Triumph Logo

Easiest Unit 650 Triumph 4-Speed Gearbox Assembly

I doubt that many workers in the Meridan repair department used the workshop manual gearbox assembly method. Why would they when they could use the method demonstrated by Hughie Hancox on his great DVD set, "Unit 650 Engine Rebuild".

Note. These instructions are for 4-speed gearboxes with spring and plunger. If you are working on a 5-speed gearbox, or a gearbox with a leaf-spring instead of a plunger, have a look at these posts on indexing by John Healy on Britbike.com.

Hancox builds up the gear cluster piece-by-piece inside the gearbox and then finishes by installing the mainshaft pre-assembled in the inner cover along with the kickstarter ratchet. If parts are lined up and ready to go, a gearbox can be assembled with inner cover in place in two minutes. (Hey! That was cool, let's do it again!).

The first time I tried the method, I didn't pre-install the mainshaft in the inner cover, I installed the mainshaft by itself followed by the inner cover and kickstart as done conventionally.

The next time I did pre-assemble the mainshaft into the inner cover and assembly went very smoothly. I also found that pre-assembling using the vice made installing the mainshaft in a tight mainshaft bearing easier then doing so on the motorcycle.

Photo 1 shows the gearbox components in their order of assembly, from left to right. A thrust washer is already installed over the drive side layshaft bearing.

Photo 1. Order of assembly, from left to right.

Photo 2 shows how an assembled gear cluster looks from the rear in 1st gear.

Photo 2. Gear cluster, rear view. We can just see the top claw of the mainshaft shifting fork in the photo.

To see a photo-animation of the action going on inside the gearbox, take a look on this page.

Order of Assembly

Here are the 12 steps of assembly in a nutshell.

  1. Thrust washer
  2. Plunger holder with plunger and spring.
  3. Camplate (1st gear postition)
  4. Layshaft/high gear
  5. Layshaft 2nd gear
  6. Layshaft 3rd gear
  7. Layshaft shifting fork & roller
  8. Mainshaft shifting fork & roller
  9. Shifting fork rod
  10. Mainshaft 2nd gear
  11. Mainshaft 3rd gear
  12. Mainshaft/inner cover


Before proceeding with the following assembly steps you must have previously 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.

Install Thrust Washer

Put in place the inside thrust washer with the grooves facing the gears, not the case. Check to see that it sits well on the locating peg and flush against the casing. Now remove the thrust washer, dab some thick grease on the back and reinstall it as before.

Photo 3. A dab of grease helps hold thrust washer in place during assembly. Azimuth of your locating peg will vary from that shown.

Gearshift Plunger and Camplate

Lubricate the camplate plunger and spring and fit them into the gearbox case with the index plunger holder.

Lubricate the gearshift camplate shaft, depress the camplate plunger with screwdriver, and fit the camplate in its 1st gear position as shown.

Photo 4. Thrust washer, and gearshift plunger and camplate in place.

The photo at right shows the gearshift plunger and camplate in 1st gear position. The red dot represents the position of the layshaft selector fork peg, and the green dot represents the mainshaft selector fork peg in 1st gear.

The Layshaft

Hancox next installs the layshaft complete with 2nd & 3rd gears. When you do it that way, the gears obscure the thrust washer and bearing hole from view.

Since a thrust washer falling off its peg un-noticed is a fairly common assembly error, I chose to install the layshaft solo and then slide on 2nd & 3rd gears afterwards. That gave me a better opportunity to check that the washer was still in place after the layshaft was installed through it.

So, using lubrication, insert the layshaft and layshaft highgear through the drive side (DS) layshaft needle bearing.

Photo 5. Layshaft installed.

Layshaft 2nd Gear

With the layshaft installed, lubricate its splines and slide on 2nd gear.

Photo 6. Layshaft 2nd gear.

Layshaft 3rd Gear

Now lubricate and slide on 3rd gear.

Photo 7. Layshaft 3rd gear installed. Note that in this photo the gearshift camplate is erroneously depicted in the position of neutral between 2nd and 3rd gears instead of first gear as pictured in all other photos.

Layshaft Selector Fork

Place a roller on the layshaft selector fork's peg and stick it on with a dab of grease.

To identify the main and layshaft selector forks, look at their 'barrels', not their 'claws'. The fork with the shorter barrel is for the layshaft and the longer barrel one is for the mainshaft.

Fit the layshaft selector fork to the layshaft 2nd and 3rd gears and slide the gears forward or back until the roller locates in the camplate groove to match 1st gear as shown by the red dot in the photo below.

Photo 8. Layshaft selector fork.

The gearshift plunger and camplate are shown in first-gear position at right. The red dot represents the position of the layshaft selector fork peg in 1st gear.

If you're interested, the camplate positions for all gears can be seen here..

Mainshaft Selector Fork

Lay the mainshaft selector fork behind the layshaft selector fork and place the roller in 1st gear position in the camplate groove as shown by the green dot in the photo above.

Photo 9. Mainshaft selector fork with selector fork rod started.

Selector Fork Rod

Lubricate and then insert the selector fork rod through both gear selector forks and push it in all the way until the shoulder contacts the case.

Also, be sure to seal the end of the rod because the rod drilling goes right through the case and is a common source for leaks. The open end of the rod drilling is already covered by the gearbox sprocket so do it now from the inside.

Photo 10. Selector fork rod tapped home.

Mainshaft 2nd Gear

Next, "fiddle" in the mainshaft 2nd gear, engaging it in the mainshaft selector fork.

Photo 11. Mainshaft 2nd gear.

Mainshaft 3rd Gear

Then, "fiddle" in mainshaft 3rd gear, engaging it in the mainshaft selector fork.

Photo 12. Mainshaft 3rd gear.

Layshaft Low Gear

Fit the layshaft low gear.

Photo 13. Layshaft low gear.

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 ratchet, 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/Camplate

  1. Fit inner cover thrust washer with a dab of grease.
  2. Coat inner cover and gearbox faces with a joint sealer.
  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.

The quadrant position should match that shown in Photo 14. 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.

Photo 14. Appearance of correctly assembled and indexed gearbox in 1st gear.
For more information on gearbox indexing, see this companion article: