Self Tapping Screws - Design Guide Considerations

Steel self tapping screws are primarily designed to be installed in metals which allow sufficient plastic deformation, enabling the thread to be formed without the removal of any material.  In some cases they can be installed into certain plastics, although it is not recommeneded without investigation and performance evaluation.  

If you are considering or using self tapping screws in any plastic parent material, we recommend reviewing our purpose-designed threadforming screws for plastics range of Polymate screws and our design guide article.  

The scope and specification of this product family includes an array of head, drive and thread form styles. This document summarises the range and preferred method of application in accordance with the specification laid down by the British Standards Insitution BS4174: September 1972.   

The following table summarises the screw type and designation.

Thread typeScrew  typeType designation 
Thread forming.  Widely spaced pitch and the point is often referred to as a gimlet (or pointed). Their preferred use is in light sheet metal and resin impregnated plywood materials.  Also  recognised under the  DIN7971 Form C designation (ISO 1481)  Click [here] to access the product specification data for this item.AB
Thread forming  These screws have the same characteristics as  type AB, but with a blunt point.  Also  recognised under the  DIN7971 Form F designation (ISO 1481)  Click [here] to access the product specification data for this item.B (formerly Z) 
Thread cuttingT formerly 23)
Thread cuttingY
Thread cuttingD (formerly 1) 
Thread cuttingBT (formerly 25)
Thread forming  This type has been declared obsolete and  BS4174 recognises this, recommending type AB instead.A (use type AB)


Table 1 illustrates industry standard self tapper size options most commonly stocked and used commercially, indicated with a shaded background. The gauge is frequently used to identify and specify the screw diameter in the format No.2 x 1/4 long.

Table 1
GaugeInches1/83/161/45/163/81/25/83/41™1 1/41 1/21 3/42™2 1/2
(Metric Dia)mm3.
4  XXXXXX      
(2.9)  XXXXXX      
6    XXXXXXX   
(3.5)    XXXXXXX   
8    XXXXXXX   
(4.2)    XXXXXXX   
10     XXXXXXXX 
(4.8)     XXXXXXXX 
12        XXX   
(5.5)        XXX   
14          X   
(6.3)          X   

Note: Due to the fact that fastening applications differ greatly, the above information is for guidance only and is correct to the best of our knowledge. The customer must satisfy themselves with the performance of the fastener and validity of the data. TR Fastenings will not be held responsible for any failure that may occur from the use of this information.