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According to Phil Irving, the Australian engineer who worked at Vincents on several occasions during their heyday and was involved in the design and building of the pre-war series "A" twin and then post-war with the Black Shadow twin, Smiths Industries were approached to build a speedometer comensurate with the expected performance of the new to be introduced Black Shadow model at the 1948 November Earles Court motorcycle show in London.
A hastily cobbled up 5" instrument based on a car model served the purpose for the show and then Smiths came up with a cast aluminium bodied, lustriously black enamelled 5" speedometer with a chronometric movement ( the same as the current 3"/80mm versions) and a scale of 150mph or 250kph for European and metric markets.
I'm not a Vincent man, so can't tell you how many Black Shadows they built, which would give you an idea of how many 5" speedometers Smiths supplied, but the demand continues, and in the years I was in the instrument business I must have build over 120, and continue to build the occasional one.
Vincents never supplied the 5" tachometer and I believe these came due to a demand after the Vincent factory closed its doors. They were likely made up by the London firm, run by Joe Shaw...Auto Tempo Instruments...but that's just my guess....
I don't intend to intimately describe how to build such a 5" speedometer or tachometer, but I will, following, set out in photographs the items you would need.
One sheet of text gives a description of the chronometric instrument and this coupled with some drawings will lead those with a technical bent on a voyage of discovery as to how these facinating instruments work....
My acknowledgments to Smiths Industries and or its decendants for some of the drawings and descriptions and to others I may have inadvertently overlooked
...the photos are my own.
Read on....
Left click on images to enlarge.....



























































































As I did the original blog late on a Sunday night, I added some more items this Monday morning.... these are comment by Phil Irving from his Autobiography, page 365 and three pages from the Smiths Equipment Schedules for Vincent Motorcycles......


















The publication "Motor Cycling in Australia" did an impression of one of the first Black Shadow Vincents sent to the Queensland Distributors and published it in their Dec.1948 edition. Below is a photo from the article which shows the prototype speedo. Obviously the first few made had these before Smiths Motor Accessories got into production.






























































A selection of dials that I made available...shown in reversed image for clarity....