You might recall I stripped the little BTH compressors and discovered a curious Scotch Yoke type of crankshaft within. Thus, I was interested to see inside one of the other type of aero air compressors I've collected over the years. They are the type that are fitted to the rear of a Rolls-Royce Merlin B bank cambox and this one is pictured in pics 1&2 at
It is obvious from their construction that they were never intended to carry lubricant in their crankcase, but must have survived on splash from the cambox, there being a ring of four slots to facilitate this in the inner crankcase wall. This inner wall supports a big (2" across), thin shell bronze bush and the other mainshaft is supported by a ball race in a blind housing. The crank itself is boringly normal except that the conn rod is cast bronze.
The ali piston has an automatic valve in the crown, two rings and runs in a fine pitched, one piece, finned steel cylinder of all-machined construction. It is secured to the crankcase in an ingenious way. There is a female ring machined in the top of the crankcase halves and this accepts a male ring that is part of the cylinder, requiring assembly when the crankcase halves are fitted together. Above it, there is a fine thread to which is fitted a toothed, internally threaded brass ring that tightens against the crankcase mouth, pushing the cylinder *upwards*. As far as I can see, the only advantage this confers to the device is that the cylinder can be turned anywhere in 360 degrees, giving an infinite variety of positions in which one can fit the outlet.
The slim, cast finned ali cylinder head with its automatic inlet valve is secured to the cylinder with six Allan screws and appears to have had a thin lead gasket, now replaced with Hylomar.
It requires filling with oil before the mounting plate is secured with a ring of 1/4" bolts, but then it is easily operated with an electric drill to ascertain its abilities - and it works just as it was intended to over sixty years ago.
Interesting to think that on this auspicious weekend, this little compressor might well have been whirring away on the back of a Merlin and Doing Its Bit to wrest the Continent from the grip of the oppressor.
Regards,
Kim Siddorn,