A very common problem on the 2000–2004 Ford Focus with the 2.0L SPI SOHC engine is the tendency to drop a valve seat. European models don't have this problem, as only the North American Focus was available with the SPI engine. These engines usually drop a valve seat with no warning, for no reason, even if the engine has been well maintained. With most dropped valve seat cases, a seat drops on the number 4 cylinder, with the next most common being the number 2 cylinder. With the factory valve seats, the typical life of the 2.0L SPI in the Focus is about 100-120k miles, but it can happen as early as 70k miles. When the valve seat drops out of the cylinder head, it falls into the cylinder and mashes up the piston and cylinder head. In some cases, the valve seat will exit the cylinder it dropped in and be sucked into other cylinders via the intake manifold until its obliverated. A valve seat dropping into the cylinder can, and sometimes will, scar the cylinder wall up and bend the piston connecting rods. When repairing an SPI with a dropped valve seat, it is very important to remove the intake manifold and have it thoroughly cleaned before bolting it up to a new cylinder head. If not cleaned, trapped particles in the manifold from the dropped valve seat will enter the repaired engine and scar the cylinder walls and sometimes ruin the new cylinder head. The best solution for avoiding the dropped valve seat problem is to purchase and install a new cylinder head that has had the valve seat problem fixed.
found it on wikipedia.com hope it helps out on u SPI drivers =D