"John Edgar" wrote in message
...
I simply do not understand the science of drilling holes in walls and
then getting the right-sized plug to fit. This morning I drilled the
first hole for a towel rail, and fitted the plug - no problem. Then I
drilled the second hole - same drill bit for the same sized plug.
However, when I came to insert the plug, would it go in? No it would
not. Why does this happen? How do you match the drill bit, plug and
screw? The usual answer is that you look for the numbers given on the
plugs/screws/bits. But what if these numbers are not present? How do
you do it by eye, which would be more convenient anyway.

Any answers appreciated.
In limine sapientiae


There at at least two different ways of specifying masonry drill sizes. In
the old days, you would buy a No. 8 or No. 10 bit (say). This would drill
the right sized hole for a fibre rawlplug for the same number of screw size.
Inn those days, you needed exactly the right rawlplug for each size of
screw.

But that has all changed withe plastic plugs - which are usually designed to
take a range of screw sizes. The plugs usually come on a plastic strip - and
you tear off each one as you use it. On the back of the strip, it usually
specifies the screw sizes for which the plugs are suitable *and* the drill
size to use. This may be a No. X drill, but is more likely nowadays to be
specified in mm - e.g. 6mm.

To come to your first question, it depends to some extent on the material
which you are drilling. In soft breeze block - or if you hit a mortar
joint - a plastic plug will usually go into a slightly smaller hole, because
it will enlarge the hole slightly as you tap it in. In brick - particularly
very hard brick - there is no give, and the plug will buckle if you try to
hammer it into too small a hole.

In hard materials, I drill the hole size specified on the plug strip. In
soft materials, I tend to use one size smaller in order to get a firm fix.
Try to use the smallest hole you can get away with. [As a rough guide, if no
drill size is specified, measure the diameter of the plug (just below the
flange) with a vernier caliper and use a drill size 1mm smaller.
Alternatively, hold the plug against the working end of a few masonry bits,
and select a bit which is just slightly smaller than the plug.]

HTH,
Roger