So what does "cylinder inside diameter size mark" shown on this diagram refer to then? I thought it was the inside diameter of the cylinder, and is what I was told by a few people here. I know the normal numbers refer to bearing sizes, but I am talking about the Roman numerals.



And so what are the differences between the 3 "standard" types of piston that Mitsubishi sell for the engines of these cars?

 

Well, when I went to pick some parts up from Mitsubishi today they had the answer from me in the form of a fax & diagram from mainland.



The Japanese characters at the top left say that the diagram is of the bottom of the cylinder block. It then refers to 4 sets of numbers which appear as Roman numerals. In the table below the left column reads "Cylinder bore size mark" and on the right is the standard piston size, so there definitely are 3 piston sizes depending on the cylinder bore size if anyone didn't believe it before.

Now what exactly happens if I put a type A piston into a mark II or III hole... well I'll find out in about 7-10 days when I fire this engine up! Have been told that on an old engine it shouldn't really have any big effect.