I'm not sure what you want me to measure, but if you can be more specific, I can measure whatever you want. Normally between the firewall and the engine, there are 4 "standoffs" that move the engine more forward to make the spinner backplate line up about 1/8 inch in front of the cowl. These standoffs can be as short as 1/4 inch or as long as one inch. Since each 100 cc engine is different, the plane manufacturers make the firewall short enought that all engines will fit with the appropriate standoffs. You can buy aluminum standoffs, or you can make your own from some hardwood dowels.