1. Place clean jars on rack in pot. Fill pot with water and cover. Bring to a boil to sterilize jars for 10 minutes. Use tongs to remove jars to a clean towel. Keep water boiling.Pickles:
2. While sterilizing jars, rinse cucumbers and scrub well with a soft brush. Trim ends and cut in half lengthwise. In a large measuring cup, prepare half of the liquid for pickles: Stir together 2-1/2 cups of the vinegar, 1-3/4 cups of the water and 4 teaspoons sea salt until salt is dissolved. Pack two jars: Add 2 tablespoons fresh dill sprigs, 1 bay leaf, 1 clove garlic, 6 whole peppercorns and 1/8 teaspoon red pepper flakes to each jar. Add 1/4 of the cucumber halves, about 8 per jar. Top each with 2 more tablespoons fresh dill.
3. Divide liquid evenly between packed jars, leaving 1/2-inch of space below rim. Repeat blending of remaining 2-1/2 cups vinegar, 1-3/4 cups water and 4 teaspoons salt until dissolved. Pack remaining two jars with dill, bay leaves, garlic, peppercorns and red pepper flakes, topped with remaining cucumber halves and more dill. Cover with vinegar mixture.
4. Warm lids in a bowl of hot water; dry lids and put on jars, then screw on caps. Place on rack and cook (process) in boiling water for 15 minutes. Pickles will appear olive-colored. Remove with tongs to a towel and cool overnight. Lids will be flat and have no give when pressed. Store in cool place for up to 1 year.