Guidance on diecast hole sizes for tapped threads is found below.
Tapped holes require special diameters, depth and draft to optimise economic production. Draft may be retained, based on allowing 85% full thread depth at the small end and 55% at the large end. A countersink or radius is recommended to provide relief for any displaced material and also strengthen the core in the diecast tool.
Greater dimensional accuracy can be achieved on tapped holes, but at higher cost. Draft may be retained, based on allowing 95% full thread depth at the small end and the maximum minor diameter at the large end.
All formed threads require the greater diecasting accuracy specified in these critical tolerances. Cored holes may be tapped without removing draft.
The cored holes specified below are suitable for both NPT and ANPT. NPT should be specified where possible, due to the additional costs and steps required. The 1°47' taper per side is more important for ANPT than NPT.
No standards exist for metric pipe threads.