Phillips (Type I cross recessed) flat countersunk head machine screw dimensions are shown in the following chart according to ASME B18.6.3 standard.

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 Phillips Flat Countersunk Head Machine Screws
(Note 5)
Height (H)
Gaging Depth
Above Gaging
Diameter, F
Dia., G
max. min. ref. max. min. M, ref. T, ref N, ref max. min. max. min.
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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.


Gages and Gaging : Specifications and dimensions for the gages used on Unified inch screw thread is defined in the ANSI/ASME B1.2-1983 standard. Types of gages including thread plug gages, snap gages, thread ring gages and plain diameter gages for internal and external threads are covered in same standard. Screw thread gaging systems for acceptability is covered in the ASME B1.3-2007 standard.

Details of gages and gaging about the machine screws heads are given in the appendices of the ASME B18.6.3-2003 standard.

Unified Screw Thread: A thread form used by the United Kingdom, Canada, and the United States to obtain screw thread interchangeability among these three nations. It has 60° thread angle and dimensions are stated in inch units.

UN form screw threads: The UN thread is intended for general purpose fastening operations.

UNC (Unified National Coarse): Most commonly used type for general engineering applications. This thread form is used in materials with low tensile strength which makes threads more resistance against stripping (internal threads) .UNC give possibility for quick assembly.

UNF (Unified National Fine): External threads of this Fine Series have greater tensile stress area than comparable sizes of the Coarse series. The Fine series is suitable when the resistance to stripping of both external and mating internal threads equals or exceeds the tensile load carrying capacity of the externally threaded member. It is also used where the length of engagement is short, where a smaller lead angle is desired, where the wall thickness demands a fine pitch, or where finer adjustment is needed.

Penetration Gaging: Penetration gaging is a test to determine the suitability of recesses in the heads of screws. Penetrations that are too deep indicate the possibility of a thin section between head and shank of screw, a weakness that might result in twisting-off screw heads during tightening of the screws. Screws with shallow penetration might result in production problems such as reaming of recess or excessive wear on driver bits.

Thread class: A classification system to classify the threads for interchangeability and manufacturability. Class 1 threads are loosely fitting thread and used for ease of assembly. Most commonly used class is 2 and this class is designed for general use. Class 3 is used for closer tolerances.

Wobble Gaging: Means to determine the compatibility of recesses in the heads of screws with mating screw drivers, and will indicate the point where deviations in the recess contours affect satisfactory driver engagement. Recesses that exhibit excessive wobble characteristics will result in poor screw driveability because of driver camout prior to have normal torque level, damage to recesses, and/or accelerated driver wear.


Link Usage
Unified Inch Screw Thread Sizes & Tolerances Calculator Calculates basic major, minor and pitch diameters of the external (bolt) and internal (nut) inch thread according to ASME B1.1-2003 standard. In addition to basic size calculations, limits of sizes calculations of different tolerance classes can be done according to same standard.
Dimensions For No.0000, No.000 and No.00 Threads Dimensions for unified thread sizes of No.0000, No.000 and No.00
General Data on ASME Machine Screws General data on ASME machine screws

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