What's the purpose of 5.5mm concrete drill bits?

Question

In (continental) Europe, all the plugs I've seen are either 5 or 6mm. So what's the point of 5.5mm drill bits, most commonly found in SDS-plus version? Is it to drill in crappy material and/or with a crappy drill/rotohammer where the 6mm hole would end up larger than expected if drilled with a 6mm drill?

I'm probably correct since some "economy" plugs, e.g. those with EAN 7332418018425, have a suggested drill hole of 5.5mm or 6mm or both (depending where you find them).

The 5.5mm might be UK-centric thing, since Bosch heavily sell sets which include that size in the UK (e.g. Bosch p/n 2607010546 has 160 reviews on amazon.uk) but it's barely sold in Germany (on Amazon, where in only has 16 reviews); instead the equivalent set with two 6mm bits is sold in Germany a lot more (Bosch p/n 2607010543 has 192 reviews on amazon.de... and a single one on amazon.uk). The former [UK-popular] Bosch set also includes a 7mm bit, for which I can find plugs on screwfix, but I've never seen them used [the 7mm plugs] in continental EU.

Answer 1

Simply, they're for 5.5mm plugs.

Red screw plugs are normally either 5.5 or 6mm, it's not the case that the manufacturer is advising a smaller drill size to account for poor quality.
5.5mm tend to be a little cheaper though as they're normally made by companies that sell only to tradespeople in boxes of 1000 without spending on packaging or marketing.
Fwiw, 5.5mm plugs in a 6mm hole will do quite a good job...which is good because 5.5mm bits are relatively rare.

It's a bit of a discrepancy in (uk) standards...in that there really isn't a standard. The UK isn't very good at adopting standards in the construction trade.

Want some blue plugs? The 8mm or 10mm ones?
Want 7mm plugs? Brown or grey?
Want some 40mm waste pipe? Sure, how big is it? 41mm, we wouldn't want to be the same as the other guys.
Want to wire a house? Can I interest you in a ring main?

Incidentally, 7mm are pretty common but not nearly as popular as 6 or 8mm.

Answer 2

5.5mm is the old 7/32 drill bit, so usual for UK/US. I find it suitable for most jobs using the 'red' plugs. If I drill a (recommended) 6mm hole for red plugs, they can be too loose in render, cement sheet and old concrete.