The oven cleaner worked great-even on the exhaust ports. I have used it on other parts as well. It may darken some aluminum parts. It will mess up your skin big time so wear gloves. When I can make the trip I will have the machine shop size the guides, cut the seats, grind the valves and surface the heads.

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Tony H
W111 280SE 3.5 Coupe
Manual transmission

Past cars:
Porsche 914 2.0
'64 Jaguar XKE Roadster
'57 Oval Window VW
'71 Toyota Hilux Pickup Truck-Dad bought new
'73 Toyota Celica GT