The brass vales are what regulate the gas flow. They include the orifice which is a separate piece that controls the maximum amount of gas that is injected into the burner tubes. When ordering, it is important that you select the correct gas type as we will ship your new valve(s) with the correct orifice.
To install you will need to unscrew the existing vales from the manifold. Normally it is best to remove everything from the inside of the chassis including the firebox. With the gas turned off, you loosen the screws on the front that hold on the manifold. Once the manifold has been removed, you should be able to remove the valves. Pipe Joint Compound is required for the threads on the valves; the compound must be natural gas and liquid propane safe. All safety precautions must be taken after reinstalling the manifold, including testing for leaks. LazyMan recommends that only a trained gas professional install the vales.
The alternative is you can purchase a pre-assembled manifold with the gas vales installed and tested for leaks. You can purchase the entire assembly > here <.