Home About Us The Star Membership Branches International Machine Dating Members Area Forum Contact Us Links

Engine and Frame Number Introduction

Identifying your machine.

On most BSA's the frame number is located in the following places. It is always stamped; raised numbers are casting part numbers. Up until 1968 it is rare for the engine and frame number to be identical and where the frame is shared by several models the codes start with the smallest model e.g. all A series models have A7 or A50 prefixes with the exception of the Rocket Gold Star and Super Flash.

Pre-war Motorcycles.

This varies from model to model but usually it is on the front top frame member either by the front seat stay or by the steering head.


On the front down tube below or in front of the petrol tank. The exception is the early Bantams where the frame number is between the front engine mounting plate. These are usually faint and small.


By the steering head reinforcing plate or front down tube. Some Bantams on the engine mounting plate, these sometimes break and re-welding obliterates the stamping.


Usually on the front engine mounting lug.

1968 onwards

Front engine mounting lug but the engine and frame numbers are the same.

1971 oil in frame models

Steering head stem. The oil in frame B-series are stamped on the prop stand bracket.

Engine markings

Engine numbers are easier to find usually by the crankcase mouth on the primary side. Exceptions are the Bantams which could be stamped on the front mounting point or just behind the crankcase mouth.

This is just a guide and will fit most situations but frames were legitimately re-stamped by dealers if the original was damaged and a replacement fitted. These could be stamped anywhere and often were not stamped at all!

Identifying engine and frame numbers, 1969 season onwards.

Owners with late model BSA's can easily identify the month and year of their machine as the factory adopted a dating system for frame and engine numbers for the 1969 season models. As with all previous BSA seasons the model year started after the factory shutdown in July so that 1969 models were those made from August 1968, through to July 1969. They would however have some machines in stock so that so that it is possible that a July model may be to either the previous or the next year's specification.

The code consisted of three parts, the first as two letters giving month and year respectively, the second was a five digit number and the third a model code. The engine and frame received the same markings. After dating and identifying many thousands of BSA's we have found that often a bike was built or shipped outside the dates given by the lettering system so this should only be used as a rough guide to the year and month of your machine.

The date letters are as follows:

1St  Letter    2nd Letter

A – January    C – 1969 Model

B – February    D – 1970 Model

C – March    E – 1970 Model

D – April     G – 1972 Model

E – May     H – 1973 Model

G – June     J – 1974 Model

H – July  

J – August    To avoid confusion

K – September    ‘F’, ‘I’, ‘O’, and ‘L’ were

N – October    not used, although it is  

P – November    is to confuse letters ‘C’

X – December    and ‘G’.

The numbering started at the beginning of each season and always started at 00100 leaving the first 99 for experimental use. The numbers were consecutively allocated throughout a model range so as an example all B group machines were lumped together irrespective of whether they were a B25 or B44. From experience in cross-referencing machines against the factory despatch books of which the club have a copy, we have found that in about 50% of cases the bike was not despatched according the date of the lettering. Sometimes they were despatched months earlier or later so the lettering system should just be taken as a rough guide.

The model designations are as follows. The years given are all model years:

A Series
Rocket Three - A75R -through all years.
Rocket Three five speed - A75RV, 1969 and 1970.
A75V - 1971 and 1972.
Royal Star A50R - through to 1970.
Thunderbolt A65T - through all years.
Police Special A65P - through all years.
Lightning A65L - through all years.
Firebird A65F - 1969 and 1970.
A65FS - 1971.

Lightning 750 A70L 1972.
Note in 1972 the frames for the twins were marked A65 with no model designation, from XG.00101.A65 on.

B Series
This is a bit complicated!
Starfire B25S - Sept 1968 to 1970.
Gold Star 250 - B25SS 1971.
Victor 250 - B25T 1971.
Fleetstar - B25FS through to 1971
Victor Special B44VS 1969 (C) November 1968 through June 1969 then 1970 (D) July 1969  to July 1970
Shooting Star B44SS 1969 (C) November 1968 through August 1969 then 1970 (D) July 1969 through July 1970
Gold Star 500 B50SS 1971 to 1972
Victor 500 B50T 1971 to 1972
Victor MX B50MX 1971 to 1973

D Series
D14/4 Supreme D14/4 1969 part
D14/4 Bushman D14/4B 1969 part
B175 Bantam D175 1969 to 1971
B175 Bushman D175B 1969 to 1970

Other points to remember:

Early post war machines up to Z series had the frame stamped similar to the plunger or sprung frames e.g. YA7.R. 9999. (The 'R' meaning RIGID). Also this carried through to some but not all of the engines.

1966 'A' series machines started the season with A50, A50B and A50C frame designations with the usual non-matching engine markings. After frame 3200 the engine and frame markings were the same. There was a short period where the engine marking matched the frame marking but the frame prefix was A50; Presumably to use up stock in store prior to the changeover. Machines still in stock in the 1967 season (about 1000) were resold in the 1969 and 1970 season. These returned machines are shown with a cross at the beginning of the despatch book record; they then reappear at the end of the book with the revised despatch date. The 1969 models are identified by the adding of an extra 0 at the end of the frame marking putting the number series into the 100,000's. Machines re-exported in 1970 were stamped with a 'Y' suffix to indicate that they were 1970 models and therefore eligible for the increased warranty.

The C15 Sports Star 80's and B40 Sports Star 90's engine markings were often abbreviated to CSS and BSS respectively.

A second series of letters and numbers starting ERS means that the unit has been replaced under the factory Exchange Replacement Service and the figures are the despatch record number.

Click on the relevant year below for a complete listing of all engine and frame numbers:

Years up to 1920 - All models up until 1920    
1921 to 1930 - All models from 1921 until 1930    
1931 to 1940 - All models from 1931 until 1940    
1941 to 1950 - All models from 1941 until 1950   
1951 to 1960 - All models from 1951 until 1960   
Post 1960 - All models from 1960 until the factory closure in 1973.

These lists have been compiled from numbers listed in the parts books from 1930 until the factory closed in 1973 and from known bikes up until 1930. The list was compiled painstakingly by the club's Librarian Steven Foden. If you know of any discrepancies please email us.

Print this page