A liner insertable into the central bore of a downhole tool includes a resilient material rolled into a substantially cylindrical shape. The outside diameter of the liner is variable to allow the liner to be inserted into a narrowed bore of the downhole tool near the box end or pin end. Once past the narrowed bore, the outside diameter of the liner self-expands within the central bore of the downhole tool. The outside diameter of the liner may expand to contact the inside surface of the central bore. In selected embodiments, a transmission line may be routed between the central bore and the outside diameter of the resilient material. The liner may be effective to protect the transmission line from materials traveling through the central bore.