The differences in our Sevens

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 sizes. 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 heels on the Big Seven.

Changes to the A7 Engine
Years	1922		1923 -1929		1929 - 1936		1936		1937 - 1939
Capacity	696cc		747.5cc		747.5cc		747.5cc		900cc
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	1 ¹_/8"	1 ¹_/8" #		1 ⁵/₁₆"		1 ⁵_/16" ##		1 ⁵_/16"
# the diameter was increased to 1⁵/₁₆" in October 1929. ## the centre bearing is 1 ¾”

In brief conversation with people having a casual interest we mention that Sevens have the 747cc engine except the Big Seven which has the re-designed 900cc engine. 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.

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 MKII 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.

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.

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 that 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.

From 1922 to March 1926 Staybrite centre wire wheels were 26" with 3.50 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.

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’ 7¹/₈" front and 3’ 9" rear tracks.

Throughout Seven production, changes were made to front and rear wings and the running boards. Between 1922 and 1932 there were ten styles 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, i.e. side and rear windows.

As Sevens were developed and the length increased, along with all the changes to engines and ancillary equipment, the weight of the cars increased. The Tourer Type AD of February 1926 weighs 8cwt 561bs (406.41 kg). 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 14cwt 2qtr 201bs (745.70kg); with the Forlite in March 1938 at 14cwt 1qtr (723.92kg).

The Austin Seven Source Book by Bryan Purves provides the detailed information on these differences to enhance our knowledge. The book Original Austin Seven by Rinsey Mills also has useful descriptive chapters on the changes.