True surfaces

Motorcycle enthusiasts may, for example, need to check the relative trueness of sliding surfaces such as carburettor slides, or check how flat mating surfaces, such as crankcase halves, are.

Going back in engineering history, workers who wanted precise, flat surfaces would use a three plate (item) method and polishing, by checking or polishing two flat surfaces against each other then regularly and randomly rotating the pair of surfaces with the third (idle) surface, a reasonable degree of accuracy was achieved.

During 1830-1840, (Sir) Joseph Whitworth (1803-1887), of British Standard Whitworth threads and Whitworth rifle, fame achieved greater accuracy of flat surfaces by again using a three surface approach with scraping methods and ‘Engineer’s Blue.’ The Whitworth techniques were well publicised in period and adopted by many engineers.

From this start, the

Whitworth techniques have been adapted for a vast number of engineering roles, from the above mentioned carburettor slide to the matching of mould cores to cavities and far more. Often Engineer’s

Blue is used, but in some instances, different coloured preparations are employed to suit the task and the colours of materials involved.

Commonly, Engineer’s Blue is a blend of Prussian Blue pigment with a non-drying thick liquid or soft solid, such as oil or soft grease. But there are many variations of the Engineer’s

Blue we might use at room temperature surfaces to those suited to warmer surfaces.

In yet other forms, Prussian

Blue is mixed with fast drying solvents (eg alcohol, such a methanol or methylated spirits) to paint on metal surfaces, which are left to dry. The surface is then precisely marked out with scraping tools for the item’s next engineering process.

Correctly, these fast drying preparations of Prussian

Blue may be called Scraping or Marking Out Blue, but some – including certain literature – annoyingly call these preparations Engineer’s Blue. Confusing, because traditionally Engineer’s Blue is non-drying, while Scraping or Marking Out Blue is usually quick drying. And our current tin of Engineer’s Blue is called Engineer’s Marking, even though it is the non-drying variety, which can be cleaned off with solvents.

To make matters even worse, engineers from different parts of the world use different terms which don’t always mean the same from country to country. Terms you may encounter in addition to above include Blue

Highlighting, Bluing, Prussian Blue, Blue Matching, Blue It and more… If reading unknown copy from which you are hoping to employ instructions, you need to ascertain in your mind what meaning the author is intending with the employed word or phrase.

A typical example of use is to rub a thin, even layer of Engineer’s Blue onto a known flat surface, such as a surface plate, then place, with pressure, the subject surface on the ‘blued surface’ or rub the work against the blued surface. If the work’s entire surface evenly picks up the ‘blue’ then it is highly likely it’s true, but if the ‘blue’ is picked up by only the high points of the work, the surface isn’t true. This example is a good starting point when checking mating surfaces such as crankcases, timing case covers, carburettor flange fit to manifold and the like. If one wants to check a round surface, then roll it over the blued surface plate, and, again, highlights will pick up the blue, and scores with be visible against a blue background.

Although it won’t often confirm an item such as carburettor slide is round, it will indicate low points and scores indicating wear and damage. In the workshop, we’ve just hit a problem with a sticking slide in the carburettor of a 1907 LMC. The slide rotates in the body and in this instance sticks at about a half open position. With the naked eye, we cannot see any high points or other cause and as the carburettor has been cleaned in an industrial sonicating tank, we know adhesive deposits aren’t the cause.

Another favourite ‘non intended’ use of Engineer’s

Blue is gasket making. Apply a modest coat of engineers blue to the mating surface and press on gasket paper to mark it. Punch holes for fasteners, dowel pegs and the like, and then cut out the outline. Clean gasket with solvent and dry before fitting.

A 1908 Brown and Barlow catalogue sketch of the instrumenting under examination. No. 7 is its slide and although there is no elbow in the subject carburettor, it was snagging at a point corresponding directly above the No. 7 and marker line tip. We’d missed this on assembly, but the Engineer’s Blue highlighted it.

What we hadn’t realised was there was a high point in the carburettor body which was snagging the slide as it rotated. Once the slide was true, it was blued and rotated in its carburettor body. The score marks in the blue indicate it is being snagged and the ‘missing’ blue marked the high point in the body. Closer examination with a magnifying glass revealed there was a small raised area near the outlet opening, caused perhaps by a past damaged slide. Once relieved, the slide rotated smoothly in the body.

A suspected warped mating face is easily checked by spreading a thin even (well, relatively even in my case…) layer of Engineer’s Blue on a true surface plate, then scrape the mating face across the Blued surface plate. The pigmented Engineer’s Blue is again picked up on the high points as photographed, while the low points remain clean. We can now decide if its gasket will take up the warp, or if the mating face can be made good with grinding paste on true surfaces such as plate glass, or we need to bring in the big guns and mill it true.

Rolling the Brown and Barlow slide on a blued surface plate clearly marks the high points (unworn areas in this case) which pick up the Engineer’s Blue while the low worn don’t. This corresponds to a large extent to the score marks on the slide. Arguably we could see these without the bluing, but it makes an example.

Left and below: Many use grease or paint to mark gasket paper. I often avoid paint because I don’t want it on casings and grease doesn’t leave that distinct impression. According to the younger person in the workshop this has more to do with my eyesight than the impression grease leaves! One’s children can be cruel at times, but of course we can always return the compliment… The move to Engineer’s Blue has aided my gasket making ability as photographed here and I will ignore the ‘humour’ of son Peter. And as you can imagine, I can give as good as I get!