The Austin Seven Gearboxes.

Note - All dimensions are in imperial, e.g. .001" = one thousandths of an inch.

The Austin 7 gearbox is generally reliable. Three speed boxes can become very noisy as the teeth wear and some will jump out of gear (mainly 2nd), often caused by faulty adjustment of the selectors. Many articles will give instructions on dismantling the box with the advice to 'Inspect all gears, shafts, bearings, selectors etcetera for wear' without giving advice on what actually constitutes 'wear', which can lead to unnecessary replacement of parts while others needing replacement are missed.

The three speed 'crash' box is an early design, fitted with inch series bearings which are no longer readily available from the major bearing manufacturers and a good search will be needed to obtain bearings from reputable manufacturers such as SKF or R&M. The selector rods and forks rarely wear out.

An initial check should be made of the teeth on the 1st and 2nd sliding gear engagement parts of the three speed box and the mating parts of the cluster gear. Any non-parallel tooth wear will 'wedge' and force the gears apart. Also check for breaking-up of the case hardening on the teeth and if any are pitted they should be replaced. Usually one tooth shows this effect first, followed by the rest.

The three speed box can be modified for sporting use by being fitted with 'close ratio gears' and a lightened engine flywheel, but keep in mind that this 'close ratio' box conversion can be quite expensive. Always use new ball bearings when overhauling.

The three speed clutch release bearing doesn't wear badly and should be carefully dismantled, washed, checked for roughness or damage, then packed with a good quality multi purpose lithium base grease. If the bearing is damaged, a rebuilt clutch assembly is available from Spares.

Before embarking on any major repairs to the Austin three speed box it is strongly recommended that you purchase a copy of 'The Austin Seven Manual' by Doug Woodrow, published by Mercury Publications and follow the very detailed instructions in section C1. If you are at all in doubt about being able to do the job, have it rebuilt professionally.

With care it is possible to quickly tell if it is worth proceeding any further with major repairs to the gearbox after removing the lid, concentrating on 2nd and 1st gears looking for excessive wear on the pitch line (½ way up - or down the tooth face) and for parallel wear on the teeth.

In 1932 there was a brief introduction of a four speed 'crash' box, quite different to the three speed box, but this was only used for some 10 months and is fairly robust, with double helical gears. Unless it is very noisy leave well enough alone.

The four speed 'synchro' box is different in design to the both the three speed and the four speed crash box. Like these other boxes, spare part problems are not readily available, although the coupling adaptor and synchro rings off some later Austin boxes up to about 1954 were interchangeable. New synchro drums can now be made.

The synchroniser assembly fitted to Austin 7 Gearbox is generally known as the 'constant pressure' type. By the later 'baulk ring' standards these early synchronisers are not unbeatable, they were an improvement on no synchro at all. With a three speed 'crash' box it can take up to 3 to 4 seconds to execute a quiet and smooth change of speed, but with four speed synchro it can be done effortlessly in 1 to 2 seconds.

If the gears are handled properly the synchro assemblies in any box should give a good life, but drivers often don't change gears correctly. In the Austin 7 synchro box it is preferable to drive as if there were NO synchro at all by 'double de-clutching' as you would with the crash box. The synchro will be there 'to assist in the noiseless changing of gears' and used to adjust any small discrepancies in speed between the two mating parts.

As noted the Austin 7 synchro is not as efficient as that fitted to more modern manual gearboxes and it not only has to speed up or slow down the gear you are changing to, it also speed up or slow down the laygear, 3rd gear, the main drive gear and the clutch plate which are all connected together. When any one of these parts it turned, all the rest rotate as well, thus it is obvious that the synchro does a difficult job, especially when called upon to do it instantly.

To check for wear on the synchro of an Austin 7 box, the bronze cone should be mated to the synchro hub (not while assembled to their shafts) and checked for 'rocking'. Ideally there should be none and in fact, only a small amount renders the synchro ineffective. The only remedy is new cones or a cone from another box.

Check also that the cone does not bottom on the synchro hub. If it does this can be machined back by the requisite amount and rechecked for "rocking". Of course if the cone is so far worn that the synchro hub rubs against the Coupling Adaptor then a new cone must be fitted. When checking for wear, the endfloat of 2nd and 3rd on the mainshaft should be between 002" - 004", however, up to .015" is still acceptable.

As with the three speed unit, the four speed box can be modified to be quick to change gears by fitting 'close ratio gears' and a lightened flywheel, although again the conversion is best carried out by someone experienced in gearbox work.

As a general rule car manufacturers keep their clearances tight in order to eliminate running noise. The tighter the clearances in the gearbox (and the engine) the quieter it will be, but keep in mind this will absorb more horsepower. As you might know, a racing motor always has loose clearances and is correspondingly quite noisy when compared to a road car. Thus it does not follow that loose running clearances will cause unreliability, but generally we should strive for the lower range of clearances wherever possible.

The mainshaft front spigot needle roller bearing usually needs replacement, which if the spigot is not worn can be replaced with a bronze bush in place of the needle roller. If the spigot is damaged the best solution is to have the output shaft nose reground and fitted with a special oversize needle roller bearing.

Looking at the problem of other wear in the four speed gearbox, with regard to the 'cluster gear' bushes, the front one is the worst for wearing out quickly. The thrust face is usually damaged and the bore worn. Renew if the clearance exceeds .005". The replacement bushes on the cluster gear, if worn, will have to be made from bronze stock. The thrust section of the front and rear bush is .125" thick and the end float of the cluster should be set at .002" to .005" by shimming the front layshaft housing. The face of this housing is usually scored and should be ground flat, with the front thrust face made thicker to compensate for the metal removed. Set the end float in the case before assembling the box. Many front thrusts wear badly allowing the cluster gear to move forward causing a lot of 'lash'. Sometimes the constant mesh gear on the cluster will even rub on the front bearing. The rear thrust face very rarely wears. When fitting, the bushes must be reamed in line or rapid failure will occur. Clearance should be .002" - .003".

Another solution is to replace the bushes with needle rollers on the shaft and thrust always using a new layshaft, again this is a specialist job.

The bushes for 2nd and 3rd gear can have what appears to be an alarming amount of slop. In fact up to about .005" clearance is acceptable. If the gearbox is known to 'stay in gear' then leave these bushes alone.

Most jumping out of gear is caused by incorrect positioning of the selector rods, which are adjustable, unfortunately meaning they can be mal adjusted. The selector rods govern the placement of the selector forks and in turn the change gears, which if out of place can be forced out of mesh. The longer any mal-adjustment causing the gears to jump out of mesh is left unattended, the more parts will need to be replaced. Initially it may only need a detent spring, but if left then it will need the coupling adaptor and finally the synchro drum and selector fork. The 'teeth' on the coupling adaptor' should be parallel and show wear across at least 75% of the face. Likewise the mating internal spline of the synchro drum should be parallel and not be worn 'wedge' shaped. Check for this wear on all gears - 2nd 3rd and 4th.

A similar check should be made of the teeth on the 1st Sliding Gear and the mating part of the cluster gear. Any non-parallel-wear will 'wedge' and force the gears apart. Also check for breaking-up of the case hardening on the teeth and if it is pitted it should be replaced. Usually one tooth shows this effect first, followed by the rest.

If the four speed layshaft is worn more than .002" and badly grooved, it can be hard chromed or metal sprayed and reground using new bushes. A new alternative is to use a needle roller conversion with a replacement shaft again by John Needham.

It is helpful to understand what happens when a gear is engaged. It is very important to make sure that the synchro drum engages the coupling adaptors of 3rd and 4th gears by an equal amount. This is fairly easy to see but is a bit harder to arrange for 1st and 2nd gears. Here all that can be done is to make sure that the 1st slider gear fully engages the coupling adaptor of the 2nd gear. Incidentally, on 2nd gear the coupling adaptor is an integral part of the gear and cannot be replaced separately.

The reverse gear is adjusted forward until it just touches the 1st gear when in neutral position; it is then backed off 1 1/2 to 2 turns i.e. about .060" to .080". Inspection shows that when engaging 1st gear the stationary reverse idler is also 'picked up'.

Always use new ball bearings when overhauling the gearbox. The front ball bearing with grooved outer ring and snap ring is similar to that originally fitted to the some 1960 Australian 3 speed manual gearboxes, although the original Austin Seven 4 speed box snap ring is narrower. Check to make sure that any replacement snap ring fits the casing correctly.

The front roller bearing on the Main Drive Gear is often in fair condition, but if not it should be replaced.

To improve the oil sealing it is a good modification to machine the front and rear covers to take a lip type seal as with the rear housing on the 1938 gearbox.

The four speed box selector rods and forks should be checked for wear. The forks can be carefully 'closed up' in a vice to eliminate any slack here, up to .030" play is acceptable, but less is better. If necessary new selector rods can be made.

Instead of paper gaskets it is usually preferable to use a light coating of Silastic or similar gasket cement, also seal the threads of the bolts and selector rods as oil leaks down the root of the threads. Do not use excessive gasket cement which can get into the box- remember two tubes of gasket cement do not seal better than one!

The four speed clutch release bearing usually lasts well and should be carefully dismantled, washed spotlessly clean, checked for roughness, then packed with a good multi purpose lithium base grease.

With care it is possible to quickly tell if it is worth proceeding any further with major repairs to the gearbox after removing the lid, concentrating on the coupling adapter teeth. If they are not parallel and engaging evenly (75%) then it will jump very likely out of gear.

As with the three speed box, before embarking on any major repairs to the four speed box, it is very strongly recommended that you purchase a copy of 'The Austin Seven Manual' by Doug Woodrow, published by Mercury Publications and follow the very detailed instructions in section C2. If at all in doubt, have it rebuilt professionally.

The original Austin Seven handbook recommendation is to use Engine Oil in the gearbox (as with many other Austin vehicles). This is usually SAE 30 viscosity oil, but it is suggested that in warmer climates a thicker oil will give better lubrication, cut down noise and not leak quite so easily. Non EP SAE 90 Gear Oil has the same viscosity range as SAE 50 Engine Oil and therefore complies with the instruction to use engine oil, but gives the protection of a good viscosity gear oil. Always remember it must not contain any sulphur/phosphorous extreme pressure additives, which could attack the bronze bushes, so no EP oils!

Bearings for Gearboxes

1923-32 three speed gearbox -

• Clutch release bearing Hoffman N 2400
• 1st motion/input shaft RHP LJ 1 ¼"
• 3rd motion/output shaft RHP MJ 1"
• Layshaft. RHP MJ ¾"

1933 spd crash box bearings.

• 1st motion shaft/input SKF 6207
• 3rd motion shaft/output RHP MJ 1"
• The layshaft has plain bronze bearings.

1933-39 4 speed synchro gearbox

• Clutch release bearing Hoffman N 1307
• Input shaft ring/rollers R&M X 1254
• 1st motion/input shaft SKF 6207 ZNR (1095)
• 3rd motion/output shaft SKF RMS 8 NR
• The layshaft has plain bronze bearings.