Hi jchildr, PM me your email & I can send you photos of the spring I installed next to the old spring. You can count coils. I can't seem to find the stronger spring wire diameter in my notes. The old spring wire diameter was .155",
I got the stronger springs from Rabers Parts Mart. They no longer sell parts. Very sad, as they were close & very good.
The strong springs did not spring bind. We deburred all parts in the outer cover so springs had a very smooth surface to glide on.
How the plungers work, I don't know of an animation, but notice when you move lever, one plunger goes under the steel plate & retracts, while the other plunger pops out. On the bench the plunger indeed pops all the way out....
With cover installed the plunger pops out & it hits the T397 cam plate operating quadrant between the teeth as you move lever. As you continue to move lever that popped out plunger rotates the cam. All the while the retracted plunger stays retracted & does nothing.
The casting of outer cover & shape of the T4089 gear change quadrant act as a stop to prevent further movement. You feel the shifter hit stop with your foot & let go of shifter.
Now shifter returns to center & both plungers are in their "home" position waiting for the next shift.
Thinking of all this if the shifter doesn't center properly, the plungers are not actually "home" so they cannot properly move cam to next gear when moving lever. So good springs, no excessive wear of steel plate T407 or plungers, or any kind of binding of shaft that restricts movement can give false shifting.
Overall the outer cover parts are fairly reliable & still work with some wear so long as plungers actually pop out & lever centers.
One thing that can throw a monkey wrench
in the works is if the cam plunger spring T3661 is weak or broken the cam is not held firmly in place. When releasing shift lever fwith foot the cam doesn't stay put, but wants to rotate as lever is released pulling trans partly or fully out of gear. I've never actually seen a broken spring, but experimented with spring tension & found that to happen. Also it happens if you try to shift gears with cam plunger T2172 removed. You can look up hole & see cam rotate backwards when you release gear shift lever. Broken leaf springs will do this from what I've heard, but again, I've not personally seen that.
Triumph had a service bulletin covering modifications to shifter T4089 stop faces & T3661 spring tension as I recall for transmissions that have incorrect gear travel which might result in over/under shifting & not selecting or popping out of gear. I'd have to dig them up, but I have them.
Also the old version cam has be superseded by a new cam profile which started with leaf spring transmission mid season 1970. While a 1969 plunger may work, comparing early & late plungers the 1973 plunger assembly should be used with the new version cams as the cam profile is quite different where plungers rest in gear. So that's yet another thing to think about.