1990 and Later General Motors C/H Body Front Lower Rail Replacement

This bulletin focuses on the redesigned front rails used on the 1990 and newer models and the inability to section these later model components due to the length and design of the internal reinforcement.

Different procedures have been researched in an effort to identify a structurally sound and efficient repair. Although this particular unirail does not lend itself to the usual sectioning technique, a practical repair can be accomplished by replacing the forward portion of the rail in the torque box area. With this procedure the majority of the replacement rail is used, with only minor disassembly of the vehicle interior required. This method of replacement also lessens the disruption of factory joints, seam sealers and OEM applied corrosion protection.

The manner in which the unirail is constructed enables the complex rear segment (the portion attached to the firewall and floor pan), to be removed from the replacement rail assembly by drilling out certain factory spot welds. This allows the entire front portion of the replacement rail to be used. Refer to photographs 1A and 1.

The damaged rail is also separated from the vehicle at this location.

All photographs contained in this bulletin refer to the right side of the vehicle. This procedure is also applicable to the left rail.

To facilitate this repair method, the drivetrain assembly must be removed. This will allow the proper access needed for drilling, quality welding and refinishing during the procedure. Depending on which lower unirail is being replaced, certain fuel, air conditioning and/or brake lines may have to be detached from the rail to gain the needed access at the factory joint location.

As a safety precaution, the interior carpeting and insulation should also be removed from the toe pan area to protect them from possible damage during welding operations. Before any structural components are removed, proper pulling operations must be performed to return the vehicle structure to its proper dimensions.

Removal of the damaged rail begins by scraping away the undercoating and caulk to locate and drill out the spot welds at the lower firewall/toe pan seam. These spot welds secure the closure panel located just forward of the base of the front hinge pillar. Refer to photographs 2 and 3.

There is also an engine cradle mounting bracket which surrounds the factory joint and is part of a support panel sub-assembly. After drilling out the spot welds that attach the bracket to the rail assembly, a 1 1/2 inch cut is made at the bottom outboard portion of the bracket (refer to Photograph 4) so that it can be separated from the support panel. Refer to photographs 5 and 5A.

After the closure panel and bracket have been removed, the damaged portion of the rail can be separated from the vehicle at the kick-up (the point adjacent to the rear cradle mounting location), by drilling out the attaching spot welds of the factory joint.

CAUTION: When drilling out the spot welds to separate the replacement rail assembly, DO NOT drill into or through the internal reinforcement. Refer to Photograph 1 above.

At the upper portion of the rail kick-up near the base of the firewall there is also a flange that must be cut through since the spot welds are inaccessible. Refer to photographs 6 and 7.

After the front portion of the rail has been removed, the original spot welds can be drilled out of the flange that was previously cut in Photographs 6 and 7. This will then allow removal of the remaining portion of the flange before installing the new structure. Refer to Photograph 8.

After the replacement rail portion has been clamped in place, and the proper dimensions have been verified, the upper flange of the rail should be scribed from the inboard or engine compartment side using the existing holes in the base of the strut tower. Refer to Photograph 9.

A 5/16" spot-weld cutter is then used to drill through the first layer of the two-piece flange. Refer to Photograph 10.

This will ensure necessary weld penetration through the multiple layers of metal.

An additional area that may require holes to be drilled through the first layer of metal is at the joint where the replacement rail is installed. If the bracket that was previously removed has holes which do not align with factory welds, it is necessary to scribe these holes that span the joint and drill through the first layer to expose the internal reinforcement in the rail. CAUTION: Do not drill into or through the internal reinforcement. Refer to photographs 5 and 5A above.

This procedure is carried out prior to welding the bracket back in its original location. Proper weld strength will not be achieved if the bracket is welded only to the mating surface of the rail.

During structural component installation, additional plug welding holes should always be drilled in the first layer of multiple layer panels wherever holes in the adjoining panel do not have a factory spot weld directly in line with them. Once the replacement rail and bracket have been plug welded in place, the bottom outboard portion of the bracket that had been previously cut to allow its removal (Photograph 4 above) is butt welded from the top and bottom sides to achieve proper weld penetration through both layers of metal. Refer to Photograph 11.

The flange on the upper portion of the rail that was previously cut through (Photographs 6 and 7) is then edge welded on both the inner and outer sides. This welding should be completed prior to installation of the closure panel. Refer to Photograph 12.

With the rail welded in place, brush prime the area of the joint which will be covered when the closure panel is reinstalled. After allowing the primer to dry thoroughly and all solvents to ventilate, the closure panel at the base of the front hinge pillar can be welded in place using the plug welding procedure with the existing holes.

In addition to the priming, caulking and refinishing of the outer portions of the structure, the closed portion of the replacement rail, as well as the repair joint must be coated with an appropriate primer and anti-corrosion compound. The area behind the closure panel must also be corrosion protected and can be coated by inserting a 360° spray wand through the cradle bolt mounting hole located at the bottom of the support panel. The exterior portion of the closure panel should also be covered with an abrasion-resistant undercoating material.

All corrosion protection procedures, including external refinishing should be completed prior to installation of the drivetrain assembly.

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 grinding into 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 0.023/0.025 in. (0.6 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 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.