James May column - May and the machine - 2010

I had to buy a new brake caliper for one of my cars the other day. I ordered it online, it came in the post, I took it to the garage and it went straight on with a few bolts.

Nothing remarkable about that. The car is an old 911, the caliper was for a 911 of the same year, so obviously it was going to fit.

But at the same time, it's a matter for extreme wonderment. At least, I think it is, and I think you should too, because if it weren't true, none of us but the very rich would own cars.

Anyone who has ever done any proper metalwork will know that, theoretically, a 10mm diameter rod won't go into a 10mm diameter hole. Either the hole must be a bit bigger or the rod a bit smaller, or maybe a bit of both. But by how much? Fettle away a bit at each part and eventually they'll go together.

In reality, of course, nothing can be exactly 10mm; it will always be 10mm plus or minus something. So your hole might in reality be 10.03mm, and your rod might be 9.92mm, so they will  go together. But that may be the other way around, and then they won't.

So what if you wanted to make 1,000 rods and drill 1,000 holes, such that any rod would go in any hole? You'd need a system of controlling maximum and minimum dimensions for each, which would be dictated by the fidelity of your manufacturing and measuring methods. This is a set of what we call ‘tolerances' or ‘allowances' or, in the lexicon of the engineering workshop, ‘limits and fits'.

The tolerances for something like a tin-opener can be pretty sloppy and it'll still work. Those for something like a fuel-injection pump have to be very tight if it is to have a hope in hell of doing its job. The former might be measured in tenths of a millimetre if it's a very posh tin-opener, the latter in microns.

"That something as complex as a car can be owned by ordinary people is, I think, one of the greatest achievements of humanity"

I love this stuff.

Working to limits and fits liberates the manufacturing process. Instead of one fitter having to make sets of rods and holes work together, a man in Shanghai can make all the rods while a bloke in Botswana drills all the holes, in the certain knowledge that any two will work together. This is the bed-rock of mass production as far as machines are concerned, if not chicken and mushroom pies.

It's a pretty simple idea, but it took a long time to sort out. The origins of limits and fits are assigned to the clock-making and gunsmithing businesses, and you can see why they'd be interested, because both these things were required in huge numbers and would break readily - the hammers on guns would shear, and the pallets in the escapements of clockwork mechanisms would wear out.

Prior to the culture of interchangeability, a new hammer for your revolver would have to be made to fit, usually with a bit of filing. The mass-produced hammer would go straight on. Job done.

The earliest cars were made like flintlock pistols, and were machines with no true commonality, even if they looked the same. Each part had to be made to fit by a, well, ‘fitter', who would remove bits of metal like the man in our original rod-and-hole example. These cars were hideously expensive to buy and hideously expensive to maintain.

It was Henry Leland of Cadillac who first demonstrated that the principles of limits and fits could be applied to internal combustion engines and other car parts, and that the business of making cars go together could be sorted out at the point of the components' manufacture, rather than at the point of assembly. Henry Ford (the two knew each other) could then develop his production line and make cars of increasing integrity and reliability while continuously reducing the price. Yippee!

This is why it annoys me to hear people whining on about how expensive cars are. In reality, they've been coming down in price ever since. The amount of engineering and manufacturing expertise in even the most basic car is almost too much to be comprehended by one person. I reckon the volume of experience and knowledge that goes into making one helical gear from a gearbox is equal to that employed in the building of a whole house, which is a mediaeval process by comparison.

That something as complex as a car can be owned by ordinary people is, I think, one of the greatest achievements of humanity. It can be attributed to improved standards of living and the relative price of Mars Bars, but it's mainly because the caliper fits the 911.

Bloody marvellous, isn't it?