Tax liability considerations when refurbishing chassis

We are overhauling a customer-owned chassis. The original chassis had a GVWR of less than 33,000 pounds, making it non-taxable. Most of the chassis will be rebuilt with new materials, but a few original components will be retained. As the renovated chassis will have a GVWR of more than 33,000 pounds, will the used components be subject to Section 4051 tax?