Thanks a lot for your help m8,
some of your reply was a little over my head at the moment but hopefully after reading a lot here, I will learn a bit .

just to clarify
is the following correct.

the piston is inserted in the sleeve with the rectangular hole in the actual piston, facing directly opposite the large rectangular hole in the sleeve which is the exhaust port, which also means the hole in the piston lines up with the "squarish" hole in the sleeve.

Thanks again, and sorry to be a pain.