
- 5/16-18 (UNC): 75% → letter F (0.257") · 50% → letter G (0.261")
- 3/8-16 (UNC): 75% → 5/16" (0.3125") · 50% → letter Q (0.332")
- 7/16-14 (UNC): 75% → letter U (0.368") · 50% → 3/8" (0.375")
- 1/2-13 (UNC): 75% → 27/64" (0.422") · 50% → 7/16" (0.4375")