Proven design, low cost and effective performance all make the Harrop Eaton Truetrac differentials the ideal choice for a wide variety of vehicle applications both on and off road. No maintenance - Just Traction. The Harrop/Eaton Truetrac was the first helical gear differential ever introduced into the automotive aftermarket. It is based on technology derived from the Eaton Detroit Truetrac® limited-slip differential (LSD) which is a pioneer in helical gear based traction technology, and remains the leading helical gear-type limited slip diff in the industry. The Truetrac’s proven helical gear design eliminates the need for wearable parts, resulting in maintenance free traction recognized not only for its toughness, but its smooth and quiet operation as well. Vehicle Applications: Vehicle models fitted with Ford 9" housing and 31 spline axles.