THE AUSTIN 7 THREE SPEED GEARBOX

(Adapted from an article by Mike Phelan which appeared in the Pre War A7 Club Newsletter for Sep 93)

There have been many texts written on the overhaul of the Austin Seven engine, brakes, steering and most other bits of our favourite car - but the gearbox has been somewhat left alone. There is, however, good reason for this - it gives little trouble and continues to give good service, even when absolutely worn out! Gearboxes rarely fail catastrophically. Nowadays, the supply of good secondhand gearboxes is slowly diminishing, so we are having to rebuild them, maybe using parts from several units.

Most of us will strip an engine with few qualms but steer clear of gearboxes and differentials, which have fewer parts than an engine! This is perhaps because of a lack of understanding of how the gearbox works so I make no apologies for giving a brief elementary description of this: after all, we have a steady influx of new members to cater for. I crave patience from our more experienced members.

General Design and Operation:  Design

The unit is a fairly conventional 3-speed sliding mesh gearbox with either a ball change or a gate change on earlier units. The gate change boxes have a ball joint at the base of the gear lever, the function of the gate being to restrict movement and to provide a reverse stop. Ball races are used on the four main bearings, bronze bushes elsewhere.

Principle of Operation

The gearbox, like most, has four shafts, viz:

1. A first motion or input shaft that is driven by the engine via the clutch. It has an integral constant mesh pinion to the layshaft (see below).

2. A mainshaft that delivers output to the propeller shaft and is locked to the first motion shaft to give top gear, which therefore has a 1:1 ratio. The mainshaft is splined and carries two sliding gears.

3. A layshaft, situated at the bottom of the gearbox, permanently driven by the first motion shaft. It, in turn, drives the mainshaft through one of the sliding gears, except when top gear is selected.

4. A reverse idler to change the direction of rotation. This meshes permanently with the reverse pinion on the layshaft.

The two sliding gears on the mainshaft are moved by selector forks, and the Seven uses a rather unconventional method of moving these. On most cars, they are moved by the end of the gear lever. On the Seven, they are moved by a sliding bar with a hole into which the gear lever end fits. This gives a slight leverage and a shorter travel on the gear lever. The sliding bar ends are stepped and fit under two notched bars screwed into the gearbox case (see Fig 2) This provides a very necessary part of any selector mechanism, an interlock to prevent more than one selector fork moving at a time. This, if it were to happen would lock the box solidly and probably cause sever damage to the car, and possibly its occupants!

The interlock operates by having steps on the sliding bar ends that prevent the bar from moving the forks until it is at the extreme left or right of its travel. In these positions, one of the stepped ends fits into the notched bar acting as a fulcrum, the other end being clear and free to move. On gate change boxes, the gate also acts as an interlock.

For first and reverse gears, the sliding first gear on the mainshaft is moved into engagement with either the first gear pinion on the layshaft, or the reverse idler.

For second gear, the sliding second gear engages the layshaft second gear pinion.

For top gear, the face of the sliding second gear carries four dogs that engage similar dogs on the face of the constant mesh pinion (see Fig 1), thus locking the mainshaft to the first motion shaft. The gearbox is now effectively 'out of circuit'.

Gear Changing

When we are in, say, first gear, doing 15 mph, the engine is rotating at some 3000 rpm and the layshaft at half this speed. If we were doing the same speed in second gear, the engine would be running much slower and the layshaft rotating at only 865 rpm. So, therefore, to change upwards silently, we must slow the layshaft down to somewhere near this speed whilst in neutral so that the two second gear pinions are rotating in step. As soon as we go from first to neutral, this starts to happen but the rotating mass of the layshaft, first motion shaft and clutch driven plate has considerable inertia and would take some time to reach the required speed. The engine slows down much more rapidly than this so we briefly release the clutch whilst in neutral to allow the engine to slow the spinning layshaft.

It is not quite so simple in practice as there are some variable such as oil temperature and grade to be taken into account. Cold gearbox oil slows the shafts down almost immediately but on a hot summer day, upward changes can take several seconds. Also, if you are on an upward gradient„ the car loses road speed while you are changing gear so the layshaft slows down even more. Conversely, road speed increases during a change on a downhill stretch, so that upward changes can be performed more quickly.

To change down, the opposite rules apply. The layshaft must be speeded up, so we rev the engine as we release the clutch in neutral, or just hold the throttle open moderately. More revs if going down hill, less if going up as the road speed helps to match the gear speeds.

Gearbox Data

Much of the data published on gear ratios etc are slightly incorrect. This is rather surprising as calculating gear ratios simply involves counting the teeth and performing some very simple arithmetic. Many of the errors have been perpetuated; as is often the case. William's book on Specials [Ed see A7 Library, Jan 93] gives the ratios for the three speed box to 2 decimal places, but there are slight errors, even allowing for he axle ratio being stated as 4.9 rather than 4.89 (44/9). I am not trying to be pedantic here, but I think if figures are stated to an accuracy of 2 decimal places, then they should be reasonably accurate.

As for the rear axle ratio, we always say 5.25, 5.125 and so on. So why not 4.89 instead of 4.9? The ratios given in 750 MC 'Co

mpanion' are correct. These data assume a 4.89:1 rear axle ration and a tyre size of 3.50 x 19. This gives 778 turns per mile of the rear wheels. Incidentally, ratios for four­speed boxes tend to be mis-quoted sometimes. The early 32/33 crash box has slightly different (wider) ratios from the later synchro boxes.

Close Ratio Conversions

Users of the 3 speed box will know only to well the wide gap between 1st gear and 2nd. Conversions to reduce this gap have been available for many years. To convert to close ratio, it is necessary to make a new first motion shaft and lay shaft constant mesh gear with a reduction ratio less than the standard 2:1 (28/14). If the same pitch of teeth is used, then the total of the teeth on the two gears must be 42¹, (the answer to life, the universe and everything). Say you made new gears of 16 and 26 teeth; this would give you a first gear of 12.91:1 and a second gear of 7.22:1. As making a first motion shaft would be extremely expensive, most conversions just machine off the old gear and press fit a new one.

 The sum of the tooth counts for each of the three pairs of gears must be the same, if the teeth are to have identical pitch. The three gears used for reverse do not obey this rule as the reverse idler can be positioned to provide the correct meshing depth.

 PROBLEMS  

 These gearboxes, as was mentioned last month, are fairly trouble free. Unlike the later synchromesh boxes, the mainshaft gears do not rotate on the shaft so there are no bushes to wear. The main problem is noise, due to the rather course straight-cut gears.

Remember that even when new, there would have been considerable whine in the indirect gears. This can be alleviated by using a thicker oil than the recommended engine oil. I use SAE 50 engine oil in mine, although thicker oils than this (such as SAE 140) can be used with no apparent damage. If you use your car in winter, though, using very thick gearbox oil will make changing upward difficult, for reason mentioned earlier. Avoid oils with sulphur-based additives, designated 'EP' (extreme pressure) although not all EP oils contain sulphur. Sulphur additives will attack any bronze~selector forks and bearings), making your oil look like gold paint!

Very severe damage or wear will result in jumping out of gear although this can also be caused by seized or broken selector springs. The gearbox tops, both types, wear rather badly as the steel gear lever ball runs directly in the alloy casting . The ball change top has the lever retained by a threaded ring and the hole in the top sometimes wears so badly that the lever comes out! The slot in the ball also wears, but this can be built up by welding. There is definitely a need for someone to start casting tops of both sorts, or is someone already doing this?

 REMOVAL   

Unfortunately, on all cars with three-speed boxes, the engine will have to be removed, either with or without the gearbox first. On earlier cars without a dummy spline, it is easier to lift out the engine separately, then remove the gearbox. If you remove both together, it will be necessary to remove the cotter from the clutch pedal to allow the later to rotate clear of the floor, having first removed the pedal top.

 DISMANTLING        

Before doing anything wash the gearbox externally then drain the oil. Collect some old containers to put small parts in. You will need four small boxes for the selector shims and a marked piece of wood with four nails for the other shims. Mark the boxes as shown in the table below:

Warm the gearbox casing with a fan heater, or use a hot-air gun but be very careful. This makes removing the bearings much easier. When taking the front cover off, turn the box front uppermost, so that the our sets of shims do not fall out. These must be put in the marked containers unless you want to start from scratch when re-assembling. Granted, we are assuming that the box has not been stripped before - true in a surprising number of cases. Some shims may be stuck to the cover. There are large shims on the two ball races and small ones about the size of a 5p piece, on the selector rails. The latter may remain in the end of the bore. The layshaft rear bearing has a pressed steel disk on top of the 'N shims. This is a distance piece and care must be taken during reassembly not to replace it in the front bearing in error. Take out the four screws holding the notch bars; you may need an impact driver for this. The selector bars will then be free and can be removed.

Take out the speedo drive bush and pinion if present. Remove the rear cover, complete with mainshaft which will slide out. Again beware of shims - four lots again!

The selectors can be removed, but carefully! Push each rail rearwards and continue until it is flush with the end of the selector fork. Then put your thumb and finger over the fork end and the small hole in the top and pull the rail out. The steel ball will now be loose inside the fork and can be retrieved, possibly with aid of a magnet. Repeat the process with the other fork. As the rods are identical, mark then so they can go back in the same positions. The springs can be pulled out with a wire hook.

The two sliding gears can be taken out now and a brass drift used to remove the first motion shaft. The layshaft is a little more difficult. The gear is in three parts, retained by two keys which will not pass through the bearings. To remove, tap the shaft rearwards until the rear bearing is free of the case. Turn the box on its front and place two pieces of brass or alloy strip between the bearing and case. Drift the layshaft back into the box and remove the bearing. Continue to drift the shaft out forwards, retrieving the two keys. The longer one is invariably broken in two pieces. The reverse idler shaft can now also be drifted out.

Remove the circlip from the mainshaft, remove the shaft, bearing and seal plus the speedo drive worm if fitted.

Examination  

Wash all parts off in a degreasing fluid or paraffin followed by petrol.   Examine the gear teeth for corrosion or pitting that has penetrated the case hardening. The engaging edges of the gears should be chamfered, but look for damage and chipping which will cause noise and jumping out of gear.

 The layshaft is frequently bent but replacements are inexpensive. The keys will need replacing, too, but the long one probably breaks in the first 1000 miles! Bearings do not wear much, replacing them may increase noise by changing the depth of engagement. Give the four ball races a final wash in clean petrol and check them for wear and roughness. If you are going to re-use them, oil them with engine oil and wrap in paper to keep the dust out until you re-install them. Any felt seals will need replacing with new ones soaked in molten tallow. The spigot bearing in the First Motion Shaft sometimes wears slightly.

 The selector forks tend to wear where the sliding bar engages them. Building these up with bronze will improve the change considerably. The notched bars can also wear, and can be built up with weld. Do not, however, give the final finish until the box is being re-assembled, as they need to be fitted exactly.

When replacing gears with those from another box, replace them in pairs if possible, or you may end up with a noisy box. Remember to include the reverse idler, and that it rotates all the time that the clutch is engaged. A severely worn idler can produce a surprising amount of noise, even in neutral. Ensure that all the ball races are tight, on the shaft and in the case. Clear out the spring holes in the selector forks with a drill or reamer, as they tend to burr. Throw the old selector springs away.

 Reassembly with Original Shims       

If you are not replacing all the shims in their original positions, refer to the next section for details of how to set up the gearbox by re-shimming and this section for detailed assembly instructions.

 The first thing to do is to place all the layshaft gears in the bottom of the box, then to feed the layshaft from the front, not forgetting to fit all the keys, which can be stuck in place with grease. The long key goes to the front. It sometimes helps if the first 1/8" or so of the keyways in the gears is chamfered with a needle file. This avoids the sharp edge planing a shaving off the key. When the layshaft is fully in position, fit both bearings after warming the casing up again. Use a tubular drift, with a block of soft metal supporting the other end of the shaft. An extra pair of hands is useful here. Tap the rear bearing in the furthest, to allow room for the distance piece.

 Replace the first-motion shaft and bearing, and temporarily fit the mainshaft into the two sliding gears. Place both selector forks in position, noting the markings that you made on dismantling. The selector shafts will then need fitting, and this is a slightly awkward job, due to the fact that the springs need compressing.

I find that the best way to do this is to make up a special tool consisting of a piece of bar as in the drawing opposite.   To use it, put the spring and ball, heavily greased, into position and insert the tool into the front of the fork bore until the end just covers the hole. Turn the tool a quarter turn; this will compress the spring.   Fit the shafts into position as follows. The 2"d/3' shaft is the one with the grooves unequally spaced. Look for the wear on the grooves caused by the ball and fit the shaft so this wear is at the top. Push the shaft in till it meets the tool, then give a tap with a hide mallet or soft drift whilst holding the tool. Once the shaft has gone in past the hole, remove the tool and tap the shaft home until 1/8" or so short of the front face of the gearbox.

 Turn the gearbox front uppermost and fit all the original shims in position, then the gasket and the front cover. Turn it over, remove the mainshaft and assemble the latter with seal, bearing and speedo worm to the rear cover. Refit the circlip and close with a vice. Tap the selector rails forward and do the same to the layshaft bearing. This ensures that there is no looseness in the shims at the front. Refit the rear shims, the distance piece on the layshaft and the gasket. Refit the rear cover and tighten the bolts.

 Reassembly with New Shims 

In an ideal world, if we replace all the shims in the positions that they came from, all will be well. Unfortunately, if the box has been stripped before, or any parts swapped, it will be necessary to re-shim the box. To do this, you really need a selection of shims as well as those fitted originally, but it may be possible to just use the original shims. The process must be carried out in the correct order, as most of the shims affect more than one thing. Like any similar process, the aim is to finish with the adjustment that only affects one item.

What are we aiming for?  Full engagement of the reverse idler and the layshaft reverse pinion.  Full engagement of 1S`, reverse and 2"d gears, and equal clearance in neutral. Correct engagement of 3rd gear dog clutch.

 Full engagement of the constant mesh pair.

Procedure: The box is assembled as in the previous section, except that the covers are left off the mainshaft and the first motion shaft, and the balls and springs are left out of the selectors. This is so that the gears can be engaged without disturbing the positions of the selector rails. Ensure that the layshaft is likewise pressed hard against the ends of the gear cluster and that the first motion shaft bearing is likewise fully home. Check that the selector rails are a firm fit in the gearbox case and cannot move easily.

 1.         Put the 1st/reverse selector in the reverse gear position (back) and pop the ball, with some grease on it, into the top hole to centralise it.

2.         Tap the selector rail either way, holding the ball in place, until the sliding gear and reverse idler appear to be fully engaged.

3.         Tap the layshaft bearing (not the shaft) with a hollow drift until the reverse pinion on the layshaft engages fully with the idler.

4.         Return the selector to neutral and check that the clearances either side of the 1st/reverse sliding gear are equal and at least 0.1". If it is too near the layshaft first gear, move the layshaft; if to near the reverse idler, move the selector rail.

5.         Repeat adjustments 2, 3, and 4 until correct.

6.         Place the 2"d/3' selector in neutral and stick a ball in the top hole with grease, as before.

7.         Tap the first motion shaft bearing outer race, not the shaft, until the constant mesh gears engage correctly and there is at least 0.1" clearance between the halves of the dog clutch.

8.         Tap the selector rail to give 0.1" clearance between the sliding second gear and the corresponding layshaft pinion.

9.         Repeat 6, 7 and 8 until correct.

10.       With a straight edge and feeler gauges, measure the depth of the two front ball races below the gearbox face and write these down. Do the same with selector rails (front only).

11.       Remove the selector rails and refit the balls and springs (see previous section).

12.       Fit a new seal to the front cover and fit the washer that retains it.

13.       Select shims to the thickness that you noted, plus the thickness of the front cover gasket, and support the box front upwards. Fit the four sets of shims and the gasket, coated in goop if you wish, then bolt down carefully.

14.       Tap the mainshaft gently until it will move no further. Also tap the layshaft rear bearing and the selector rails, the latter only gently.

15.       Measure the depths as you did the front. Add the thickness of the gasket to the figures and from the mainshaft bearing depth, deduct 0,020" for spigot bearing end float. From the layshaft figure, deduct the thickness of the distance piece.

 
Remove the mainshaft and bearing, refit the cover with a new seal and washer. Squeeze the circlip into its groove in a vice - it is very soft, fit the shims and gasket and replace the cover. This completes the shimming process.

 Reassembling the Selector Bars

 Place both selector forks in neutral, then refit the sliding bar, well greased. The notched bars should then fit so that the sliding bar slides freely, with equal clearance either side of the square portions where they enter the notches. Try swapping the bars around for the best fit. If the latter have been built up with weld, the notches can be filed for an exact fit. Clearance should be no more than necessary to allow free movement.

 Replace the screws using an impact driver and punch metal into the slots to lock them, as they were before. Check finally that all gears can be engaged after fitting the top temporarily.

This should be self-explanatory. If the clutch pedal has been removed, make sure the cotter is refitted from the rear. Cotters of this type have a slope of some 10° so fitting the cotter the wrong way round will mean the loss of some 20° of clutch pedal movement - on a Seven this is an extravagance! Note this also when replacing other cotters, particularly those in brake cam levers.

 Lubricate the clutch release bearing with castor oil. This tip was given to me by someone who worked on Sevens during the Twenties and Thirties and seems to work well. The bearing seems to last forever on a three-speed but the four-speed bearings (that do not rotate continuously) are generally noisier.

 If you are fitting the gearbox, then the engine, there are two. points to watch:

First, turn the engine and feel that each toggle lever is in its slot in the release ring before bringing the engine and gearbox finally together.

Second, fit the nuts on the bell housing before sliding the engine back into position, as two of them are pretty well inaccessible afterwards.

The job is easier if the gearbox top is fitted and the gearlever can then be used to manoeuvre the box into position. Do not forget the oil!

(Adapted from an article by Mike Phelan which appeared in the Pre War A7 Club Newsletter for Oct 93 with many thanks)