DIN 76 Standards for Thread Undercuts | PDF | Metalworking - Scribd
: For internal threads (Form C and D), the undercut diameter must be larger than the nominal thread diameter. Din 76-1 | PDF | Screw | Metalworking - Scribd din 76a pdf
The standard is typically divided into sections based on the type of thread: DIN 76 Standards for Thread Undercuts | PDF