Initially the base was the familiar pre-1932 style but with rough cast 3/4" mounting holes. In around 1923 or perhaps earlier the mounting holes were reduced to 1/2" (i.e. Clearance for a 7/16" BSW bolt). The reason is not known. Breakages might be the cause but a seemingly more likely explanation is the advent of a range of cast bases for Petter-Light and other combination sets. The smaller machined holes would have given more accurate location and bolts/set screws with heads larger than old size 7/16"BSW would not have fitted the base foot and also have been somewhat clumsy.

Some rationalisation of the component product range evidently occurred around 1931 and from around that time a common base was used across the 5hp M and S range. This has cast bosses for both the large fuel-filler orifice and the S type fuel lift pump. The large filler used an aluminium cap rather than the earlier knurled brass item.

Although visually similar the crankcase underwent a number of changes. Initially a cylinder oiler boss was included and used. The boss remained long after its use had been dropped (1915?) and finally disappeared in around 1922. Approximately the first 1000 engines used a smaller exhaust flange whose dimensions are the same as the 2 1/2 hp engine.

The bottom flange of early bases is arched and to seal the fuel tank a sheet steel "bed cover" is interposed between the base and crankcase feet. In around 1923 the solid base was introduced and remained through to the end of production. The later style needs no bed cover and is bolted directly to the base.

Having been designed for an uncooled head there is only one waterway in the front top flange of the early crankcases. This changed with the arrival of the fully cooled "Jellymould" head

From the early 1920s the crankcases show a casting date on the upright of the reed valve aperture.

The 1932 model required only machining changes to allow the new main bearing housings to be fitted.