Coach bolts, carriage bolts, coach screws etc.

A coach bolt normally has a square under the dome to prevent it turning when the nut is tightened.

Bolts are only threaded part-way along the shank. If they are threaded all the way along, they're a machine screw. A man in a brown coat patiently explained this to me in a splendid little hardware shop about 3 decades ago.

In message , "Dave Plowman (News)" writes

Indeed, but is a bolt with a hex head and therefore no square/dome still a coach bolt?

In message , Huge writes

I agree, BUT some 'coach bolts' are threaded the full length of the shaft, yet retain the domed head with square section below the head. They're not machine screws? Would it be very broadly correct to say machine screws generally have a finer thread than coach bolts?

Why is a machine screw called a screw when it is a bolt?

Because it isn't a bolt.

Is that all? I've been wondering about that --- thanks.

You met Ronnie Corbett?!?

OK. I have always assumed that anything with a parallel shaft or thread is a bolt, but anything with a tapered shaft and thread is a screw, but apparently not.

I think Huge's man in a brown coat is correct. Back in the days when Britai n supplied the empire with fixings and thus set the standard, a bolt had a parallel thread for a portion of its shaft with a hex head. Coach bolts wer e also partially threaded with domed heads and a square section underneath to prevent turning. Coach screws had a tapered thread and in the past had a square head although these days hex heads are available. Machine screws ha ve parallel threads down the full length of the shaft and can have any kind of head from slotted to hex head and anything inbetweeen. Now that these t hings are sourced all over the world manufacturers seem to apply their on t erminology which adds to the confusion.

Richard

Not sure. One which has no means of holding the domed head might be considered a liability. ;-)

Huge's definition is certainly the one I remember - a bolt isn't threaded all the way. But just when a machine screw becomes just a screw I dunno.

Bolt probably comes from the same root as a bolt for the door. It locates the two parts due to being a good fit before being tightened.

Not a shop with a counter 3 score years ago?

The (more interesting) question that popped into my mind was:-

"Did you happen to bump into Ronnie Barker by any chance?" :-)

A bolt is a screw if screwed into something rather than having a nut attached.

Not so - an obvious example being car cylinder head bolts. Which screw into the block in most cases.

Untrue.

This is a bolt;

And this is a screw (despite what the URL says!);

No, it isn't.

A bolt is a part threaded machine screw, A machine screw is designed to attach into a pre threaded hole, possibly in a nut. Both have constant diameter and thread.

A Woodscrew is self cutting and so are self tappers for metal sheet and plastic.