Measure of perfection: Bentley’s in-house metrology team under the microscope

Deep in the heart of the Bentley factory in Crewe is a large, airy and air-conditioned workshop that visitors, even out of a government lockdown, will never see. It’s packed with the kind of precision instruments you’d expect to find in a space agency facility or university science lab. It’s here that Head of Metrology Michael Stockdale and his 25 colleagues measure every part of a Bentley to the highest standards of precision.

Metrology is the science of measurement, and it’s fundamental to the quality, performance and longevity of every Bentley that each component is made to consistently precise dimensions. Stockdale and his colleagues can measure every part of each model Bentley makes, from the smallest washer to body panels and interior trim, and ensure that no component strays from the close tolerances prescribed for it. “We have the tools to measure everything from the graining of leather to the surface of a cylinder bore, down to fractions of a micron” explains Stockdale.

Having measured components individually, they are measured again as sub-assemblies and once more as part of the finished vehicle. Thus, the Metrology team plays a vital role in ensuring that near perfection is repeatable.

The precise choreography of the Flying B

Measuring and enforcing the highest standards of dimensional accuracy is especially important for a vehicle, where multiple components become sub-assemblies. To take just one example, the retractable Flying B mascot that adorns the bonnet of the Flying Spur, which smoothly deploys and retracts only because each component in its complex control assembly is made to close and consistent dimensions. It is illuminated and linked to the keyless entry system as the driver approaches the car, yet it must also retract automatically in the event of an accident. To achieve this precise choreography and to ensure that it sits perfectly centred within its plinth, elements of the Flying B system have tolerances as low as 0.15 millimetres.

Tolerances finer than a single red blood cell

Laymen use ‘a hair’s breadth’ as a description of the tiniest imaginable measure, but such a term would be too imprecise for Bentley’s Metrology team. As Stockdale points out, a human hair can be anything from seventeen microns to over one hundred and fifty microns thick. In contrast, there are instruments within Metrology that can measure down to 0.5 microns.

A micron is one millionth of a metre, and a human red blood cell is 5 microns in diameter. Not every component of a Bentley needs to be measured to tolerances of less than a micron, but there are some.

As an example, Stockdale cites the crankshaft at the heart of Bentley’s 6.0-litre W12 engine, the most advanced 12-cylinder engine in the world which powers the new Flying Spur. Spinning at up to 6,000 rpm, the crankshaft converts the immense downward forces generated by the pistons into rotary movement that powers the wheels. Though invisible to the naked eye, each of the twelve machined bearing journal surfaces in which the crankshaft sits features minute grooves that hold a microscopic film of oil.

By using a high accuracy Perthometer (a tool designed to measure surface finishes) the Metrology team can verify that those minute grooves are within their defined tolerances, in turn ensuring that each W12 engine produces both the immense power and lifelong durability its owners would expect.