The oiler has been relocated away from the exhaust.
The No.1 mixer is clamped down while the 1A mixer is bolted down. Note also the different mixer screw and the addition of a priming hole on the 1A.
The No.1 has the early single mount governor detent while the 1A is mounted on both sides.
Another view of the detent. On the No.1 timer blade is mounted to the hole in the housing to the left of the detent while on the 1A it is mounted off the hole on the detent.
Here is a major change in the casting. The 1A has the mount for the pump-jack cast into the crankcase while the No.1 has no provision for this.
The No.1 is fitted with a chain driven magneto while the later engines had a cast mount with a gear driven magneto.
The No.1 has a two piece sump and base (allowing for a pump jack base) while the 1A is a one piece casting.
The No.1 is fitted with a pulley constructed out of timber bolted together and mounted on the crankshaft while the 1A as a cast pulley bolted to the flywheel.