The 40-series was an extension of the 20-series short with the 25 becoming the 40, and the longer wheelbase 28 model becoming the 43. In the later stages of the 20-series a long bodied 35 becoming the 45.
Thereby the 40-series inherited by body and chassis from the 20-series, and most of the changes focused on the power train. The 40-series for the first time took on a 2-speed transfer with a LO range, so in addition to being able to switch from 2WD to 4WD, now you were also able to switch from HI to LO range. On the 40-series the switch from 2WD to 4WD was made by engaging or disengaging the front wheel drive using an engine vacuum actuator; whereas the HI/LO switch was a remote control type by the mechanical operation of a lever. To switch from the 2WD to the higher 4WD range you pulled a knob, and to switch into the LO range you operated the lever. It was built in such a way that moving the lever automatically triggered the switch in a single smooth operation.
The change to a 2-speed transfer was also linked to the change to a 3-speed transmission. The 20-series did not have a LO range, so it required a 4-speed transmission, but with the 40-series the gear ratio adapted to very low speeds reduced the need for an extra gear in the transmission. The operational interaction of the transfer and the transmission also underwent changes that led to increased reliability of operation. The front drive switch knob remained on the instrument panel, but the HI/LO switch was moved to a floor shift type directly on the transfer. By that time changes in the transmission reflected a shift to the 4-speed and 5 speed.
The F-type gasoline engine was switched to a B-type diesel engine, with improvements in the piston displacement, and other accommodations to increasingly strict emissions control regulations. The 3.0-liter engine that had been found in the BJ40/43 was enlarged to a 3.2-liter power unit, and this B-type engine was put in the BJ41/44. Then an even larger 3.4-liter 3B-type engine, which cleared increasingly strict emissions regulations, was mounted in the BJ44/46.

On the upper grade LX of the BJ42 the dashboard panel had a crash pad to protect passengers in the event of a collision, as well as a switch panel board for cosmetic effect, a digital clock, and a tachometer. The M/T and transfer shift levers had already both been moved to the floor when the change was made from the FJ to the BJ, and there were no changes made to the drive train mechanisms.

The Zebra pattern fabric seat was standard equipment in the LX grade.

The LX grade of the latest model BJ42/BJ46 had resinous paneling on the walls, and carpeting on the floors. The springs were set soft, which made for a softer ride. The 40-series retained the tough image, but also added some features of the passenger car.

The early model FJ engine specs were a maximum output of 125ps/3,600rpm, and a maximum torque of 29.0kgm/2,000rpm, which later was upgraded to 130ps/3,600rpm and 30.0kgm/2,200rpm.

The front springs were made of 7 plates, each 70mm wide and 6mm thick. The rear springs were made of 6 plates, each 70mm wide and 7mm thick, reinforced by an 11mm helper leaf to be able to bear heavier loads. These were the same specs as on the long wheelbase FJ45V.

The 3B-type diesel engine in the BJ42/BJ46 had a larger bore than the 2B-type engine that had been used in the predecessor BJ41/BJ44, and it had a piston displacement of 3,431cc. The engine room for the 6-cylinder F-type engine was too wide for the 4-cylinder 3B engine. The cylindrically shaped part in front of the passenger seat is the heater unit, which drew air in from a slitted panel behind the front fender.