Triumph Unit 650 Gearbox

Assemble & Index a Unit 650 Spring & Plunger 4-Speed Gearbox

Hughie Hancox Method

The guys at the Meridan repair department didn't follow the Triumph workshop manual when they were refitting gearboxes. They used the way demonstrated by Hughie Hancox on his great DVD set, "Unit 650 Engine Rebuild". Hancox assembles the gear cluster quickly and effortlessly, piece-by-piece inside the gearbox, and finishes by installing the inner cover pre-assembled with mainshaft and kickstart ratchet.

The first time i used the method, the gear cluster (less mainshaft as per the Hancox method) went into place amazingly easily. But instead of pre-installing the mainshaft/ks in the gearbox inner cover and pushing them onto the assembled gearbox as one, I installed the mainshaft through the cluster and then pushed the inner cover on over it. The next time I pre-assembled the mainshaft/ks/inner cover and all went swimmingly well. And using a vice made installation of mainshaft through a tight mainshaft bearing in the cover much easier then it would have been on the machine.

Assembly Order

  1. Camplate (1st gear postition)
  2. Layshaft/high gear & 2nd/3rd gears
  3. Layshaft shifting fork & roller
  4. Mainshaft shifting fork & roller
  5. Shifting fork rod
  6. Mainshaft 2nd gear
  7. Mainshaft 3rd gear
  8. Mainshaft and inner cover

Photo 1 shows the gearbox components and their assembly order from left to right. In the photo, a bronze thrust washer has already been installed over the layshaft bearing.

Photo 2 shows how an assembled gear cluster looks from the rear. Note mainshaft gear shift fork in photo.

Before proceeding, you need to have already installed the mainshaft high gear bearing and its circlip, the high gear bearing oil seal, and the gearbox sprocket with its tab washer and 1 1/4" nut.

Detailed Instructions

Install Plunger, Camplate, and Gear Cluster

  1. Use grease to hold the inside thrust washer (grooves towards gears) being sure that it is well seated over its locating peg and flush to the bottom of the casing. Photo 3.
    In Photo 3, note position of new locating peg after repair.
  2. Fit camplate spring and plunger with a spot of oil.
  3. Lubricate the camplate shaft, depress the camplate plunger with screwdriver, and fit the camplate in the 1st gear position (Photo 4 & Photo 9.
  4. Insert the layshaft/layshaft highgear into the DS layshaft needle bearing.
    Hancox installs the layshaft complete with 2nd & 3rd gears, but the gears obscure the view of the thrust washer and bearing hole going in. I find it easier to install the layshaft (Photo 5), and slide 2nd & 3rd gears on afterwards. Practially no chance of the thrust washer being disturbed and falling down this way.
  5. After installing the layshaft, lubricate layshaft splines and slide on 2nd gear (Photo 6) & 3rd gear (Photo 7).

    Note error in Photo 7 - the camplate is shown in neutral between 2nd and 3rd gears, not first gear as it should be. Refer to other photos. All of the camplate positions are illustrated in these photos.

  6. Place roller with grease on layshaft selector fork.
    To identify the main and layshaft selector forks, look at their 'barrels' (not the 'claws'): the shorter one is for the layshaft, the longer one for the mainshaft.
  7. Fit the layshaft selector fork to the layshaft 2nd and 3rd gears (Photo 8), and slide the gears forward or back until the roller locates in the camplate groove to match 1st gear - the red dot in Photo 9.
  8. Lay the mainshaft selector fork behind the layshaft selector fork Photo 10.
    Place the roller in the 1st gear position in the camplate groove, shown by the green dot in Photo 9.
  9. Lubricate and then insert the selector fork rod through both gear selector forks and all the way home into the gearbox case. (Photo 11).
    Later, don't forget to smear some sealer over the opposite, open end of the rod's drilling.
  10. Next, "fiddle" the mainshaft 2nd gear in, engaging it in the mainshaft selector fork. Photo 12).
  11. Then, "fiddle" in mainshaft 3rd gear, engaging it in the mainshaft selector fork. (Photo 13).
  12. Fit the layshaft low gear. (Photo 14).

Install Mainshaft into Inner Cover

  1. Hold mainshaft "firmly" vertical by the splines in a soft-jawed vise.
  2. Place the inner cover over the mainshaft being held in the vise.
  3. Install kickstarter assy: washer, sleeve, spring, pinion, top rachet, tab washer, and nut.
  4. Tighten nut and fix with tab washer (check to insure kickstart ratchet turns freely).

Install Inner Cover with Mainshaft & Index Quadrant to Camplate*

  1. Fit inner cover thrust washer with a dab of grease.
  2. Coat inner cover and gearbox faces with jointing compound.
  3. Lubricate the mainshaft, which is fixed in the inner cover.
  4. Fit inner cover with mainshaft to the gearbox.
  5. Tap the inner cover on with the quadrant on gearbox floor until it contacts the teeth on the camplate.
  6. Then, back off the cover just slightly, enough to free the quadrant and then turn the quadrant clockwise ever so slightly until it just engages the camplate.
  7. Tap the cover on.
  8. The quadrant position should match that in Photo 15. Verify this by noting the position of the nearest tooth to an imaginary line extending from the center of the mainshaft along the shaft of the camplate.
*These steps are for a 4-speed gearbox with spring and plunger. If you are working on a 5-speed gearbox, or a gearbox with a leaf-spring instead of a plunger, see John Healy's posts on indexing.