The improvements made by Parilla to the early tensioner were at best marginal & poorly thought out. There is no point in tensioning a chain on the drive & “slack” side. Other manufacturers with chain drive cams rely on a single sided tensioner on the return side of the chain run. Tensioning a chain with a spring is OK as long as there is a mechanism to take the load off the spring once the slack has been taken up- some Japanese engines use spring tensioners but they have a locking system to keep the slipper in contact with the chain & relieve the spring of the cyclic loading & unloading.
Other manufacturers, such as Norton, use a slipper which is tensioned by hand & locked in position as the engine is built.
Both types have a good track record (when maintained) so I’m of the opinion that the Parilla tensioner can be further improved. My next engine will have a prototype single sided “rigid” tensioner.
The other cam drive system is, to my mind, the perfect solution. Parilla themselves utilised this method in their works & production racers. There are no tensioners to worry about or chains to thrash about plus the system uses no fibre gears (I didn’t mention those yet) in the timing or oil pump/points drive.
A more expensive solution in its day & no less so today. The 175cc chain drive engines can be converted to gear drive, indeed Parilla offered just such a conversion for the engines. Even the very early type with no cast in bosses for the idler gear.
The gear drive system uses straight cut gears from the oil pump/points gear, crank pinion, 2 piece conversion gear, idler & cam. The primary drive is also by straight cut gear (not shown in this picture). I have a gear drive 175 & can vouch for the reliability of the system.
Gear drive conversions are available today for both 175 & 250 engines. All gears, bearings, X1 cam & spindles in the drive chain are included, only the primary drive remains as a helical drive (in the case of the 175 engine). Supplies of these conversions are limited & inevitably expensive, see Parts-Engine for availability & current prices.
Mention was made earlier of the usage of fibre as a material for some gears in the cam drive of the “chain” engines. To my knowledge, none of the gear drive units use any fibre cogs at all. No doubt the use of fibre ( a composite of a fabric lattice with a resin binder) was an economical & suitable material for low stressed drives such as points/oil pump. Easy to machine & silent in operation, it would have been ideal but for two disadvantages, one was predictable the other less so.