Step 1In a medium saucepan, bring the cider, salt, vinegar, pepper flakes and allspice to a simmer. Stir to dissolve the salt completely, then remove from heat and cool to room temperature.
Step 2Place the pork loin in a 1-gallon resealable plastic bag and pour the cider mixture over the top. Squeeze out any air and seal the bag tightly. Refrigerate overnight.
Step 3The next day, start a fire with 10 pounds of charcoal. Remove the pork from the bag and pat it dry. Skewer the pork on the spit and secure it tightly.
Step 4When the coals are covered in gray ash and the fire has settled to the point that you can hold your hand at skewer level for 3 to 4 seconds, begin roasting the pork over indirect heat. It will take about 1 hour for the pork to reach 155 degrees.
Step 5Remove the pork to a platter and set aside for 10 minutes to finish cooking and to allow the juices to redistribute. Remove the pork from the spit, slice one-fourth- to one-half-inch thick and serve along with its juices. The pork may be slightly pink inside, but it will be thoroughly cooked.