Saturn Rear Unirail Sectioning Procedure

In the interest of providing increased customer satisfaction, the Saturn vehicle line was both designed and engineered with reparability in mind. This is evident not only from the use of bolt-on plastic exterior side panels, but in the construction of the underlying space frame as well.

This bulletin will focus on a sectioning procedure for the rear unirail that also includes replacement of the rear body panel and rear floor pan.

The basic unirail section applies to all Saturn models, since any differences in underbody measurements are found in the center sections of the 4-door and station wagon derivatives. Although the section referred to in this bulletin was performed on the right side of the vehicle, the procedure also applies to the left rear unirail.

To gain access to spot welds securing the structural members that will be replaced, the bolt-on quarter panels are removed. Removal of these panels will also allow a better visual inspection of the space frame, since the plastic outer panels can rebound after an impact and hide damage to the vehicle structure. Refer to Photograph 1.

Prior to removal of any damaged structural components, the space frame must be returned to its proper dimensions through appropriate pulling methods.

The section procedure begins by drilling out the spot welds that attach the rear body panel to the space frame.

With the rear body panel removed, spot welds that attach the rear floor pan to the unirails, rear quarter panel reinforcements and rear crossmember can be drilled out. Removal of the rear floor pan will allow proper access to the unirail where the section will be performed. There is an internal rail reinforcement located approximately 14 inches (356 mm) from the energy absorber mounting flange. This reinforcement acts as a reference for the section joint. Refer to Photograph 2.

The damaged rail is cut straight through 1/2 inch (12.7 mm) rearward of the internal reinforcement. Refer to Photograph 3.

The section is performed using a 1/4 inch (6.4 mm) overlap with the replacement rail located 1/4 inch (6.4 mm) from the internal reinforcement in the existing rail. The overall measurement when the replacement rail section is clamped in place is 23 7/8 inches (605 mm) from the inside of the energy absorber mounting flange to the center of the 1/4 inch (6.4 mm) diameter hole located on the inner portion of the existing rail, near the rear suspension crossmember mounting point. Refer to Photograph 4.

After fit-up of the replacement rail section, dimensions should be verified prior to welding. Refer to Photograph 5.

The replacement rail is attached by lap welding on the inside to avoid overhead welding (refer to Photograph 6), while welding of the replacement floor pan and rear body panel is performed using the MIG plug method.

Once the replacement rail section has been welded in place, cleaning and priming of the weld joint should be performed prior to installation of the floor pan. Refer to the Welding Information for general welding guidelines.

When welding has been completed, plug weld nuggets that may interfere with proper mounting of the exterior panels should be ground flush.

As indicated in their Body Repair Manual, Saturn Corporation requires the use of a two-part epoxy primer such as PPG DP90/DP401 Catalyst, or the equivalent on all bare metal surfaces to achieve proper corrosion protection, and also to match the color of the coated structure.

After priming and caulking of any exposed seams (such as the outer portion of the section joint, trunk floor and rear body panel), any exposed exterior surfaces of the space frame that may be subjected to harsh conditions, such as the underside of the rear floor pan, should be coated with an appropriate abrasion resistant material.

The closed portion of the sectioned rail should also be primed using a wand, then coated with an anti-corrosion compound.

Welding Information

Prior to any welding operations, the weld site must be cleaned to remove grease and dirt as well as primers and paint, without removing factory corrosion protective coatings or thinning the base metal. Do not remove any preexisting galvanized coatings. Do not use an oxyfuel (oxyacetylene) gas torch. After cleaning, a weld-through coating must be applied to all bare metal in the overlap area of the weld sites. All welding should be performed with a M.I.G. welder (Metal Inert Gas, now called G.M.A.W. or Gas Metal Arc Welding). Welding wire should be either 0.023" (0.584 mm) or 0.025" (0.635 mm) diameter which meets or exceeds the American Welding Society Standard (AWS-E-70S-6) with a mixture of 75% argon and 25% carbon dioxide shielding gas. Do not concentrate all welding efforts on one area, but alternate sites so that each welded area has time to cool down before the welding is continued. All continuous welds must be made in 1/2" - 3/4" (13-19 mm) increments which helps to minimize heat build up which can cause metal strength loss and/or panel distortion. The welding equipment must also be properly adjusted by making and testing practice welds on similar materials.

Corrosion Protection, including the refinishing of replacement components should be performed as outlined in Tech-Cor Bulletin 1986-7, Restoring Corrosion Protection During Vehicle Repair.

The information provided in this bulletin is for educational purposes only. Although every effort has been made to ensure the accuracy of the information contained in this bulletin, Tech-Cor assumes no responsibility or liability for any repairs performed using information from any publication issued by Tech-Cor.

Any person performing repairs must determine whether any suggested or recommended procedures or repairs are suitable or appropriate for the particular vehicle being repaired. The repairer remains solely responsible for such determination, as well as for the proper completion of the repairs.

Reproduction of this bulletin is not permitted without the written approval of Tech-Cor.