A lot of 302 GMC engines had rotating valves. If you have them, you will need new valves, springs, spring retainers and keepers. The rotating valves were difficult to adjust and required a special tool. We used them back in the day, but they are not necessary for the way vehicles are driven today. The rotating valves can be identified by the tapered groove for the keeper. I have a bunch of these left over from over 60 years ago and they are scrap. Attached is a photo of a rotating valve.
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Rotating valves.jpg (47.57 KB, 21 downloads)
GMC Rotating Valve