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Self Tapping Screws - recommended hole and drill sizes

Menu of Self Tapping Screws

Type B Self-Tapping Screws hexagon head in structural steel

It is important that the correct hole size is used and the recommendations below should be followed, but if very hard material is being used a hole size slightly larger may have to be used, and in very soft material a smaller hole may be necessary. If any difficulty is encountered send a sample of your application to us and we shall be pleased to carry out tests to determine the optimum screw type, gauge and pilot hole.

recommended hole and drill sizes

Type B Self-Tapping Screws

in non ferrous castings or sections, aluminium, magnesium, zinc, brass, bronze etc.


  1. Cored holes. A side taper of 1°11' is permissible. The diameter of a cored hole should equal the nominal hole size shown in the table to the right at one half the screw penetration depth.
  2. Porous castings may require the use of a smaller hole and/or increased depth of engagement.
  3. 18-8 quality stainless steel self-tapping screws are much softer than case hardened steel screws and therefore care must be exercised in using them. They cannot be used in very hard material.

Type Y Self-Tapping Screws

in thermo-setting (hard) plastics and cast iron


  1. Because conditions differ widely it may be necessary to vary the hole size to suit a particular application.
  2. Type ‘Y’ screws are not generally suitable in materials other than those listed in the table to right.

Type AB, B Self-Tapping Screws in thermo (pliable) plastics

Because of the vast differences in these plastics, the following table is intended only as a guideline. It may be necessary to increase or decrease the recommended hole size to obtain optimum fastening conditions.

Type AB, B Self-Tapping Screws in thermo (pliable) plastics

Type ‘AB’
Fig. 2. Firstly make a hole to suit the diameter of screw being used. The hole can be clean-punched or drilled as shown in Fig. 1 or plunged as shown in Fig. 2.
Type ‘B’
Fig. 5. For light sheet metal assembly, no clearance hole is necessary in the top sheet.
Figs. 6 & 7. When fastening to solid section of casting or die-casting, or to heavy gauge sheet metal, a clearance hole in the top piece is required. If a clearance hole is not provided, the total thickness of the material must be taken into account when deciding upon the tapping hole size. Remember that as the thickness of material increases, it becomes more difficult to pull the two sheets together tightly.

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