The downside of this is today's high performance thumpers are far more costly to maintain than they used to be. Though they generally don't require as much regular maintenance as a two stroke - but when they do there are more parts to replace, therefore costing you more.
The 4 stroke dirt bike engine consists of a piston and barrel, and the cylinder block which holds the valve train. The number of valves vary with different bikes - The '08 YZ450F has 5, while the '07 CRF450 has 4.
Just like the 2 stroke, the piston moves up and down in the cylinder block powered by explosions from combusting fuel/air mixtures triggered by the spark plug.
But, unlike the 2 stroke, the thumper engine only gets one ignition, or explosion for every four strokes of the piston.
YZ450F Piston & Crankshaft
The four stroke engine has four unique strokes or movements that each perform a specific task.
This whole process then repeats...
The powerful YZ450F
The 4 stroke dirt bike engine, unlike a 2 stroke does not have fuel inside the crank case. Therefore, you are able to fill the crankcase (where the crankshaft bearings and moving parts of the piston rod are) with a heavy duty oil to lubricate them.
So, that is why you do not need to mix oil into the fuel with a four stroke engine. No burning oil - less damage to the environment.
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