Incorrect or correct diameter of screw?

[MoonMan2]

I require to drill screws into wall board (to hold a cistern in place) and believe 4.5x40mm screws will be sufficient (with wall plugs). This is the screws and wall plugs I have.

I draw your attention (circled in red) to the specified diameter of screw on the instructions of the pack.

I measured the diameter of the screw and took this photo.

As you can see, using a ruler, with each black vertical line representing a mm, that the diameter of the screw is in fact 4mm.

Is this normal practice to overstate the diameter of a screw or is this a mistake? The reason why I ask is I wish to purchase stainless steel screws to use with these wall plugs but am unsure of whether to buy 4mm or 4.5mm diameter screws?

 

[big-all]

its the shank size at the head end, iff its 4.5mm the actual head will tend to be double at 9mm

 

[foxhole]

Plastic cistern ? Seem short for a porcelain one.

 

[MoonMan2]

Just for clarity, here is the unmarked and enlarged diagram of the screw in question, that claims to be 4.5mm but I measure (see photo above) to be 4mm.

 

[big-all]

as i say you measure the other end it will be shown that way for clarity

in general plugs will put up easilly with a screw plus or minus 0.5mm even + or- 1mm iff not excessive load

 

[MoonMan2]

Here is photo of 'shank end'?

It's still looking like 4mm.

Anyhow, I take your advice that the wall plug will take a 4.5mm screw even though the wall plugs came in a pack with 4mm screws (claiming to be 4.5mm).

 

[endecotp]

I require to drill screws into wall board (to hold a cistern in place) and believe 4.5x40mm screws will be sufficient (with wall plugs).

Plasterboard? With regular plugs, not plasterboard fittings, and not into a stud? To support a cistern? Hmmmmm.

 

[Harry Bloomfield]

Plasterboard? With regular plugs, not plasterboard fittings, and not into a stud? To support a cistern? Hmmmmm.

It needs more support than offered by plasterboard, besides, the instant the PB gets moisture on it, it will let go...

Find where the studs are located, fix a ply panel to those, spanning at least two, then your cistern to the ply.

If the PB is dot and dab, then drill through the PB and fix to the brick/block behind it.

 

[MoonMan2]

The cistern rests on the toilet pan so should be supported there? Aren't the cistern screws at the back more to hold it in position than to provide support? I mean the cistern screws won't be taking any of the weight of the cistern?

 

[endecotp]

The cistern rests on the toilet pan so should be supported there? Aren't the cistern screws at the back more to hold it in position than to provide support? I mean the cistern screws won't be taking any of the weight of the cistern?

Yes, but…

Do the plugs you have say they are suitable for use in plasterboard? Personally I would use this type, or stronger:

https://www.screwfix.com/p/fischer-plasterboard-wall-plugs-hdf-10mm-x-28mm-100-pack/58219

 

[flameport]

As you can see, using a ruler, with each black vertical line representing a mm, that the diameter of the screw is in fact 4mm.

The screw is more than 4mm, even compared to that crude measuring device.

Left red line to the green line is 4mm. Screw extends to the second red line, which is pretty near 4.5mm

In any event, the actual dimensions are irrelevant.
What matters is that the screw and plug result in a secure fixing in the wall you have.

Suggest making a trial hole in the centre of the wall behind the cistern to confirm - if the screw and plug holds there, fix the cistern in the same way.
If not, then use other screws/plugs. In any event, the trial fixing will be hidden once the cistern is in place.