As a Dennis apprentice from 1950-1955, I had the good fortune to finish my time in the Road Test Department, where I had the opportunity to drive vehicles fitted with the Dennis 06 engine and the 'O' type 5 speed gearbox. It is on the basis of that experience, together with that gained subsequently driving Dennis vehicles fitted with that type of powertrain, that I have detailed below some suggestions on how to handle the 'O' Type 5 speed gearbox.
At the time I was an apprentice the 'J' series Lancets and 'K' series Lances were finished, 'LU' series Lancets very rare and Lolines had yet to come. The Max, Max 6 and Jubilant were still being built fitted with the Dennis twin plate clutch and 'O' type gearbox. As a 21 year old apprentice in Road Test I drove Max 6's on several occasions. That experience proved very useful when I became one of the team of drivers who drive Norman Hamshire's East Kent P3 Falcon and J3 Lancet buses. Some of you may have ridden with me in the J3 at the 2004 ADBIG Guildford Running Day, and may have noticed that I used overdrive quite frequently, but always made sure I was back into 4th before approaching, for example, a 'giveway' situation such as at a roundabout. That was to ensure that if I needed a lower gear quickly, I was already back to the main gearbox.
The Dennis twin plate clutch incorporates a clutch brake, which, if adjusted correctly, serves two purposes. Firstly when the vehicle is at rest it will stop the rotation of the primary reduction gears and the layshaft in the gearbox, thus making it easier to engage a gear. Secondly it can be used to slow down rotation of the layshaft when changing up, making the change more rapid than if you double-declutch and wait for the engine revolutions to drop.
Basically the 'O' type box is a 4 speed and reverse crash gearbox with the overdrive fifth gear in an additional compartment on the front of the main box. Until 5th gear is selected, everything operates in a conventional manner, the drive from the clutch being taken to the primary reduction gears at the front of the main gearbox where they drive the layshaft. 1st gear is at the rear of the gearbox, 2nd further forward, 3rd further forward still and 4th a direct drive with the output shaft engaged, by means of a dog clutch, directly to the input shaft. Thus in 4th gear the constant mesh primary reduction gears at the front of the box continue to rotate the layshaft, but the layshaft has no influence on the output shaft. Quite conventional.
Only when fourth gear (direct drive) is engaged can fifth gear (overdrive) be selected by pressing the gear lever against a spring further to the left and then forward as far it will go. This will operate a spring loaded selector mechanism in the additional compartment in front of the basic four speed gearbox, which houses the overdrive, comprising a train of 2 spur gears, one in line with the main gearbox input shaft and the other in line with the layshaft. The gear in line with the mainshaft is fixed to the output shaft from the clutch, and until 5th gear is engaged, a dog clutch engaged with it takes the drive to the input shaft of the main gearbox. This dog clutch disengages in the first stage of making a change into 5th gear, after which a second dog clutch engages with the other spur gear, thus taking the input to the main gearbox via the overdrive gear train and to the layshaft, and then back to the mainshaft via the main gearbox primary reduction gears working in a reversed manner to normal and giving a 0.69:1 ratio overdrive.
Because of the manner in which changes from 4th to 5th are pre-selected and completed, and also from 5th to 4th there are basic rules which experience suggests should be observed.
One advantage of the pre-select arrangement is that if you come to rest in 5th gear, you can quickly de-clutch, bring the gearlever back through 4th to neutral and re-engage the clutch. The overdrive will then sort itself out ready for you to engage a suitable gear and pull away. Another makers gearbox had a reputation for locking in 5th in such circumstances.
I hope these comments will be of assistance and value to drivers of vintage Dennis vehicles who are unfamiliar with the Dennis 06 engine and 'O' Type gearbox.