Triumph Unit 650 4-Speed Gearbox Assembly

Three Methods Side-by-Side

Below are descriptions of three different assembly methods for the unit-construction Triumph 4-speed 650 gearbox.

Workshop Manual

Hughie Hancox


Mainshaft and layshaft bearings
Inner gearbox cover

(Mainshaft ball journal bearing S35-7 - 3/4 x 1 7/8 x 9/16")
  1. Heat inner cover to 100C.
  2. Press or drift in mainshaft bearing.
    WS Man says use 2 1/2" drift, but that would be for the high gear bearing.
    The mainshaft ball journal bearing is 1 7/8, not 2 1/2.
  3. Install circlip.

  4. Open end needle roller bearing (57-1614/TIMKEN B1112 - 1 1/6 x 7/8 x 3/4")

Mainshaft and layshaft bearings
Inner gearbox cover

(Mainshaft ball journal bearing S35-7 - 3/4 x 1 7/8 x 9/16")
  1. Cool the mainshaft (S35-7) and layshaft (57-1614) bearings.
  2. Thoroughly heat inner gearbox cover.
  3. Fit the mainshaft bearing (S35-7) (Tool-Z15)
  4. Fit the open end needle roller bearing (57-1614): using a soft-faced mallet and the (high gear end) of layshaft as a drift, drive the bearing in from the inside of the cover, leaving it protruded just below the depth of the thickness of the thrust washer (57-1607).


High gear ball journal bearing

(57-0448 - 1 1/4 x 2 1/2 x 5/8")

  1. (Old bearing driven out from inside gearbox using a shouldered drift (Z15) on inside race).
  2. Drift in new bearing with (<)2 1/2" drift on the outer race.
  3. Put in new circlip.
  4. Press in new oil seal.

High gear ball journal bearing

(57-0448 - 1 1/4 x 2 1/2 x 5/8")

  1. High gear roller bearing
  2. (with the right-hand half of the crankcase lying on its side on the workbench...)
  3. Thoroughly heat the area around the bearing housing.
  4. The bearing should then 'drop straight in' (as it does in the video)
  5. Fit the circlip.


Layshaft closed-end needle roller bearing

(57-1606/TIMKEN M11121 - 1 1/6" x 7/8" x 3/4"

  1. Heat casing to 100C. (Cool needle bearing).
  2. From inside the gearbox, use drift with shoulder profiled to give proper bearing protrusion for the thrust washer.
  3. Seal the outer portion of the bore into which the bearing seats.

Layshaft closed-end needle roller bearing

  1. Put crankcase in a vice (more or less normal orientation)
  2. Use the layshaft with a soft-faced mallet as a drift to 'gently tap it home' (I count a dozen medium blows with the soft-faced mallet).
  3. Insure that the bearing lip is below the face of the bronze thrust washer.
  4. High gear bearing oil seal, open side to the bearing - tap it all around (like 25-30 times) with a ball peen hammer (beat the sucker in!).


High gear and drive sprocket

  1. Drive a new oil seal up to the main bearing with lip and spring facing the bearing.
  2. Press the high gear into the bearing.
  3. "...ensure the oil seal is correctly engaged over the protruding nose of the high gear". (D15, WS-M).
  4. Lubricate and tapered boss of the sprocket and slide it into the high gear.
  5. Apply sealant to the splines.
  6. Put the sprocket nut and tab washer on finger tight.
  7. Install drive chain and use rear brake to hold the sprocket so you can "tighten the sprocket securing nut as tight as possible" (Tool Z63).
  8. Tab over the lock washer.
  9. Oil "the extended high gear with oil" and replace the door with a new paper gasket.

High gear and drive sprocket

  1. From the inside of the gearbox, insert high gear through the bearing and oil seal.
  2. Push on sprocket with drop of oil where the flange runs in the oil seal.
  3. Place the case over an open vice, sprocket down, and use a 'suitable drift' (think big) to drive the sprocket all the way onto the high gear.
  4. Fit tab washer and large sprocket nut.
  5. Tighten the nut

    Note: Neither shown nor quite clear to me how he does this as the case is not in the frame and therefore can't be held by the chain and rear brake.

  6. Replace and tighten camplate plunger holder.
  7. Replace and tighten gearbox drain bolt.

High gear and drive sprocket

  1. (Mainshaft main bearing & circlip installed)
  2. Install (57-0946) high gear bearing oil seal w spring to gearbox.
  3. Lubricate and "drive" the mainshaft high gear (57-2316 26T) through the oil seal from inside the gearbox without damaging the seal lip.
  4. Lubricate and insert the final drive sprocket (57-1918, 19T), tapered end first, over the sleeve of the high gear.
  5. Install new tab washer and hand-tighten the large BS 1 1/4" nut (57-0440).
  6. Connect the final drive chain.
  7. Apply brake to tighten the nut (50lbs torque, WS-M).

Install mainshaft into inner cover HH

  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).

Camplate and gear cluster WSM

  1. Install camplate plunger, spring, and holder.
  2. Lubricate camplate spindle and introduce into gearbox with the camplate plunger engaging in the notch between 2nd and 3rd gears. (Figure D12, not Fig. D14 as specified in the WS-Man).
  3. Install the left thrust washer w grease - grooves towards layshaft.
  4. Lubricate all moving parts in the gear cluster.
  5. Assemble the entire gear cluster, including shifter forks and rollers, and introduce into the gearbox.
  6. While sliding the two shafts into their left-hand bearings, move the gears in and out to position the rollers into the camplate groove.
  7. Lubricate and slide in the shifter fork spindle and seat it in the case.

Camplate and gear cluster HH

  1. Use grease to hold in place the inside thrust washer (grooves towards gear cluster).
  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.
  4. Lubricate layshaft and fit layshaft 2nd and 3rd gears.
  5. Insert layshaft complete with 2nd and 3rd gears into the gearbox and into the closed-end needle roller bearing.
  6. Place roller with grease on layshaft gear selector fork.
  7. Fit the layshaft gear selector fork to 2nd and 3rd gears, locating the roller in the camplate.
  8. Lay the mainshaft gear selector fork behind the layshaft gear selector fork and locate the roller in the camplate.
  9. Insert the selector fork rod through both forks and all the way home into the gearbox case.
  10. "Fiddle" the mainshaft 2nd gear in and engage the selector fork.
  11. "Fiddle" the mainshaft 3rd gear in and engage it with selector fork.
  12. Fit layshaft low gear.

Gear Cluster Hayes

  1. Lubricate camplate spindle and install camplate in gearbox.
  2. Position camplate so that camplate plunger will engage the camplate BETWEEN 2nd and 3rd gear notches.
  3. Install camplate plunger, spring, fibre washer, and nut.
  4. Check and maintain camplate position.
  5. Install brass thrust washer over protruding (closed end) layshaft needle bearing with grooves facing into the gearbox. Hold in place with grease.
  6. Assemble the gear cluster complete with fork selectors and rollers and install into gearbox as a unit.
  7. Lubricate and insert the selector fork spindle through both fork selectors and all the way home into the casing.
  8. Using grease, install 2nd thrust washer over needle roller bearing in inner cover.

Inner cover WSM

  1. Install the right-hand thrust washer with grease - grooves facing in towards the left-hand thrust washer.
  2. Apply joint compound.
  3. Push on the inner cover, stopping 1/4" away to "position the camplate quadrant in to the middle point of its travel and push the cover fully home".
  4. Installed this way, "the middle tooth of the quadrant should align with the mainshaft center line".
  5. Check shifting.

Inner cover Hancox

  1. Fit inner cover thrust washer with a dab of grease.
  2. Coat inner cover and gearbox faces with joint sealer
  3. Lubricate the mainshaft, which is fixed in the inner cover.
  4. Fit inner cover with mainshaft to the gearbox.
  5. Hancox taps cover on with quadrant on gearbox floor until it touches.
  6. Then, taps back the cover slightly to free the quadrant, and dips the quadrant (rotates it clockwise) ever so slightly until it just engages the camplate.
  7. Finally, tap the cover home.

Inner cover Hayes

  1. Apply joint compound to casing and inner cover.
  2. Confirm camplate plunger is between 2nd and 3rd gear camplate notches.
  3. Slide casing on while correctly positioning camplate quadrant with the camplate.
  4. Install two screws and bolt holding on the inner cover.
  5. Reconnect oil pipe junction with new gasket.
  6. Temporarily install outer cover, check for proper gear operation.
  7. Remove outer cover.
  8. Install the kickstarter assy (washer, spring, sleeve, pinion, ratchet).
  9. Install a new tab washer (57-2240) and the 21-0594 retaining nut.
  10. Using brake to hold the mainshaft, torque the nut to 45lb, but don't over tighten otherwise it may distort or break the kickstarter pinion spacer.
  11. Replace clutch rod.
  12. Replace right-hand engine mounting plate.

Outer cover WSM

  1. Install kickstarter washer, spring, pinion, ratchet, and washer. Then put on nut with blue Loctite and torque to 45lbs with brake locked and engine in 4th gear.
  2. Refit outer cover.


Outer cover Hayes

  1. Apply joint compound to inner/outer cover matching surfaces.
  2. Pre-tention the kickstarter and slide on outer cover.
  3. Replace outer cover screws and bolts.



Finish up Hayes

  1. Install a new oil seal (70-7565) in the chaincase inner cover plate (open lip/springs face primary chain case).
  2. Fit a new joint washer (70-3832).
  3. Install inner cover plate being careful not to damage oil seal lip.
  4. Fasten inner cover plate with 6 counter-sunk screws (70-3821).
  5. Install transmission.
  6. Replace foot rest, exhaust pipe and muffler.