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I've heard that if using an SDS drill I should go a size down for drilling for rawlplugs.
I want to use red rawlplugs which source vary on saying I either need a 5.5 or 6mm hole for. I am based in the UK if there if is any difference in the colours.
What size SDS bit should I use to drill a suitable hole.
Thanks
5.5 or 6mm sounds about right for a red plug, however not all reds are created equal, drill bits also vary and the material you are drilling into makes a difference as well.
If I'm drilling into a softer material like plaster (especially old plaster) I will always start with a slightly smaller bit than called for in the instructions, even 1mm less, this is because softer materials can sometimes allow the bit to drift and you end up with a larger hole than the bit. Even with harder materials like brick I will also go slightly smaller, .5mm less, than called for just in case my bit takes out more material than expected. I drill, then I try to hammer the plug in, then I will step up a size if needs be. I'd rather have a really snug fit and trim a bit of the plug off at the wall then have one that goes in but is slightly loose.
With SDS bits I would use the same approach, how much depends on the bits and how aggressive the hammer drill is (presuming you are using one). If it's brick or concrete you can start off with a 5mm and work your way up.
Both 5.5 and 6mm plugs are available and both are red.
Rawlplug brand are typically 6mm.
Annoyingly while 6mm bits are very common and 5.5mm bits are pretty rare, 5.5mm plugs are more readily available.
Does this make a difference?
Only a little, you should follow the instructions for best results but there should be enough wiggle room to make it work.
A 5.5mm plug in a 6mm hole will work unless you're right at the limits (which you shouldn't be anyway) - especially if using 4.5mm instead of 4mm screws.
Ideally plugs should be tight in the hole and it's normal for them to need a gentle tap to get them in, but if you have to swing the hammer there's something wrong.
If they're too loose then they'll spin in the hole when you try to screw into it.
Note on use:
The talk of materials above caught my attention. It seems to be a thing in some places to use plugs for plasterboard - the should strictly only ever be used for masonry (which I assume you are if using a SDS drill).
For render, I use 5mm. For 13mm cement sheet, tiles, old brick or concrete I use 5.5mm. for new brick I use 6mm. 5.5 mm is 7/32, which is readily available. Drill with the smaller size first hole, and if too tight, go up a size for the rest. My red plug pack says suitable for 6mm
