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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 (sometimes known as the gearchange
quadrant). In turn, the quadrant moves the camplate
and the camplate operates the gearshift selector forks which
slide the gears back and forth on the mainshaft and layshaft.
Figure 1, left
When the inner gearbox cover is pushed on, meshing the teeth of the quadrant with the teeth of the camplate, the quadrant and camplate must be correctly positioned relative to each other. If not, one or more gears will be inoperable. Correctly positioning the quadrant relative to the camplate when installing the inner gearbox cover is what's referred to as "indexing the gearbox".
When the gearbox components are assembled into the gearbox, the camplate is fitted first and it is positioned into a particular gear. Then, after the gear cluster and selector forks with their rollers have been installed, the quadrant is held in a position that corresponds to the same gear into which the camplate has been set, and it and the inner cover are pushed into place.
Part of the difficulty with this somewhat notorious procedure stems from having to use the small, light end of the quadrant to position its larger, heavier end that meshes with the camplate.
Which gear to use for assembly and indexing is personal preference, but for anyone with little or no experience, these descriptions may help.
Four-speed spring and plunger gearboxes can be indexed in
The Triumph Workshop manual method uses Neutral between 2nd/3rd gears. Positioning the camplate that way is easy enough, but visually lining up the quadrant using an imaginary line between two slightly vague and un-level points while trying to avoid parallax error can be a bit dodgey.
John Healy on Britbike.com pointed out that using 1st gear or 4th gear adds the tactile sense to the visual.
Stein Roger finds it considerably easier to index leaf-spring gearboxes in 4th gear, as opposed to 1st gear.
I used the WS Manual method several times, sometimes successfully, sometimes not (pull apart and try again). Then I tried using 1st gear with my spring and plunger 1969 Triumph 650. Much better, and now perfect results using 1st gear. I tried 4th gear once or twice and it worked well also. I feel more control lifting the quadrant than dropping it. Personal preference.
Photos at left show the quadrant's position in all four gears and two neutrals.
The late and great Hughie Hancox assembled gearboxes with the camplate in first gear position and indexed them the same way.
As explained by Hancox on his DVD, the technique is to tip the quadrant just enough that it slides along the floor of the gearbox as you push on the inner cover. When the cover is nearly on (1/4-3/8" gap) you can feel the quadrant touch the camplate inside. Then back the cover off just enough to free the quadrant so you can tip it up and hold it the position shown in Photo 1 as you push the cover all the way home.
In the WS manual method you must maintain the quadrant in a position that corresponds to the camplate and gears in the Neutral between 2nd and 3rd gears, as shown in Photo 4 as you push on the inner cover.
Indexing in 4th gear is like using 1st gear except that you run the quadrant against the top of the gearbox and then lower it to the position shown in Photo 6 and push on the inner cover.
After you index a gearbox, always check to see whether it's done correctly before going on to install the outer cover. This can be done by operating the quadrant with a screwdriver (or a robust pair of wide-mouth pliers) to verify that all four gears are working properly. Note that it will probably be necessary to gently bump the rear wheel in order to make the gears slide into place.