I have standardised on Metric Sizes (M5, M6, M8 & M10 Sizes) as this reduces my investment in the necessary Drills and Taps and at the same time makes sourcing Screws and Fixings that much easier.

It is important if you are to achieve a good fixing that the correct size Drill is used at all times to ensure that the Thread Profile is of the right proportions and therefore strength, the table below gives the relevant data for the more common Thread Types.

Drills are generally available in 4 Size Ranges:-

• Inch (Fractions)

• Gauge (Number)

• Gauge (Letter)

• Metric (mm)

The difference in size between Inch Calibrated Drills is too wide for use in Thread Sizing and therefore we have both Gauge Number and Letter Drills which bridge the gaps and we also have Metric Drills calibrated in mm, it will be very rare to achieve an exact match for the above required Drills in every case and therefore the nearest size within reason will always have to be used.

Although Metric Drills are available in steps of 0.10mm they are difficult to obtain from Commercial Outlets and the more common sizes only are included in the table below.

For instance if we take the 2BA Thread above a 4mm Drill is a common Size but the 4.85mm Clearance Drill Size is not, so you will need to use one of the following for Tapping and Clearance Drill.

The Table below gives the most common Drill Sizes available and the equivalent Decimal Dimension in Inches:-

The Table below gives the Decimal Inch Conversion for the various sizes of Metric Drills shown in the Tapping Drill Size tables above, this will enable you to select the nearest equivalent Drill Size for your requirements. Standard Sizes which are readily available are Highlighted.