Rinse cucumbers to remove any debris, lay on towel to dry.
Slice a thin strip off the flower end of each cucumber if desired, to help keep them crunchy.
Gather all your equipment and fill a large canner pot with water, leaving enough room for increase in level as you lower jars in. Start bringing to boil as it takes a while for that amount of water.
Combine vinegar, water and salt in large pot and bring to a boil.
Start with clean hot jars. Place a head of dill and a clove of garlic in the bottom of each jar. Pack tightly with as many cucumbers as possible. Add a second head of dill to the top of each jar and add ¼ tsp of alum.
Pour boiling brine carefully into each jar, leaving a headspace of about ¼ inch. Wipe rims of jars and ensure no dill is hanging over.
Warm seals in canner pot just for a few seconds, then place on jars. Cover with rings and tighten by hand, not too tightly.
Boil jars in large canner pot, about 8 minutes for medium sized jars, up to 15 if using large jars.
Remove from water and let sit at room temperature to cool. Lids will pop as they seal. The seal will be indented if sealed properly. Any jars that don't seal should be stored in the refrigerator.
Allow to cure for at least 3 weeks for smaller pickles, up to 6 weeks for larger, before opening. Will last in the pantry for at least a year.