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Spark plug type

Engine

Vehicle Information: Model: 911; Year: 1968; Body Type: Coupe; Engine (type, size, modifications): 2.0L Type 901/14; Total Mileage: 110k;

Ed, I have read your past posts regarding spark plugs and they have been a great help in understanding temperature grade. My question is regarding when to use a projected tip. For example NGK recommends a standard tip for cars up to '68, but projected tip for '69 on onward. I don't think the pistion to head clearance changed between those years (affecting tip clearance to pistion), but CD was added. Does this mean that projected tip plugs should be used when CD is used and the plug clears the pistion at TDC? Thanks for your thoughts.

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