There are many components in the Daihatsu Hijet engine. The engine valves are what lets the fuel travel from the carburetor to the combustion chamber. The Intake Valve lets fuel in, and the Exhaust Valve lets the burnt fuel or exhaust leave the combustion chamber. The pistons have piston rings fitted to allow the engine to build compression. When a Daihatsu Hijet mini truck is run with out an air filter it will suck in dust that causes extreme wear of the piston rings and thins the engine rod bearings and engine main bearings. If this happens the engine needs a rebuild kit to restore the compression and allow the small 660cc mini truck engine to perform at its best.