Dave,
Continuing your thread on the old Stuart twin cylinder engine.
I did some research and found that I had posted a Thread (.
https://www.modelboatmayhem.co.uk/forum/index.php/topic,31619.0.html ) on a similar engine that I had done some refurb on, I can't remember where my engine came from but when refurbed it was sold to a collector on e-bay.
The pics that I have posted may give you some idea as to whether you paint it or not, click on to the pics for a larger image.
I am surprised that there is no Crank case vent so lubrication to the cylinder is by splash pressure in the Crank Case and pumps oil from the sump to the underside of the pistons and also lubricates the small end in the piston.
The ring grooves are there to collect oil and steam from above to help in the sealing of the pistons to the bore. You will find that all of Stuart engines that don't have piston rings rely on this method, the sump oil is normal Auto engine oil, as I can't see any method to replenish the oil in the sump, I have a feeling that one of the central Crank case fixing bolts when taken out allowed oil in and it was total loss system when running,
I personally when building Stuart Models withought rings use Silicone "O" rings and never have had any failures.
If you go to the end on the posts on the 2nd page you will find a History of this engine and how it developed into the "Sun"
George.