the first mk2 square 4 was 1953 and the last 1959, and that is as stated, a mk2 head. The valve collars suggest that head is 53 or 54 as a different top valve collar was adopted from 1955.
You can just see part of one crankshaft behind the head. The individual cranks are one up one down and are counter rotating which when properly balanced make for an incredibly smooth vibration free ride (every bit as good as an isolastic commando) All the 1000cc 4s have the crankshafts coupled by a pair of gears external to the crankcase.
The original OHC 4s however had teeth cut on the central flywheels which formed the coupling gears and in the original design then coupled to a unit gearbox which meshed into the rear flywheel. The unit gearbox was however too costly to produce so a separate gearbox was adopted for production. The original OHC 4s had several major faults. The head is only fixed with 8 bolts. think about that 4 cylinders and 8 head bolts so not hard to blow a head gasket. And the cylinders were too close together which resulted in overheating and often seizing of the rear cylinders. The later pushrod 4s have a tunnel between cylinders which cured the overheating but the reputation for seizing the rear cylinders remained to the end. The final problem was again heat related as the magnetos were prone to failure because of the heat from the engine.
The mk2 square 4 is in reality a very nice bike and with the head shaved, 50 hp is achieved in a bike that weighs about the same as a commando.
ando
