On DI cars it is common to walnut blast the valves and ports. I have also done this on my DI car -- walnut shells will not damage the metal but will take carbon and buildup off and make it look like new. You will need a good compressor and ensure nothing gets into the combustion chamber. If the head is off, seems like a decent way to clean it.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth