Slotted flat countersunk head machine screw dimensions are shown in the following chart according to ASME B18.6.3 standard.

The flat countersunk head has a flat top surface and a conical bearing surface with a head angle of approximately 82 deg for one style and approximately 100 deg for another style.

For short lengths of 82 deg of head angle, machine screws have heads undercut to 70 % of normal side height to have greater length of thread on the screws.

Some important notes from the ASME B18.6.3:

- Thread dimensions for the No.0000, No.000 and No.00 sizes are also included in the standard.

- The general data about the head height, bearing surface, gaging of recess, slot, length, tolerance on length, length of thread, points, finish, workmanship and designation of machine screws are given in the standard.

Head Angle
Nominal screw size


Dimensions of Slotted Flat Countersunk Head Machine Screws
Nom. Size --- ---
Basic Dia. --- --- ---
Head Angle --- --- ---
Length [Note 5] --- --- ---
Head Diameter (A) max --- ---
min --- ---
Head Height (H) ref --- ---
max --- ---
min --- ---
Slot Width (J) max --- ---
min --- ---
Slot Depth (T) max --- ---
min --- ---
Protrusion Above Gaging Diameter (F) max --- ---
min --- ---
Gaging Dia., G --- --- ---
Note --- --- ---

Note 1: For 82 deg countersunk head machine screw, the head angle shall be between 80 deg - 82 deg. For 100 deg countersunk head , the head angle shall be between 99 deg - 101 deg.

Note 2: Dimensions in this calculator are in inches.

Note 3: Machine screws shall have a definite underhead fillet large enough to ensure that full fastener strength is achieved. For countersunk head screws, the radius of fillet shall be no greater than 40 % of the basic screw diameter.

Note 4: Not practical to gage.

Note 5: Screws with these length and shorter shall have undercut heads as shown in the figure.

Note 6: No tolerance given for gaging diameter. If the gaging diameter of the gage is differs from tabulated value, the protrusion will be affected accordingly and the proper protrusion value must be recalculated using the formulas given in the Appendix of ASME B18.6.3.

Note 7: For 82 deg countersunk head selection, 2nd row gives the dimensions of machine screw with undercut head. See Note 5 for more information.

Note 8 : For 100 deg countersunk selection, dimensions of normal and close tolerance machine screws are given in the 1st and 2nd row if applicable.



  • ASME B18.6.3 - 2003, Machine Screws and Machine Screw Nuts