Or, the stem clearance was too tight. Running iron guides with tight clearance will do that. Depends on the materials used in the valves and the guides. Last set of heads that I had rebuilt where the shop used a reamer hung open an exhaust valve. That was a shop in Houston that was supposed to know what was right, being that they had Top Fuel blocks with prominent names on the tags.

The shop I use now hones the guides to get the clearance. If If your builder used a reamer and it hadn't been checked for size in a long time, that would do it.

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1967 Falcon 4 door 351C - Owner built, owner abused.
70 Mustang 351C / 06 Ranger, 04 SuperCrew parts hauler