Bolt Heads

Anybody know why bolt heads are either square or hexagonal? I recall reading that some older English cars (Rolls-Royce) had octagonal-headed bolts. I would think that the hex shape is logical, since it has a regular shape, but why not pentagonal heads?

A pentagonal is an irregular shape. You could only position the wrench one way. In a tight spot you’d be doomed.

There is a regular pentagon.

All sides equal length. All interior angles 108 degrees.

There should be five positions for the wrench.

WAG as to why no pentagons is that early designers sought a compromise between maximizing the number of grip positions and minimizing slipping of the wrench.

My geometry teacher said that the reason fire hydrants have pentagonal bolt heads is that they figure no one is going to have a pentagonal spanner. So by keeping the bolt head shape so rare, they ensure that only the right people (the city water folks) will have the “key”.

As to why pentagonal is less popular, he said it was because 108 degrees is a little harder to make, perfectly. A regular hexagon has nice 120 degree angles which can be machined more easily. I don’t know why this is, I mean, the folks who make wrenches must have protractors…? But I wasn’t one to question the geometry teacher. Maybe it has something to do with the fact that a regular hexagon has three pairs of parallel sides, while a regular pentagon doesn’t have any parallel sides.

The first one is that for a really definitive answer, you should probably have asked this over in GQ.

But, let me give it a go. Pentagons don’t work for general purpose bolt heads because of the lack of parallel sides. For things like sockets and box-end wrenches this doesn’t mean much, but for an open end wrench you have to have parallel sides. Think about a joint on a pipe where there is no way to get a closed-in type wrench on it. Also, in many tight places, the only approach to the bolt head is straight at it from the side.

I think that the square headed bolt gives the greatest surface area between the head and the wrench, plus was easier to machine before modern methods were in use. But it lacks a lot in flexibility. In tight quarters you have to turn a full 90[sup]o[/sup] to be able to get the wrench back on at the same angle. Even with the wrench head offset, you still had to turn it 45[sup]o[/sup]. When you get up the 8 sides, I think you loose too much surface area to be able to put adequate force on the bolt with an open-end wrench.

Once you reach a compromise for the use of an open-end wrench, all the closed-in type like box-end and sockets, plus the inside type like allen and torq just follow suit.

That’s the way I see it anyhow.