1/8" Blue Poly Fiber-Fill Insert 3-7/8" length 31inc 12-219 / BOWES TR 15025S Box of 501 8" Blue Poly Fiber Fill Insert 3 7 8" length 31inc 12 219 BOWES TR 15025S Box of 50. These inserts are gray vulcanizing gum coated, wrapped in blue protective poly, and have a reinforcing core of fibre, designed to allow their easy insertion through steel belted passenger or truck radial tires. Unlike competitive products that are fabric throughout (forcing the injury to take the shape of the insert), Fibre Seal have a larger amount of rubber