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Mark Jeffrey of Trans Am Racing performed most of the port work on this 289 head casting. Note the dimensions Mark is compelled to follow. Intake-valve sizing is up to 1.94-inches from the stock 1.54-inch size. Exhaust-valve sizing increases to 1.60-inches from the stock 1.45 inches. Increased valve sizing helps-so does extensive port work. If you're interested in keeping a stock appearance externally, porting factory heads is a good idea.