Jules. Here are my thoughts.
The Rigid and Plunger A Range were always the bike for sidecar use. So, a robust transission was a obvious requirement. The Plunger type scores here with a smooth running duplex chain and a driven centre splined directly to the gearbox input shaft. The "6 Spring" type with the pressed steel centre, used on B,M and Swinging Arm A Ranges, transmits the drive to the clutch adaptor over a very small area, most used centres will have some wear to the splines, and corresponding wear to the very tips of the adaptor splines, allowing slop and further wear. Considering the higher power output of later models this is a big ask for these two parts. Tightening the adaptor and clutch centre onto the gearbox input shaft taper can be hit and miss, evidenced by sheared keys and damaged slots.
The Plunger chainwheel is a pretty tough construction, supported on rollers, and shielded from ingress of oil by the domed cover which must also have a welcome function of adding further rigidity. The driving plate tongues must not extend out further than the outside of the chainwheel fingers, otherwise they will bind against the inner surface of the domed cover. Look for witness marks next time the cover comes off.
The 6 spring chainwheel has a spot welded band, but in most cases this does not contact the plate carrier fingers over their entire surface to offer support. If it did, there is a further prospect of the driving plate tongues binding against the band in the same way to the Plunger dome cover. So the function here can only to be to keep the oil out. Both clutch types suffer wear and burring of the chainwheel fingers by the narrow edges of the driving plate tongues and these areas are highly loaded.
A Plunger centre is a finely engineered component, of heavy construction with machine cut splines. Wear to these splines by the driven plate tongues is less than with the softer pressed steel of the centre of the 6 spring.
Compare this with the 6 Spring pressed steel type, The fingers here are thin, and all must be perfectly aligned to allow the pressure plate and driven plates to slide. A tall order. As for the spring pins, all need to be vertical compared to the bottom face, replacement types come with varying degrees of perfection, many simply poorly peened over. A problem here when they come loose and rotate freely is that the spring tension cannot be altered, and the only way to dismantle the clutch is with a hacksaw blade between the plates to part off the offending pin.
To standardise across the range and with the demise of the sidecar market on the horizon, a decision to switch entirely to a cruder and cheaper clutch took away the well loved but expensive Plunger type. The later 4 Spring Triumph Type has several features similar to the Plunger Clutch and again is a robust and favoured type. The spring pins are free in the clutch basket, but have squared ends to prevent rotation, yet allow a degree of float to align with the pressure plate and allow its free movement.
Swarfy.
