Unlike the rolling chassis, the Minor power plant was subject to very radical change half way through the models life span. The early overhead valve engine was costly to build and maintain due to its relative complexity when compared to the engine of its main rival, the Austin Seven. This led to the introduction of a newly designed side valve power unit which was introduced in late December 1930 and then installed in the mid season range of three cars. These models were then sold alongside the existing range of four overhead valve models for the remainder of the 1931 model season. (Apologies for the poor quality of the scans used - these were taken straight from the pages of the Operations Manuals concerned, many of which have seen much better days.)
This image appears in all three (1929, 1930 & 1931) handbooks for the overhead valve engined cars despite it displaying the early and smaller dynamo throughout. It also shows an integral bell housing, not featured on production engines. This unit was replaced in 1929 by another Lucas unit with an improved output. This was larger in diameter and required the block to be re-cast with a small concave indentation to accommodate it.
This photograph appears in the 1931 and 1932 Operations Manual and shows the near side of the newly introduced side valve engine. The 1" S.U. carb is still fitted with the bottom feed float chamber and the gearbox is unchanged from the OHV engined car although the transmission handbrake mechanism has gone.
This is the OHV engine fitted to the 1932 season only Family Eight. For the remaining two seasons the car was sold with the by then all pervading side valve unit. Note the oil filler breather pipe missing from earlier OHV engines and the twin bladed cooling fan. (only a single blade on SWB OHV engined cars)
For the 1933 Operations Manual photo the engine was shown with the new four speed gearbox option together with the twin bladed fan and top feed float chamber on the S.U.
For its final year in the Minor car range (1934) the engine looks unchanged from 1933. However the new button operated handbrake lever is clearly visible while the S.U. carb has a larger dome and the fully synchromesh gearbox appears to be longer than that on the 1933 car.