Hi Phylo,
Firstly from the Workshop Manual for the K5, and to quote " piston skirt fitting has been increased from .0025"-.0028" to .004"-.0045" to make engines with new pistons immediately suitable for wading through water without fear of siezure. Pistons with the original smaller clearance can be used for engines which it is known will not be called on for wading".
OK, I first interpreted that as only those vehicles with waterproof servos having the increased clearance as they might have been the one specially set aside for wading. There is a contract of K5's which just have the word "Water" after the vehicle type, I am certain these were not water tankers and can only think that these were the ones prepared as above. There appears to be 500 lorries in that contract. Reading the manual again and it is dated 1946, so after the event, and after final production so it might indicate if vehicles were being prepared for water, then this is the data.
Just to confuse things further, I have a document, Inspection Standards (Provisional) for the Austin K6 6x4, issued by REME Central Inspectorate. Handwritten on the cover are the words "Comments by 26 Feb 1955". In there is some interesting wordings and bear in mind this is 10 years later;
"Pistons must be selectively fitted to obtain clearance. Feeler gauge applied to thrust side of the piston skirt, tight .0045" and loose .004" will suffice for practical purposes. When assembled and on test, piston slap will be apparent and is permissible"
The clearances mention are the increased ones stated for wading, so maybe they set all the 4 litres up this way, in case of the eventuality. Further on in the engine testing data there is a another mention;
"NOTE: Due to increased piston to bore clearance piston slap will be apparent and is permissible."
I have more thoughts on all this but will have to find time later on to relate them.
regards, Richard