What is the minimum thread engagement for structural bolts?

For structural steel connections, 100% thread engagement is standard per ASTM A325/A490. However, '100%' in this context means full thread depth in the nut, not 100% of the screw thread height. For tapped holes in machine components, 65–75% is adequate for 90% of applications per machinery handbook guidelines.

Does thread percentage affect class of fit (6H vs 6g)?

Thread percentage is determined by the tap drill size and affects the internal thread minor diameter, which is part of the 6H classification. The 6H tolerance band has upper and lower limits — a higher thread percentage uses the lower portion of the band. The pitch diameter (which determines fit quality) is determined by the tap's geometry, not the drill size.