A traditional air cooled 650 cc twin cylinder with a single large bore Amal Monobloc carburettor, it did not suffer from the same level of vibration as earlier BSA twins and could comfortably cruise at 70 mph (110 km/h).

Fitted with 12 volt electrics, a Zener diode voltage regulator and twin coil ignition, the Thunderbolt sold well in the important US import market and with the 3.5-imperial-gallon (16 L; 4.2 US gal) fuel tank gave a range of 210 miles (340 km).

[1] From 1968 the Thunderbolt benefited from a number of minor improvements including a longer kick start to make starting easier and metal tank badges to replace the earlier plastic ones, which had a tendency to crack.

An Amal Concentric carburettor dealt with the problems of fuel flooding experienced with the earlier monobloc carburettor, by having the float bowl arranged centrally around (concentric with, hence the name) the main jet to remove the sensitivity to fuel surge inherent in all the earlier designs.

Stiffer suspension improved cornering but poor quality control and production problems on the BSA assembly line with later models led to oil leaks and rusting parts which damaged the reputation of the Thunderbolt, which was already struggling to compete against the emerging Japanese motorcycles of the early 1970s (such as the Honda CB750) and production ended in 1972.