The ‘J’ type was introduced in 1912 as a 4½ tonner having a 30 h.p. 4 cylinder engine, cone clutch and 4 speed gearbox. It was the first Thornycraft to have a ‘live’ rear axle – previous models were chain driven. Like its competitors a straight chassis frame was used and both goods and passenger bodies were fitted. In 1918 a more powerful. 40 h.p. engine was introduced and in the following year, a longer wheelbase was offered increasing the variety of bodywork that could be fitted.
In 1919, Portsmouth Corporation bought 10 ‘J’-type chassis and had them equipped with 36 seat double deck bodies. These were typical of the period and followed horse bus design down to the narrower lower deck to allow the front wheels to turn! This resulted in having 8 inward facing seats on each side but allowed 9 pairs of forward facing seats in the wider upper deck. In addition 2 more passengers could be accommodated alongside the driver. This was an exposed position as there were no windscreens but very often the driver had a tarpaulin which he could tie round his neck in wet weather, similar protection was provides in the upper deck, for the passengers. A porch was built over part of the platform to give the conductor shelter.