Be careful not to get the contra piston too far down into the cylinder. You can get the fit too loose with repeated up and down with the contra in the cylinder. Once I get the contra piston in the top of the cylinder I use the compression screw to lower the contra a bit. Wait to you are ready to run the engine before adjusting the contra up and down so as not to get the fit too loose. Go to the SAM web site for a good article on the basics of model dieel operation. Best to find someone near you who has experience running diesel for some help in this. Jack