Screws Explained:

Screw Gauge, Metric, Size, Clearance Holes, Pilot Holes & Wall Plugs

Which one to choose?

I’ve explained this screw data in an easy to understand table because I find manufacturers recommendations are often vague and general, I mean; why not be specific? I understand there’s a ‘range’ but why not give us the ‘ideal’ measurement, because mostly we have a choice?

The table below shows you:

• Metric equivalents to imperial gauge.
• What size pilot hole to drill in timber substrates for the best screw grip.
• What size clearance hole to drill through the timber being fixed.
• Which size plastic wall plug to use with each screw size or gauge.

Righty-ho, lets jump right in…. (you can download a free PDF of this table at the bottom)

Gauge

Metric equiv.

Pilot hole

Clearance hole

Masonry plug size

3 2.5mm 1.0mm 3.0mm Yellow (5mm hole) 4 3.0mm 1.5mm 3.5mm Yellow (5mm hole) 6 3.5mm 2.0mm 4.0mm Red (6mm hole) 8 4.0mm 2.5mm 4.5mm Red (6mm hole) or Brown (7mm hole) 10 5.0mm 3.0mm 5.5mm Brown (7mm hole) 12 5.5mm 3.5mm 6.0mm Brown (7mm hole) 14 6.5mm 4.0mm 7.0mm Blue (10mm hole)

(click to download and print the screws explained table as a PDF to keep for reference)

Pilot holes and clearance holes in more detail…

(I got a few emails about this….. :-))

If you’re not sure what all these holes do; here’s a brief explanation….

How tight is tight?

The pilot hole recommendations in the chart are typical for softwoods. Hardwoods may well need a slightly larger pilot hole. If the screw starts to get too tight, then it’s advisable to back out and re-drill a larger pilot hole rather than risk breaking a screw (you’ll be having a very bad day if you snap a screw…..).

In fact, if any screw starts to get very tight, it’s best to stop and have a rethink before you break the screw or burst the material. Screws should be tight, but not crazy, “sweating to get them in” tight! Simply hit reverse and take it out.

Then you’ll need to consider whether to use a smaller gauge screw or drill a larger pilot hole if it’s in wood. In masonry you always have the option to re-drill the hole one size bigger (drilling through the existing wall plug) and use a bigger plug, red up to brown for example.

General Tips

Incidentally, gauge relates to the thickness of a screw’s shank or body and only relates to imperial sized screws. Metric screws are simply measured and the thickness is given in millimeters. The metric screw equivalent size is approximate, but since we’re heading that way, we need to learn and get used to them!

By coincidence, the size of an imperial screw head also roughly equates to the gauge. For example an 8-gauge screw has an 8mm wide head…..handy huh?

How deep does a wall plug need to be? I talked about this a lot on another post, here’s the link: How to Properly Use Wall Plugs

Parting note about wall plugs: Be aware that not everyone follows the colour ‘rules’, one leading brands plugs are all grey for example, and others are, well; they can be any colour under the sun (as the above photo of my box shows). Blue wall plugs are another example, they can be either 8mm or 10mm, go figure! If in doubt always measure the plug at the thicker end to make sure it’s right for the size hole you have drilled.

Hope that helps you sort out your screw dilemmas. Feel free to send this to anyone you know who needs their ‘screws explained’ lol! And don’t forget to let me know how you get on. Incidentally if you want to help support this site, grab your… screws etc. from ebay… Ta muchly!

Stay well

Ian

Originally published at handycrowd.com.