Note: These articles pertain to a 1969 Triumph T120R 650, as well as other late-60s 4-speed gearboxes using a plunger and spring. If you're researching a 5-speed gearbox, or a gearbox using a leaf-spring, be aware that there are important differences. See these Britbike.com links on Leaf-spring and 5-speed gearboxes.
The quadrant is operated by the gearchange lever and the gearshift spindle and plunger assembly (also known as the gearchange quadrant). In turn, the quadrant moves the camplate (see Figure 1 at left), and the camplate operates the gearshift selector forks which slide the gears back and forth on the mainshaft and layshaft.
The teeth on the quadrant and the camplate teeth must be meshed while the quadrant and camplate are correctly positioned relative to each other. Only then will all four gears be operable. Finding the correct relative position between the quadrant and camplate is what's refered to as "indexing the gearbox".
Photos at left show the quadrant's correct position (as determined by an imaginary line beginning at the mainshaft center and following the camplate's shaft - see Photo 1) in all four gears and two neutrals of a properly indexed gearbox.
Note on leaf-spring gearboxes. Four-speed gearboxes with a spring and plunger are easily indexed in 1st gear, 4th gear, or neutral between 2nd & 3rd gears. John Healy maintains that either 1st or 4th gear camplate timing are preferred because they are tactile rather than visual. However, Stein Roger (who has considerable experience in these matters) informs me that when indexing a gearbox with a leaf-spring, he finds 4th gear much easier than using 1st.
After the camplate, layshaft, mainshaft, gears, and selector forks have been placed inside the gearbox, the quadrant is indexed to the camplate as the inner cover is pushed into place. While pushing the inner cover on, the quadrant is held in a position matching the gear determined by the camplate position.
Hughie Hancox assembles the gearbox with the camplate in first gear, therefore he indexes the quadrant to the camplate in its first gear position.
Do this by tipping the quadrant just enough that it slides along the gearbox floor as you push on the inner cover. When the cover is nearly on (1/4-3/8" gap) the quadrant will touch and be held by the camplate inside. Back off the cover just enough that you can move the quadrant and then hold the quadrant in the position shown in Photo 1 as you push/tap the cover all the way home.
Verify correct indexing by using a screwdriver to move the quadrant and confirm that you have all four gears and neutral(s).
For a complete, illustrated, step-by-step tutorial on assembling the gearbox using Hancox's method, see Assembling & Indexing the Unit 650 Gearbox, Hughie Hancox Method.
The workshop manual assembly method involves stuffing the gearbox with the camplate positioned in the neutral between 2nd & 3rd gears.
The only difference between indexing the assembled gearbox with the camplate between 2nd & 3rd gears and the camplate in 1st gear is that you hold the quadrant in the position shown in Photo 4 as you push on the inner cover.
Again, the only difference is the position in which you hold the quadrant while pushing on the inner cover. With the camplate positioned in 4th gear, hold the quadrant in the position shown in Photo 6.
Studying these photos should help greatly in acheiving the proper relative positioning of the camplate and quadrant regardless of which assembly method you choose.
For an annotated side-by-side comparison of three Triumph 650 Unit gearbox
assembly methods, see Gearbox
Assembly, Three Methods.