Well, we are and we aren’t. Ultimately we are still measuring pressure (and the VC-10 trials were focused on pressure, not loading) whether in lbs per square inch or how much water or mercury that can push. It is conventional to measure manifold pressure with a gauge, but the mystery gauge is not conventional in the application as it is marked in m/mH2O, where mmHg is the closest ‘conventional’ measure used in this context, while inches Hg seems far more common (in the US, UK and subservient dominions 😉 . Not saying you’re wrong, I am only a dabbler!

Regarding bearings, the specs for non-engine bearings for late-1930’s British aircraft might include a decimal of an inch, especially where imported bearings were initially specified (and most of them were, initially) but I haven’t seen mm. Thinking about it now it might be interesting to convert these inch decimals (I am thinking specifically of some ‘mystery’ SKF and Hoffman bearings in the Whirlwind parts catalogue) into mm, see if I get whole numbers.. OK, maybe not worth-a-TV-documentary-interesting, but..

I imagine the mix is down to the issue of shaft diameter – surely there wouldn’t be a knock-on effect of forced metrication of other components just because the bearings are foreign spec? Would it in fact be likely that it’s the balls, the tricky bit done by cunning Swedes, and the subsequent O/D that might be metric?

Trouble is, if the British using the imperial system wanted to more finely calibrate something more usually measured in inches, they would normally resort to fractions of an inch.

Having said that, there are some 1950’s technical reports from Farnborough that use mm of Mercury. Just can’t find anything test-related using mm H2O. Pressure differentials are most usually expressed as a figure devoid of units.

I think it is quite likely a hydrometer.. as TonyL says, measuring specific gravity, ie the pressure at the base of a column of a fluid relative to the pressure at the base of a column of water. Hence the pneumatic connectors. In aviation it would make sense to measure density in this way as the ratio would be constant irrespective of attitude, altitude, temperature or acceleration.

I can only think of two things in aviation that might require monitoring of specific gravity- a glycol/water cooling mix and battery water. I am sure there are others?

Great result then! Lucky for my kudos I left 5% uncertainty..

The definitive guide is taking shape.. thank you very much indeed Ed.

I am particularly interested in visible shape changes bought about by using 2024 where 2014 was originally specified for a design. Or did you mean the non-UK manufacturer would stoop to 2014 where necessary?

Bit mean spirited of you there. You’d never see those four made into four Bolingbrokes, but one more Blenheim being made possible has to be a good thing. They really aren’t all that different – varying marks of Spitfire have far less in common with each other. I recall someone who worked for ARC telling me that some drawings had Blenheim crossed out by hand and Bolingbroke scribbled in if they were Canadian, while some GA’s had Bolingbroke crossed out and Blenheim IV written in as this was always meant to be the new British name for what became just a new mark.

The way de Havilland have it, at least in the 1941 bracket manual, the means of getting the oil as far as the piston is not provided as a propeller part, rather the two-position valve, the collector ring on the shaft and a conduit for the oil are provided by the engine manufacturer. The text goes on to say that engine manufacturers were ‘currently making provision’ for the transfer of oil via the shaft, and ‘If it is desired to make use of a controllable pitch airscrew with an engine not provided with these adjuncts the engine manufacturer should be consulted” Certainly they provide no other detail or part numbers for this part of the system. So the thing would be to find something from Fiat on the models of that engine designed for use with bracket props. Oh dear, I can see your problem!