is shorter than "basic". The maximum diameter (within tolerance) at the crest of the external threads (called the max. major diameter of external thread) is that of the basic thread form and equals the "nominal diameter", D, stated in the screw's designation. The minimum diameter (within tolerance) at the crest of the internal thread (called the min. minor diameter of internal thread) is that of the basic thread form and equals the nominal diameter minus twice the basic thread height (i.e. D − P). There is also a "Stub Acme" thread standard, identical in all respects to the one just described except for the height of the basic thread being 0.3P. Metric trapezoidal thread characteristics In case of the trapezoidal thread form the angle is 30° instead of 29°. All dimensions are in millimeters. Trapezoidal threads are defined as follows by ISO standards: Tr60×9 where Tr designates a trapezoidal thread, 60 is the nominal diameter in millimeters, and 9 is the pitch in millimeters. When there is no suffix it is a single start thread. If there is a suffix then the value after the multiplication sign is the lead and the value in the parentheses is the pitch. For example: Tr60×18(P9)LH would denote two starts, as the lead divided by the pitch is two. The "LH" denotes a left hand thread. Other trapezoidal threads For maintaining air conditioning systems using R134a gas, a non standard "ACME" thread is specified for gas canisters. See also Buttress thread Leadscrew Ball screw Notes References Bibliography . . External links Standard Acme threads Nominal dimension for trapezoidal thread profiles Trapezoidal coarse thread Trapezoidal fine thread Stub ACME Thread Sizes and Data Chart ASME: Acme Screw Threads B1.5-1997 Manufacturer of trapezoidal screws Metric Trapezoidal Threads Screws Thread standards Threading (manufacturing)