Guarantee We know that having a transmission rebuilt isn’t a pleasant experience. We want you to feel confident that you are getting the best service for the best price – and a quality guarantee. That’s why Custom Transmissions warranties all rebuilt transmissions for twelve months or 12,000 miles. Our warranty policy demonstrates our trust in the parts we use and the labor our technicians perform. We do what it takes to ensure your satisfaction and continued confidence in our shop’s services. Please view our service standards for a more in-depth explanation.