Prepare boiling water canner. Heat jars in simmering water until ready to use. Do not boil. Wash lids in warm soapy water and set aside with bands.
Tie pickling spice in a square of cheesecloth, creating a spice bag, if desired.
Combine vinegar, water, sugar, pickling salt and spice bag in a large stainless steel saucepan. Bring to a boil over medium-high heat, stirring to dissolve sugar and salt. Reduce heat and boil gently for 15 minutes, until spices have infused the liquid.
Place 1 bay leaf, 1 garlic clove, 1/2 tsp mustard seeds and 1 head of dill into a hot jar. Pack cucumber slices into jar leaving a 1/2 inch headspace. Ladle hot pickling liquid into jar to cover cucumbers leaving a 1/2 inch headspace. Remove air bubbles and re-measure headspace. If needed, add more pickling liquid to meet recommended headspace. Wipe jar rim. Center lid on jar and apply band, adjust to fingertip tight. Place jar in boiling water canner. Repeat until all jars are filled.
Process jars 15 minutes, adjusting for altitude. Turn off heat, remove lid, let jars stand 5 minutes. Remove jars and cool 12-24 hours. Check lids for seal, they should not flex when center is pressed.