Our club display at events
usually consists of ‘Tourers’; ‘Box Saloons’; the Ruby types and a ‘Big Seven’,
a good across-the-range exhibit. The general public with a casual interest will
only see the different body shapes, colours and size. Anyone with mechanical
knowledge will notice many differences, the result of continued development that
all manufacturers undertake to keep ahead of the competitors.
These easily seen changes
under the bonnet involve the magneto which came and went; the positions of the
starter motor, dynamo and distributor were altered; different carburettors; the
size of the fuel tank was increased and its position moved from scuttle to the
rear; the diameter and position of the lights; the chromed radiator surround
replaced by the enveloping radiator cowl on the ‘Ruby’; the smaller wire wheels
and the change to the pressed steel wheels on the ‘Big Seven’.
Engine
In brief conversation with people having a casual interest we mention that
‘Sevens’ have the 747cc engine (left)
except the ‘Big Seven’ which has the re-designed 900cc engine
(right).
However, making that simple statement
does give the impression that the same engine was used for all models
from the
‘Chummy’ to the ‘Ruby’. There were many internal changes to the engine,
and the discerning enthusiast is able to
look at the outside of an engine and say which ‘Seven’ model it is from.
Capacity
696cc
747.5cc
747.5cc
747.5
900cc
Years
1922
1923 - 29
1929 - 36
1936
1937 - 39
Model(s)
All
All
All
Ruby MK2
Big Seven
Bore – mm
54
56
56
56
56.77
Stroke - mm
76
76
76
76
88.90
Compression
5.25:1
5.25:1
5.25:1
5.8:1
6.25:1
Output - bhp
10
10.5
13
17
25
at RPM
2400
2400
2600
2600
4000
RAC
Rating - hp
7.2
7.8
7.8
7.8
7.99
Crankshaft
Bearings
2
2
2
3
3
Diameter
11/8″
11/8″
#
15/16″
15/16″
++
15/16″
# the diameter was increased to 15/16″
in October 1929. ++ the centre bearing is 1¾″.
The 747.5cc ‘Chummy’ engine
produced 10.5 bhp at 2400 rpm and with modifications to crankshaft and
carburetion in late 1929 the output was increased to 13 bhp at 2600 rpm.
Significant changes came with the 3-bearing, higher compression, ‘Ruby MK2’
engine, introduced in June 1936, supplying 17 bhp. The 900cc engine fitted to
the ‘Big Sevens’ in July 1937, and further modified for the ‘Eight’, has larger
cylinder dimensions and higher compression to deliver 25 bhp at 4000 rpm.
Gearbox
The first gearbox was the 3-speed crash box fitted from 1922 to 1932. The
4-speed box came in September 1932, still without synchromesh. ‘Twin-top’
synchromesh was introduced on 3rd and 4th gears in August
1933, and the 4-speed box with synchromesh on 2nd, 3rd and
4th came in July 1934, the gearbox being marked ‘2 SYN’. The overall
ratios were also changed. The ‘Big Seven’ gearbox is also ‘2 SYN’.
Carburettors and
Electrical equipment
The detail of the changes is more the interest of the expert rather than for
casual conversation. Austin fitted Zenith carburettors throughout the
production of ‘Sevens’ except for some cars in November 1930 were fitted with
Amal. CAV electrical equipment (dynamo, cut-out and switch panel) was fitted
from 1922 to 1925 with Lucas equipment fitted from 1926 to 1939.
Wheels and tyres
From 1922 to March 1926 ‘Staybrite’ centre wire wheels were 26” with 3½” tyres,
and were changed to 19” with 3.50 tyres until March 1933 when 17” wheels with
4.00 tyres were fitted until the end of the ‘Ruby’ production. When the ‘Big
Seven’ was introduced in July 1937, it was fitted with pressed steel ‘Easy
Clean’ 16” wheels with 4.75 tyres.
Wheelbase and track
From 1922 to October 1931’Sevens’ have a 6’ 3” wheelbase with 3’ 4” front track
and 3’ 7” rear track. The wheelbase was then increased to 6’ 9” with the track
at 3’ 4” front and 3’ 7” rear. The Big Seven has a 7’ 3” wheelbase with 3’ 7⅛”
front and 3’ 9” rear tracks.
Wing, Running Boards,
Windscreens and Body Lights Throughout ‘Seven’ production changes were made to front and rear wings and the
running boards. Between 1922 and 1932 there were tenstyles of the rear wing,
fifteen of the front wing, and twelve running boards. From 1923 to 1937 there
were sixty-one different windscreens and body lights, ie side and
rear windows. (Photos: left - 1934 AJ Tourer:
right
- 1935 Ruby MK1)
Weight
As ‘Sevens’ were developed and the length increased, along with all the changes
to engines and ancillary equipment, the weigh of the cars increased. The Tourer
‘Type AD’ of February 1926 weighs 8cwt 56lbs (406.41kg). The ‘RP’ Box Saloon of
October 1932 is 10cwt 2qtr (533.42kg) with the ‘Ruby’ models from July 1934
weighing 12cwt 14lbs (615.97kg). The ‘Big Seven’ ‘Sixlite’ of July 1937 is 14
cwt 2qtr 20lbs (745.70kg); with the ‘Forlite’ in March 1938 at 14cwt 1qtr
(723.92kg).
References:
‘The Austin Seven Source Book’ by Bryan
Purves provides detailed information on these differences to enhance our
knowledge.
‘Original Austin Seven’ by Rinsey Mills also
has useful descriptive chapters on the changes.
This article, written by Doug Castle, originally appeared in CA7C Seven Focus in
Aug 2008 pp18-20.
