Extruded Solutions Front Rail Seal XL 5 1/2ft EPDM Rubber Fills a height gap of .488" to .937" from the sidewalls to the front rail when mounting a truck cap If the height of the gap is .251" to .487" install Front Rail Seal (ASIN: B00C0WVV72) If the height of the gap is .250" or less, just install one ESI high quality EPDM rubber truck cap seals on all 3 rails (see descriptions below) No need to replace this seal every time your cap is removed.