If you are rebuilding your engine's cylinder head (or heads), then there comes a point when you might have to replace the valves or valve guides. There are many reasons why this would need to happen, but the most common is when the inside diameter has become worn and oil is starting to leak though to the combustion chamber. At this point, you have two options. You can either ream the inside of the guide and install valves with thicker stems, or you can replace all of the valve guides with new ones. That decision is usually made based on how much time and money you want to spend on the engine overhaul, but if your valves are good then usually you will just replace the guides. Either way, it is absolutely critical that the clearances between the inside diameter of the guide and the outside diameter of the valve stem are tight enough to prevent an oil leak but not so tight that the valve gets stuck in place. Oil blow-by will result in reduced octane levels and pre-detonation, and a stuck valve can ruin the rocker or lifter if it's stuck closed and it could get smashed by a piston if it is stuck open.
Your engine's valve guides are pressed into the cylinder head so that each valve has a clear and controlled path in and out of the combustion chamber. They are typically made of bronze or steel, and should last tens (or hundreds) of thousands of miles. Since they are located at the top of the combustion chamber, they tend to absorb extremely high temperatures. This eventually wears out the oil seal and causes excessive oil consumption. So, one key thing to consider when your engine is burning oil is the condition of the valve stem seal and the valve guide.
Integrated valve guides cannot be replaced, as they are part of the cylinder head. They can be reamed and filled with oversized valves, but they cannot be swapped out. Older engines have pressed valve guides that can be popped out and changed. Once they are removed, the open space they once occupied should be honed and smoothed. After that, they can be filled with a valve guide with a thicker outside diameter. The inside diameter should be maintained so that you can reuse your original valves. You can also get new valves if necessary... after all, it couldn't hurt.
Buying replacement engine components doesn't have to be a difficult process. Sure, you want to get the highest quality parts for your cylinder head - like new valve guides. But you don't want to get junk just because you find a good deal. Car Parts Discount carries excellent engine internals from the top name brands in valve guide manufacturing such as Sealed Power (Federal Mogul) and Canyon to name a few. So don't go anywhere else when looking for new engine parts for your car or truck. Buy from CPD and do the job right the first time.