1974 MGB Single Groove
2002 Subaru Head 83,000mi on engine before it was grooved. 2.5L Horizontally opposed engine. Used a Dremel tool and a file.

MGC Head Single Groove Head Shaved .060 Four Groove
Head removed: Note Clean valves after 20,000 mi. Also cleaner burning plugs. The head came off to be replaced by multiple grooves.
Notice how clean the valve trane is with so many miles on this 40 yr old engine.
Head now in use on 1969 MGC
Somewhere in Wyoming......