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.
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