Can you make your yz125 a 250 by switching cylinder and head?

It is not possible to directly convert a Yamaha YZ125 to a YZ250 by simply changing the cylinder and head. While both the YZ125 and YZ250 share similar engine designs and layout, there are significant differences between them that require more than just a cylinder and head swap.

The YZ250 has a larger displacement engine, achieved by having a larger bore and stroke compared to the YZ125. The cylinder and head are designed specifically to accommodate the larger piston bore and combustion chamber of the 250cc engine. Simply putting a YZ250 cylinder and head on a YZ125 will not result in the bike becoming a 250cc motorcycle, as several other components would need to be changed as well.

Some of the additional modifications required to make a YZ125 into a 250cc motorcycle include:

1. Engine cases: The engine cases of the YZ125 and YZ250 are different to accommodate their respective engine configurations. The YZ250 cases are designed to fit the larger cylinder, piston, and crankshaft, while the YZ125 cases are not.

2. Crankshaft: The crankshaft of the YZ250 is different from that of the YZ125, as it has a longer stroke to match the larger piston displacement.

3. Piston: The piston used in the YZ250 is larger in diameter and has a different compression ratio than the YZ125 piston.

4. Camshaft: The camshaft of the YZ250 is different from the YZ125, as it has different cam profiles and timing to accommodate the larger displacement engine.

5. Carburetor: The carburetor of the YZ250 would need to be changed to one that is appropriate for the larger displacement engine.

6. Exhaust system: The exhaust system would also need to be upgraded to match the larger displacement engine, as the stock YZ125 exhaust is designed for the smaller engine size.

In conclusion, converting a Yamaha YZ125 to a YZ250 is a complex operation that requires not just swapping the cylinder and head but also involves significant changes to the engine internals and other components. Without comprehensive modifications, it is not accurate to claim that installing a YZ250 cylinder and head will transform the YZ125 into a 250cc motorcycle.