When do you replace valves/guides?

[wes]

I have had my race bike for 2 years now, and when I got it the block I bought if off said the valves had been replaced 2 years ago and were good for another 2 years. Which would mean I need to change them all now and presumably new valve guides too.

How often do you change your valves or do you measure/inspect each season?

I rebuilt my engine last winter, completely refreshing the bottom end and on checking the vales were surprised to learn that I needed almost the biggest shims to achieve the correct clearance. I would have thought with wear or vales stretching that the gap would get smaller and require thinner shims?