[Viper2000]

Early Merlins used 100 per cent ethylene glycol colling which allowed 135ºC coolant temperature, but with the advent of the Merlin XII a major move was made to pressure water cooling using 30 per cent gycol as anti-freeze. The advantages of water as a cooling medium are considerable. With 135ºC coolant temperature the mixture at 15 lb per square inch reduces the cylinder head metal temperature by some 30ºC when compared with 100 per cent glycol.

Harvey-Bailey, A. H. 1995. The Merlin in Perspective - the combat years. 4th Edition. Derby: Rolls-Royce Heritage Trust.

However, Appendix VIII of the same book notes the Merlin IV as the lowest Mark number of Merlin designed for pressurised water cooling, and describes the Merlin XII as

Basically as Merlin IV but with higher supercharger gear ratio and uses 100 octane fuel. Coffman starter, pressure water cooled. For Spitfire II.

Meanwhile, Appendix VII, which is the list of Mark numbers from the Production Drawing Office & Service Department is not in complete agreement. It states that the Merlin IV was pressure glycol cooled, whilst the Merlin XII is listed as pressure water/glycol cooled.

Merlin III is listed as glycol cooled, as is Merlin V.

However, in yet another list which forms part of the same appendix, Merlin V is given as "converted Merlin III" and as either pressure or glycol cooled.

So, what I think was actually going on is this:

1. Originally, all Merlins were glycol cooled.
2. Glycol causes all sorts of trouble and is flammable, so it was phased out in favour of pressurised water cooling from the Merlin XII onwards.
3. Existing Merlin III engines were gradually converted to Merlin V standard, using pressurised water cooling, presumably some time after the arrival of the Spitfire II in September 1940.
4. The switch to pressurised water cooling reduced the maximum coolant temperature from 135ºC to 120ºC.
5. Pilot's notes were amended accordingly, but the engines weren't allocated a new Mark number because they weren't new engines; it's easy to imagine the pen-pushers saying "You can't possibly give these engines a new Mark number without filling in the following 17 forms for each engine in triplicate - we have a system to maintain!"...
6. Therefore my best guess is that Spitfire I and Hurricane I during the Battle should have the older 135ºC limit, whilst Spitfire II should have the 120ºC limit, and then some time in late 1940 the older aeroplanes should be converted in dribs and drabs during overhauls.

Note however, that this is nothing more than informed speculation. It is possible that the temperature reduction was independent of the change in coolant.

However, my best guess would be that the original intention was to retain the 135ºC limit, but that the pressurisation required to prevent the system from boiling over was such that leaks became a problem (indeed, coolant leaks were quite well documented in this period) and therefore the degree of pressurisation and thus the maximum safe temperature was reduced at some point in order to mitigate the problem.

Obviously the arguments for standardisation are pretty strong, so I imagine that once the decision was made to convert the installed fleet to pressurised water cooling with a 120ºC limit this new limit was imposed across the board irrespective of the progress of the conversion programme, resulting in the strange combination of pure glycol cooling and a 120ºC limit in some sources.

BTW, the Spitfire I notes in the CE state that the limit is 120ºC and the coolant is 70% water and 30% glycol; but they state the engine as Merlin II or III not Merlin V.