Wood screw specifications

[Pamela]

Despite jars of old screws, I had trouble finding the right one to attach a PIR sensor to a wooden fence so I decided to buy an assortment of small wood screws for the future. Rather than risk some badly made Chinese rubbish from Amazon, I checked out Screwfix but got confused by their spec.

The diameter is for the screw shaft but does that include any protruding thread?

If so then does that mean the normal spec doesn't define the head diameter?

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For example:

[dennis]

It defines the shank diameter. Its normally the size of the clearance hole you should drill in the wood it goes through.

Lidl had some mixed packs in a week ago so you might still get one if you look.

[John Rumm]

Usually they quote the major diameter - so that includes the thread.

Correct. (its normally 2 - 3x the shank diameter)

[newshound]

It's confusing because you still get imperial sizings (length in inches, diameter as 4, 6, 8, 10, 12) and metric ones.

Tradespeople seem to go for top grade: Turbogold, Spax, etc. Personally I find the ordinary Screwfix "quicksilver" and the Toolstation equivalent are fine for most jobs.

You *do* want to stick with pozidrive heads. You have a choice of single and twin-start threads, the latter are perhaps a bit better although I have lots of the other.

Mixed packs are normally all countersunk, sometimes dome headed screws are better for fixings. This claims to have mixed head types

Screwfix has several other small "mixed packs" that give you more for your money.

I almost invariably throw away the fixing screws that are supplied with things which are often just "mild steel".

[dennis]

8<

I disagree torx head are better but cost more.

[newshound]

Perhaps theoretically, and certainly for machine screws into steel but how often do you need the extra strength when going into wood? And most stuff will be PZ2, and three screwdrivers covers everything. Who has a full set of Torx screwdrivers and hex bits in every location?

[Pamela]

I don't need top grade but el-cheapo screws are more trouble than they're worth, so the ranges you suggest look right.

Assorted packs often have to many big screws. I'm unlikely to need anything longer than 30mm and probably have those longer screws anyway.

The Spax link you gave contains too many long screws:

3.5 x 16 (x50) 3.5 x 30 (x30) 4 x 30 (x50) 4 x 35 (x40) 4 x 40 (x30) 4.5 x 50 (x25) 5 x 60 (x20)

Out of interest, how different can the 30mm screws in 3.5mm and 4mm diameter be because there's hardly anything in it.

[Brian Gaff]

I've converted to those thread all the way up screws these days myself. Brian