Finally the last part to be made is the crankcase breather, based on Westbury's Kiwi design.

One last tapped hole in the crankcase for the breather.
Then a total strip down and final clean of all the parts.
Before final assembly begins with the crank and con rod.
All the bits for the carb go together.
And hey presto we have a finished engine.
 

BUILDING THE SIMPLEX PETROL ENGINE

Part 8 by Phil Procter

Part one here Part two Part three  Part four  Part five  Part six  Part seven eight

See also engine description.

https://www.sarikhobbies.com/model-engineer-builder/