How do you change engine valve stems?

To change engine valve stems, first remove the cylinder head from the engine and take out the valves. Use a valve spring compressor to carefully remove the valve springs and retainers, then extract the valve stems from the guides. Replace the old valve stems with new ones, ensuring they fit snugly, then reassemble the valve springs and retainers, and reinstall the cylinder head. Always check for proper clearances and ensure everything is lubricated before reassembly.