1966 AJS Model 31 650 Specifications

Name: 1966 AJS Model 31 650
Brand: AJS

Classic • Specs last verified 31 October 2026

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1966 AJS Model 31 650 Overview

AJS Model 31 650 is a classic motorcycle, displacing 646 cc. Power comes from a Four stroke, pushrod parallel twin, OHV, 2 valves per cylinder, engine, a compression ratio of 7.5 : 1. On the performance side, it delivers 31,4 kW @ 6000 rpm of peak power. On the road, the 1966 Model 31 650 tips the scales at 176 kg and carries a 16 L fuel tank. The full 1966 AJS Model 31 650 spec sheet below covers every technical detail, from suspension travel to tyre sizes.

1966 AJS Model 31 650 compared with other bikes in the Classic category

We checked the recorded figures for the 1966 AJS Model 31 650 against up to 909 bikes in the Classic category in our database, counting each model once at the model year nearest to this one.

Only 179 of the other 591 bikes in the Classic category have a higher power output than the 32.1 kW of the Model 31 650.
With 646 cc, the Model 31 650 has the joint 315th largest engine displacement of 909 bikes in the Classic category, 25% above the category average of 516 cc.
Narrowing the field to the 143 models within five model years of it, the power output of 32.1 kW is higher than that of 116 of the other 142.

1966 Model 31 650 - Pros & Cons

Strengths

Capable power output: The Model 31 650 produces 31,4 kW @ 6000 rpm - respectable for its class and era.

Considerations

Carburetor fuelling: Expect to spend more time keeping the carbs in tune, especially after long storage periods or a change in altitude.
Drum rear brake: Rear braking comes from a drum, not a disc - it does the job but won't match the feel or fade resistance of newer equivalents.
Chain final drive upkeep: Budget time for routine chain maintenance - it's cheap to replace but needs consistent attention to avoid premature sprocket wear.

Wear Points & Servicing Notes

Heat management in traffic

Because the Model 31 650 relies on airflow rather than a coolant jacket, expect noticeably more radiant heat at low speed and in traffic. It's inherent to air cooling, and ventilated gear largely offsets it.

Carburetor maintenance

Carburetors on the Model 31 650 need periodic attention - pilot jets and float needles are the first parts to gum up after the bike sits, and the first thing to check if idle or throttle response degrades.

Suspension condition on used examples

The Model 31 650's suspension runs Hydraulic telescopic fork at the front. The rear uses Dual chocks. Fork-seal condition and rear linkage play are the two things worth checking before buying used.

Final drive maintenance

The Model 31 650 uses chain final drive, which is cheap to replace but needs regular cleaning, lubrication and tension checks to get a full service life out of the sprockets.

Frame & chassis check

Built around a Single cradle, the Model 31 650's chassis is straightforward to inspect: look for corrosion, cracks or repair welds around stress points like the head-stock and swingarm mounts.

Model 31 650 - Buying & Maintenance Guide

What to check before buying

Before handing over money for a used 1966 AJS Model 31 650 with its 646 cc classic, cover the fundamentals: service book and oil change history, drivetrain wear, and any sign of oil weeping around the casings. This model runs chain final drive, so check chain slack and sprocket tooth profile specifically - a worn sprocket is a cheaper fix now than after it's chewed up the next chain. Now 60 years old, this classic is as likely to show age-related wear (perished rubber, seized fasteners, degraded seals) as mileage-related wear. Cold-starting it yourself tells you a lot - a healthy Model 31 650 should catch quickly and settle into a smooth idle.

Oil and filter changes

Nothing protects the Model 31 650's 646 cc four stroke, pushrod parallel twin, ohv, 2 valves per cylinder, like disciplined oil changes - stay on the manufacturer's schedule (roughly every 6,000 km or annually). With a 7.5 : 1 compression ratio, running the octane grade specified in the manual matters more than on a lower-compression engine. Match the viscosity grade to what the owner's manual specifies, not what's cheapest.

Carburetor upkeep

Deposits in the carbs are the usual culprit behind poor running on a stored Model 31 650 - strip and clean the float bowls before the first start. Pilot jets clog first and get overlooked most, so make a point of checking them.

1966 AJS Model 31 650 - Frequently Asked Questions

What engine does the 1966 AJS Model 31 650 have?

The 1966 AJS Model 31 650 uses a Four stroke, pushrod parallel twin, OHV, 2 valves per cylinder, engine with 646 cc of displacement. It is air cooled.

How much does the 1966 AJS Model 31 650 weigh?

The 1966 AJS Model 31 650 weighs 176 kg. This is the manufacturer's dry figure.

How much power does the 1966 AJS Model 31 650 make?

The 1966 AJS Model 31 650 produces 31,4 kW @ 6000 rpm of power.

What is the fuel tank capacity of the 1966 AJS Model 31 650?

The 1966 AJS Model 31 650 has a 16 L fuel tank. At around 5-6 litres per 100 km, that is roughly 266-320 km between stops.

What brakes does the 1966 AJS Model 31 650 have?

The 1966 AJS Model 31 650 uses drum at the front and drum at the rear.

What tyre sizes does the 1966 AJS Model 31 650 use?

The 1966 AJS Model 31 650 runs 3.25 -19 front and 3.50 -19 rear tyres. A good selection of sport, touring and all-weather rubber is available in these sizes.

What gearbox does the 1966 AJS Model 31 650 have?

The 1966 AJS Model 31 650 is fitted with a 4 Speed transmission.

Key Specifications

Engine

Four stroke, pushrod parallel twin, OHV, 2 valves per cylinder,

Power

31,4 kW / 43 hp @ 6000 rpm

Torque

Not recorded

Top Speed

Not recorded

Weight

176 kg / 388 lbs

Fuel Capacity

16 L / 4.2 US gal

Full 1966 AJS Model 31 650 Specifications

Year 1957 - 66

Model Model 31 650

Make Model AJS Model 31 650

Dry Weight 176 kg / 388 lbs

Fuel Capacity 16 L / 4.2 US gal

Max Power 31,4 kW / 43 hp @ 6000 rpm

Displacement 646 cc / 39.4 cu in

Engine Detail Four stroke, pushrod parallel twin, OHV, 2 valves per cylinder,

Bore x Stroke 72 x 79.3 mm

Compression Ratio 7.5 : 1

Cooling System Air cooled

Fuel Induction Carburetor

Ignition 6 V (After 1964 12 V) Magnet (Lucas K2F)

Starter Kick

Clutch Multi Plate (dry, Burman)

Final Drive Chain

Transmission / Drive 4 Speed

Frame Type Single cradle

Suspension - Front Hydraulic telescopic fork

Suspension - Rear Dual chocks

Brakes - Front Drum

Brakes - Rear Drum

Tyres - Front 3.25 -19

Tyres - Rear 3.50 -19

Wheels Steel, wire spokes

Specifications are compiled from manufacturer documentation and cross-referenced technical sources, and corrected on an ongoing basis. Spot an error? Let us know.