Front Lower Unirail Section Procedures for General Motors 10 Series Vehicles

Introduction

The purpose of this research bulletin is to convey collision repair information regarding sectioning procedures for the front lower unirail on General Motors 10 Series vehicles. Introduced in 1988 as front-wheel drive models, this completely redesigned platform included carry over nameplates such as the Buick Regal, Oldsmobile Cutlass Supreme and the Pontiac Grand Prix. In 1990, the Chevrolet Lumina coupe and sedan models, that share the same vehicle platform were also added to this model series.

The section procedure described in the following text uses an overlap method of attachment that has undergone extensive testing and is a proven, effective method of repair. This procedure can also save both time and labor when structural damage is confined to the front portion of the rail, Photograph 1.

Since the lower rail is considered to be a primary load bearing member of the structure, it should be replaced partially or completely (depending upon the amount and location of damage) if the metal is badly kinked, cracked or torn as a result of deformation.

This bulletin describes in detail the section procedures performed on the Driver Side lower unirail. The same procedure can also be applied to the passenger side using different measurements.

Please read the bulletin completely before starting the procedure.

General Repair Information

Normal safety precautions must always be observed when working with collision damaged vehicles and related repair equipment.

Prior to replacement of any front unibody rails, the following procedures should be completed to allow for proper sectioning techniques:

• Remove necessary exterior sheetmetal and trim to gain proper access to the damaged area.

• Pull and align front unibody structure to factory specifications to insure proper dimensions. This must be done prior to cutting and removal of any primary structural member.

When the rail section procedure is completed, the remainder of the assembly and reinstallation of other unibody parts can be completed. All adjoining seams and welded areas should be prepared as described in the WELDING INFORMATION section of this bulletin. 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.

Sectioning Procedures - Driver's Side - GM 10 Series

Read this bulletin completely before making any section cuts on the damaged or replacement rail. All measurements should be treated as Guidelines Only, ongoing manufacturing changes may affect the placement of holes during component stamping operations.

The lower rail section procedure can be performed with the drivetrain in place, which greatly reduces the amount of time and labor required for repairs. Prior to performing any measuring or cutting operations, the battery tray must be removed by drilling out the attaching spot welds that secure it to the lower rail. Removal of the air conditioning receiver-dryer is also necessary, (if the vehicle is so equipped) as well as its mounting bracket that is also secured to the rail with spot welds, since this bracket spans the area of the rail where the section joint will be located.

If this procedure is performed with the drivetrain in place, and the vehicle is equipped with air conditioning, space limitations will prohibit two of the six spot welds that secure the receiver-dryer mounting bracket from being removed. In this situation the bracket can be bent out of the way far enough to allow cutting, fit-up and welding operations, Photograph 2.

The initial cut line is scribed on the outer portion, or wheelhouse side of the damaged rail. This places the cut line 25 millimeters ahead of the leading edge of an 18 millimeter hole that is stamped in the outer portion of the rail, measuring towards the front of the vehicle. There is also a 10 millimeter hole stamped directly beneath the 18 millimeter hole for reference, Photograph 3.

The cut line for the replacement rail is scribed 18 millimeters ahead of the same 18 millimeter hole that is stamped in the outer portion of the rail. This will allow the appropriate overlap at the section joint.

Driver side rail section dimensions:

Damaged Rail "A" - 25 mm

Replacement Rail "A" - 18 mm

To achieve this overlap, carefully split the two formed corners on the original structure at the exposed end. These splits should not exceed 1/4". Any portion of the splits that are exposed after fit-up are to be welded closed. The upper and lower pinchweld flanges are assembled with a butt joint. This can be easily formed by carefully cutting or grinding back the flange area of the original rail as the replacement rail is fitted and aligned, Photograph 4.

Care must be taken to remove only enough metal to create a uniform butt joint with a 1/32" root gap for proper welding. The flanges must also be welded from both sides to achieve proper penetration and strength.

Since the internal reinforcement in this particular rail is not located in the section joint area, the cuts can be made straight through both the damaged and replacement rails, Photograph 5. This applies to both the Driver Side and Passenger Side front rails.

Sectioning Procedures - Passenger Side - GM 10 Series

The 18 millimeter hole stamped in both the Driver Side and Passenger Side lower rails are not placed in exactly the same locations. Measurements taken on the Driver Side are made towards the front of the vehicle, while those taken on the Passenger Side are made towards the rear of the vehicle. Refer to Photograph 6 and the dimension chart.

Passenger side rail section dimensions:

Damaged Rail "B" - 3 mm

Replacement Rail "B" - 10 mm

All other steps in the section procedure are the same as the Driver Side.

Welding Information

Prior to any welding operations, the weld site must be cleaned 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 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 and panel distortion. All plug welds are made by welding around the edge of a 5/16" (8 mm) diameter hole. The welding equipment must also be properly adjusted by making and testing practice welds on similar materials.

Remember, 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.