1959 AJS Model 31

1959 AJS MODEL 31 BACKGROUND
AJS and sister-brand Matchless were late to the vertical twin party. Triumph started it all with the 1938 Speed Twin, the world's first 'modern' vertical twin (aka: parallel twin). It took the world by storm, and suddenly every other British brand had to have one of their own. Alas, World War II intervened, delaying progress until after the war. BSA followed first in 1946 with their 500cc A7. Ariel, now owned by BSA, got their twin, itself based loosely on the BSA A7, in 1948. Next up was Norton with their 1949 Model 7.
AJS & MATCHLESS HAVE TWINS!
That same year, 1949, AMC launched it's own set of twins, both 500cc, the Matchless G9 and the nearly-identical AJS Model20. While both were fine machines, they had a hard time competing in the all-important American market against the combined might of Triumph, BSA and Norton.
1959 AJS MODEL 31 ENGINE DESIGN
Despite being modeled after the pattern of the proven Triumph vertical twins, AMC decided to do what no one else in Britain had even thought of: installing a center main bearing. Most vertical twins had enormous central flywheels suspended between the two rod journals, which was a recipe for deflection (bending of the crankshaft at high speeds). It caused all sorts of problems, the most obvious of which being wicked engine vibration. AMC solved all that by installing a center main bearing in both it's sister twins, the Matchless G9 and the AJS Model 20, and all subsequent models as they climbed in displacement all the way up to 750cc. The 1959 AJS Model 31 was the 650cc variant. Because of the added strength afforded by the center main bearing, AMC was able to get away with building their cranks out of cast iron, rather than steel. Cast iron was easier to work with and far cheaper than steel. And lighter than your typical British twin crankshaft with their huge central flywheels. All of this combined to make a very smooth-running engine, even at higher RPMs.
Another unique feature of the AMC twins was that the entire top end was split into left-and-right halves. So, the cast iron cylinder block was split in two each with one cylinder, and the same with the cast aluminum cylinder head (s). It probably helped with cooling, and allowed only one head or cylinder to be removed during repairs.

1959 AJS Model 31 SPECIFICATIONS

Engine type

Displacement

Bore & Stroke

Compression ratio

Fuel system

Power output

Torque output

Clutch

Gearbox

Wheelbase

Overall Length

Seat Height

Width

Fuel capacity

Suspension, F/R

Brakes, F/R

Tires, F/R

Curb Weight

Air-cooled OHV twin

646cc / 39.3ci

72mm X 79.3mm / 2.83" X 3.12"

7.5:1

Amal Monobloc 389/50 carburetor

34hp @ 6500rpm

33 lb-ft @ 5500rpm

Multi-plate, wet

4-speed, right-foot shifted

55.25" / 1403mm

86.0" / 2184mm

31" / 787mm

27" / 686mm

5.1 US gal / 4.2 Imp gal / 19.3L

Telescopic fork / swing arm w/2 shocks

SLS drum / SLS drum

3.25" X 19" / 3.50" X 19"

441 lbs / 200kg


Check out these AJS BOOKS


AJS: The Complete Story (Motoclassics)



AJS and Matchless Post-War Singles and Twins: The Complete Story



BOOK OF THE AJS SINGLES 1955-65 350cc & 500cc



AJS 7R3A 'TRIPLE KNOCKER' ISLE OF MAN TT WINNER: This bike beat all of the European opposition to win the 1954 Isle of Man TT (THE MOTORCYCLE FILES Book 22)




Illustrated Ajs and Matchless Buyer's Guide (Illustrated Buyer's Guide)



The AJS 7R



AJS and Matchless Gold Portfolio 1945-1966



AJS and Matchless: The Post-war Models



BOOK OF THE AJS 350cc & 500cc OHV SINGLES 1945-1960



The AJS 7R (Redline Motorcycles) (Redline Mptprcycles) by Mick Walker published by Redline Books (2002)



PLEASE BUY MY NEW E-BOOK HERE
12 Chapters & over 100 original photos. My best work yet.
ONLY $4.99!
Thank you,
Andy Tallone, your humble author

Click here to buy now:
The Random History of Classic British Motorcycles, Volume 1




Comprehensive Vintage Motorcycle Price Guide 2020-2021

Please visit our
MOTORCYCLE BOOK STORE


Take a tour of our site...

HOME PAGE
Triumph Motorcycles
.....Bonneville
.....Tiger
.....Trophy
.....TR6
.....TR7
.....Trident
.....X-75 Hurricane
.....6T Thunderbird
.....5T Speed Twin
.....Daytona
.....Twenty-One
.....3T 350 Twins
BSA Motorcycles
.....A65
.....A10
.....A7
.....Gold Star
.....Bantam
.....Rocket 3
.....B50
Norton Motorcycles
.....Commando
.....Atlas
.....Dominator
.....Model 7
.....Manx
.....Navigator
AMC-Associated Motor Cycles
Matchless-AJS Motorcycles
.....G3
.....G9
.....G12
.....G80
Royal Enfield Motorcycles
.....Interceptor
.....Bullet
Ariel Motorcycles
.....Square Four
.....Red Hunter
.....Cyclone
Vincent Motorcycles
.....Black Shadow
.....Rapide
.....Comet
.....Egli-Vincent
Velocette Motorcycles
.....KSS
.....KTT
.....LE
.....MAC
.....Thruxton
.....Venom
.....LE
Rickman Motorcycles
Japanese Motorcycles
.....Honda
.....Kawasaki
.....Suzuki
.....Yamaha
Cafe Racers
....Triumph Cafe Racers
.....Norton Cafe Racers
.....Other Cafe Racers
Choppers & Bobbers
.....Choppers
.....Triumph Choppers
.....BSA Choppers
.....Rat Bikes
Classic Racing Motorcycles

Classic Motorcycle Events
.....Auctions
.....Shows
.....Rides
Classic Motorcycle Restoration
Classic Motorcycle Shops
Classic Motorcycle Clubs
Send us Pictures of YOUR BIKE
YOUR BIKE Pictures

HOME PAGE
.....About Us
.....Site Map
.....CONTACT US
.....Please visit our BLOG
.....Subscribe to our FREE e-zine BRIT IRON
.....BRIT IRON E-zine Back Issues

Visit our STORES
.....Motorcycle Books
.....Motorcycle Magazines
.....Motorcycle Calendars
.....Motorcycle Accessories
..........Motorcycle Helmets
..........Motorcycle Rain Suits
..........Motorcycle Locks & Security
..........Motorcycle T-Shirts
..........Motorcycle Diecast Models
..........GoPro Cameras
.....Motorcycle Parts
..........Motorcycle Tires
..........Motorcycle Chains
..........Motorcycle Batteries
..........Motorcycle Handlebars
.....Motorcycle Tools
..........Whitworth Tools
..........Tire Tools
..........Chain Tools

Crazy about cars, too? Check out our sister site:
www.American-Muscle-Cars.net




Share this page:
Enjoy this page? Post it on your Facebook page.

Would you prefer to share this page with others by linking to it?

  1. Click on the HTML link code below.
  2. Copy and paste it, adding a note of your own, into your blog, a Web page, forums, a blog comment, your Facebook account, or anywhere that someone would find this page valuable.

Back to top - 1959 AJS Model 31
Updated 12/11/20

Learn how to BUILD YOUR OWN WEBSITE, like I did.