4 to 5-speed Gearbox Conversions, Assembly Hints

I first wrote the following information sheets in 1989 and have edited them a number of times since. I hope they are interesting and helpful.

Instructions and suggestions for fitting a 5-speed gear cluster to 4-speed unit construction crankcases.

There are two modifications to make to a 4-speed gearbox casing to allow the fitting of a 5-speed gearcluster.

Firstly, the shield wall of the main casing, behind the sprocket main bearing must be opened up to 1-11/16", or slightly more. This is to allow the sleeve gear (also called the high gear) to fit the new roller bearing correctly. It is also worth checking that there will not be any foul between the outer face ends of the sleeve gear teeth and the case when the gear is in position. If needed, rectify by the careful use of a rotary burr on the casing.

Secondly, the shield wall behind the mainshaft bearing in the inner cover needs opening up to 1-1/8". This is to allow the nose of the combined mainshaft 1st and 2nd gear to abut the bearing. It is this gear, and the arrangement of the kickstart ratchet, that positions the mainshaft. It is permissible to remove all the kickstart equipment and replace with a spacer of sufficient length to ensure that a tightened nut at this end of the mainshaft locates it firmly against the inner gearbox cover bearing, if a kickstart is not required. Special spacers are available from Triple Cycles that have the spline included to allow continued use of the lockwasher.

Two types of 5-speed camplate were produced by Triumph. They are fully interchangeable in service. The earlier type is "fully round" and must fitted before the sleeve gear. The late type, the "low inertia" camplate, has excess material removed from the periphery and has "ears" at the end of the index plunger track. One of these ears was used in the T160 Trident to operate the neutral switch. If an eared camplate is being fitted to an earlier Trident or Rocket lll crankcase, a small notch will have to be cut in the inner edge of the inner cover joint of the main crankcase to allow the camplate to fully rotate. There may also be slight foul conditions if this camplate is used in earlier and pre-unit gearbox casings. Any interference can be easily modified by removing material from the casings or the camplate.

It is always advantageous to use the latest type of T160 Trident index plunger, plunger housing, and spring, when the low inertia camplate is used. This plunger and plunger housing is the longest types produced, and the plunger has a rounded nose. This assembly gives the sweetest action to the selector mechanism, however it seems the fully round camplate operates best with a pointed plunger.

It is obvious that the gearbox inner quadrant needs changing for a 5-speed component, but changes that are required for the footchange quadrant and plungers are often overlooked. In a 5-speed arrangement both the footchange quadrant and plungers are modified from their 4-speed predecessors and failure to use these components will result in over or under selection. The 5-speed footchange quadrant can be visually identified by having extra material left on it, compared to the 4-speed item, in way of the positive stop. This is because of the reduced travel required between each gear in the 5-speed selection arrangements. The 5-speed footchange plunger has a larger cutout than its 4-speed counterpart. The top edge of the pawl is much nearer to the diameter of the plunger than in the 4-speed component, where the top edge of the pawl is closer to the circumference of the plunger.

Instructions and suggestions for fitting the fixed gears to a Triumph 5-speed layshaft.

When fitting the fixed (top) gear and spinning (fourth) gear to a layshaft, it is possible that various machining operations will have to be carried out, to ensure correct positioning of the gears on the shaft, and the shaft in the gearbox.

In a Triumph gearbox the length of the layshaft assembly, between the outer face of the first gear driving dog and the outer face of the top gear pinion must be such that a small amount of end float exists, between the thrustwashers, when the inner cover is in position. From experience it seems a good rule of thumb length is 4.907", but this may vary for any particular gearbox shell.

The layshaft is located by the circlip in the top gear pinion and the circlip situated between the first gear pinion and the first gear driving dog.

So, the first step to layshaft assembly is a "dummy run". With the top gear pinion circlip fitted, and the spinning gear fitted to the layshaft, carefully push the top gear on to the splines, until the circlip abuts the end of the splines. Be careful not to push too far or too hard, as it is very easy to broach the circlip with the splines. Fit the first gear dog circlip and dog and measure the overall length. This will then allow calculation of the amount the layshaft has to be "shortened". This material may not all have to come from the layshaft itself because a clearance has to be maintained between layshaft fourth and layshaft top. Fifteen thou (0.015") seems to be a suitable figure.

The effective length of the shaft can be shortened in three ways depending on how the shaft has to move to adjust the clearance between top and fourth: Grinding the end of the spline that the top gear abuts, this will close the clearance between top and fourth gears, and shorten the layshaft by moving it further in to the top gear. Surface grinding the outside face of the top gear will shorten the layshaft assembly without altering the clearance between top and fourth gears. Surface grinding the inside face of the top gear will increase the clearance between the top and fourth gears without altering the layshaft assembly length. A combination of the above may be used.

Ideally, if this operation is being carried out, and the gearbox in which the gears are to be fitted is available, it is very worthwhile to make a further check before deciding where to modify the above components.

When the layshaft is in the gearbox, and first gear is selected, the gear itself moves into engagement with the driving dog. The layshaft should be so positioned that the first gear does not contact the circlip, which is positioned between first and the driving dog. A measurement taken between the circlip and the first gear during the dummy assembly will allow accurate decisions to be made as to where and how to modify the layshaft and top gear to obtain the perfect fit.

Early 5-speed layshafts are different to the description given. There was no circlip inside the layshaft fixed (top) gear, and the splines on the layshaft underneath this gear were longer (when compared to a later layshaft) These layshafts were used in some 1972 and early 1973 models. If later gears are fitted to these earlier shafts then the splines will have to be considerable shortened. Gearbox Assembly

Instructions and suggestions for fitting a 5-speed gear cluster to a 4-speed pre-unit gearbox.

There are further modifications to be carried out to a “pre-unit” gearbox shell, in addition to the modifications that are common to those needed for “unit” construction gearboxes.

Firstly, if a late camplate (the "Low Inertia" camplate) with "ears" is to be used, the shell will have to be relieved inside, to allow full rotation of the camplate. A fully round camplate may or may not require the operation. Judicious use of a rotary burr will have the desired effect.

Secondly, it is necessary to machine a little material from the inner surface on the inner cover, around the layshaft thrust washer. In a 4-speed gearbox, the 1st gear on the layshaft has a very short, small diameter nose that abuts the layshaft thrust washer. The 5-speed driving dog that now sits in this position is flat and larger in diameter. In the original machining the thrust washer is slightly below the surface in the inner cover. It is only necessary to remove 20 thou or so of aluminium for a diameter slightly in excess of the flat surface of the driving dog.

Thirdly, it is worthwhile making two alloy plugs. One for the speedo drive and one for the camplate spindle bush, this latter one to be quite shallow.The 5-speed camplate, being meant for fitment in the "blind" hole of a unit crankcase, has a flat on it to allow the escape of trapped air. In a Pre-Unit application it would allow the escape of oil. The speedo drive plug must be machined to replace the brass speedo gear housing, and the thread of the outer cover screw near the new plug may require to be re-tapped after fitting the blank. This method has the extra advantage of holding the blank in position.

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Triple Cycles
95 Wanstead Park Road
Telephone: +44 20 8215 4583 (answerphone only, unless by arrangement)
Facsimile: +44 20 3137 2295

Copyright © 2000 Philip Pick
Version 0.1 Date January 2001