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TAS Country : September 2nd 2010
22 Tasmanian Country Friday, September 3, 2010 Machinery Money never issue for VERSATILE: In a standing position, the view of a Marshall 12-20 from the drivers platfo HAZARD: The cylinder head on the Marshall Model 12-20 tractor showing the ignition paper holder (a remade example), the injector and the cable operated decompression valve. ENGAGE: A view of the clutch/belt pulley on my Marshall Model 12-20 MANOEUVERABLE: 1938 Marshall Model 12-20 diesel tractor which came to Australia on the 30/03/1938. It carries erection number 137 and stamped as serial number 642. Displayed at the Vintage Tractor Shed Museum at Westbury. TRAGIC Tractor Glenn Shaw BY 1934 the original Marshall Company was for all intents and purposes bankrupt. They were developing a more versatile new tractor that bore a marked difference to the previous Marshall tractors. The biggest change to the appearance was borne through the change to thermo-siphon cooling which dispensed with a water pump. .The cooling fan was now mounted between the radiator units above the cylinder block and belt driven directly off the flywheel. It was the cylinder dimensions that would lay the pattern for all following Marshall tractors --- a 6½ inch bore with a 9 inch stroke. Originally rated as a 10-20 horsepower tractor running at 600rpm, changes were made to coax a further two horsepower on the belt pulley and from that point the new tractor was known as the 12-20. Gone was the old ratchet type decompression device and replaced with a simple roller that ran in a threaded groove in the flywheel. This starting system was carried over to all Marshall single cylinder tractors until 1957. The Model 12-20 used the same type of lubrication system as the Model 18-30 that featured a gear driven pump with multiple plungers delivering oil. Oil was carried in a sump on the right-hand side of the tractor and holding around one gallon. Used oil from the engine was ejected by crankcase compression through a one way valve to a reservoir fitted to the LH side of the cylinder block where it was drained off.Reed valves were used in the crankcase to allow air in and hold it there to create crankcase compression for cylinder scavenging. The fuel injection pump was fitted on the right-hand side of the tractor behind the flywheel and was a Bosch- C.A.V design fed by diesel. The fuel injector in the cylinder head was also a Bosch-C.A.V designed unit. Nozzle cleaning was something the owner would get used to though, as carbon build up would result in poor starting and erratic running. Engine power was delivered to the transmission by a cone type clutch operating in the belt pulley The centre of the pulley contained the clutch cone onto which was riveted a Ferodo friction lining that contacted the inside edge of the belt pulley that took the drive to the transmission. The Model 10-20 tractor was originally designed with a two speed transmission which was carried into the re-rated Model 12-20. This incorporated a parking brake which was an expanding type that used two shoes in oil inside a drum. This brake was surprisingly effective and engaged by a hand lever. With receivership looming large on the horizon, Marshalls completed only one 12-20 two-speed tractor which was given erection number 1 and serial number 207 and it was promptly sold to a long standing Marshall customer Mr Millegan of Stalham in Norfolk; the provision made for the tractor to be used as an evaluation tractor for engineers. On the face of it, the prototype 12-20 was a great little tractor, its nine foot length made it manoeuvrable and at 4700 pounds it was nowhere near as cumbersome as the earlier models. In 1935 Marshalls were taken over and became a part of the Thomas W Ward group of Companies and would be re-named as Marshall Sons and Company (Successors) in March 1936. At last the grand old company had some much needed cash behind them and great faith placed in the 12-20 tractors to become an export earner --- once the flaws had been ironed out. Mr Millegans experience with the prototype 12-20 revealed that it would need a three forward transmission which would be incorporated into the second prototype tractor built early in 1936 and given erection number 2 and serial number 210. Tractor serial number 210 incorporated this and a number of other improvements to make the tractor a more user friendly. The prototype 12-20 suffered problems with the oil from the air cleaner finding its way into the engine crankcase and also emitted oil spots over the driver. A similar problem existed with the exhaust. These problems were addressed by changing to a dry type air cleaner that used coconut fibre for filtering and a new expansion type exhaust/muffler that included an oil trap. The prototypes lubrication system was carried over to the second modified tractor in modified form. This filtered oil was now admitted back into the lubricating oil sump to be recycled. Traction was proving to be a problem in certain conditions due to the Model 12-20's comparatively light weight but this was to be solved by the addition of a differential lock which was activated by the driver using a lever.. Mechanically the 12-20 specifications were virtually set,
August 26th 2010
September 9th 2010