Which size allen key bolt do I need?

[rebeccat]

I need to buy some Allen key-headed (hex I think they're also called) bolts to replace some that are missing from a chair. I already have two the right size in the chair, but I'm confused by what size that is when I look online to buy them. The correct Allen key is labelled CR-V 5mm; the bolt appears to be 5mm in diameter (and 20mm in length under the head which is about 12mm in diameter). Strangely, the online table shows the bolt for a 5mm Allen key to be an M8 which has a head diameter of 17.6mm and that won't fit the socket.

Logically, I need an M5 20mm. Am I right?

 

[Burnerman]

Confusion time!
Allen key sizes are measured across the flats of the hexagon (AF).
For a 5mm threaded screw, the allen key size is actually 4mm AF and for a 6mm threaded screw the allen key size needed is 5mm.
For an 8mm thread, the allen key size is a 6mm hexagon.
So, it's the thread diameter that's important in your case, so ascertain that - and buy an allen key set that will have what you need.
I hope that hasn't confused you, but I will add that allen keys are available in imperial and metric sizes - but metric is almost certainly what you'll need.
5mm threads are pretty small!
John

 

[rebeccat]

Ummm... even more confused. I have the correct Allen key for the bolt. The key has inscribed on it CR-V 5mm. So which bolt is that for, as in which M number is it?

 

[JohnD]

the bolt appears to be 5mm in diameter (and 20mm in length under the head

That is called an M5 20mm hex-headed screw.

It is probably either cheese-headed (shaped like a wheel of cheese) or pan-headed (shaped like a cooking pan) but it might be countersunk (shaped like a funnel)

You do not order screws by the size of the spanner or wrench
(Except sometimes for antique or American machines)

As it is for indoor use I'd get BZP which has modest rust resistance. Grease or oil the thread.

Stainless are available but most unbranded stainless screws are Chinese and deliberately made of low-quality alloy without nickel to keep the price down, and liable to break.

 

[Burnerman]

Ummm... even more confused. I have the correct Allen key for the bolt. The key has inscribed on it CR-V 5mm. So which bolt is that for, as in which M number is it?

An allen key marked 5mm actually fits a 6mm threaded allen screw......of course there may be non standard sizes but that's unlikely.
CrV = chrome vanadium.
John

 

[JohnD]

Maybe the screw diameter needs to be measured again?

Or Rebecca could buy a few of each. They're cheap.

I mostly get such fastenings from Kays on ebay

 

[rebeccat]

An allen key marked 5mm actually fits a 6mm threaded allen screw......of course there may be non standard sizes but that's unlikely.
CrV = chrome vanadium.
John

OK. Which M number is that?

 

[Stuckinarut]

Why not just buy an Allen key set? Iirc 2mm up to 10mm in size.

 

[rebeccat]

Maybe the screw diameter needs to be measured again?

Or Rebecca could buy a few of each. They're cheap.

I mostly get such fastenings from Kays on ebay

That is called an M5 20mm hex-headed screw.

It is probably either cheese-headed (shaped like a wheel of cheese) or pan-headed (shaped like a Wok) but it might be countersunk (shaped like a funnel)

You do not order screws by the size of the spanner or wrench
(Except sometimes for antique or American machines)

As it is for indoor use I'd get BZP which has modest rust resistance. Grease or oil the thread.

Stainless are available but most unbranded stainless screws are Chinese and deliberately made of low-quality alloy without nickel to keep the price down, and liable to break.

M5 20mm was what I thought, only for me to become confused when I read the chart on the Kay's ebay site. So I'll get those (there'll be trouble if they don't fit!)

Thank you.

 

[rebeccat]

Why not just buy an Allen key set? Iirc 2mm up to 10mm in size.

I have the key and two bolts. I need two more bolts but the sizing chart is confusing me.

 

[JohnD]

There'll be trouble if you measured if wrong.

 

[Stuckinarut]

I have the key and two bolts. I need two more bolts but the sizing chart is confusing me.

Maybe take the existing one to a local hardware store if you have one? Could you contact where you got the chair from/maker?

 

[rebeccat]

Maybe take the existing one to a local hardware store if you have one? Could you contact where you got the chair from/maker?

Local hardware store? Sadly, they no longer exist. My town here in north-east England had perhaps four or five when I was young in the 70s. None remains, the last being the Wilko chain that infamously went bust a couple of years ago. Anyway, I'm fairly sure it's an M5 20mm, so I'll order those.

 

[Harry Bloomfield]

I have the key and two bolts. I need two more bolts but the sizing chart is confusing me.

Size of allen key, which fits, is not a guide to the thread size of the bolts. Your best bet is to take the bolt size you need, to a nut and bolt stockist. Even then, you might struggle to match the head of the bolt, some manufacturers of furniture, have bolts specially made for their use.

 

[JohnD]

Local hardware store? Sadly, they no longer exist. My town here in north-east England had perhaps four or five when I was young in the 70s. None remains, the last being the Wilko chain that infamously went bust a couple of years ago. Anyway, I'm fairly sure it's an M5 20mm, so I'll order those.

Get some M6 as well. The cost is trivial and it may save you a second order.