Hi Joey.

I had a look at one of my i/c engine books (miniature internal combustion engines by Malcolm Stride).

And according to his reference you could use the following :

Cast : LM51

Machined from barstock: HE 15 OR HE 30.

For small compression engines (Diesel ) He recommends cast iron but no spec supplied .

I imagine these designations change from one country to the next etc so you may need to search for an equivalent on the internet .

There have been many articles in Model Engineer regarding engines so try there .

Alternatively try the MODEL ENGINE NEWS website as they have masses of info on many types of engines .

With a 40MM bore i would guess that you are using piston rings ?

Are you sure that it is not them that is seizing in the bore (ring gap too small )

And if you are using rings try a little more piston to cylinder clearance say 3-4 thou .

Are the pistons oval ? Should be smaller accross the wrist pin (gudgeon pin hole ) to allow for deflection or any flexing .

Ian