GM N-Body Left Front Lower Rail Section

Introduction

Beginning with the 1992 model year, General Motors redesigned the front lower unirail on its N-Body vehicles as part of an overall platform update. The N-Body series includes the Buick Skylark, Oldsmobile Achieva and the Pontiac GrandAm.

This bulletin will specifically address the section repair of the left-front lower rail on 1992 and later GM N-Body vehicles, equipped with a V-6 engine and automatic transmission; but the basic procedure will also be applicable to vehicles equipped with four cylinder engines and standard transmissions. The general location and configuration of the section joint used for the left unirail may also have some possible applications for the right unirail. However, to properly complete the right-side repairs removal of the drive line is likely. The location and severity of the right-side damage will also be affected by the motor mount reinforcement on the top side of the unirail.

The section joint on this vehicle which is equipped with an automatic transmission is located approximately 19 1/2 inches (495 mm) from the front edge of the rail under the raised area of the mounting bracket for the transmission.

The raised area allows the rail to be cut from the outboard side, after the closure panel is removed without the cutting tool contacting the bracket. This also places the vertical cut under the mount, where the raised portion will allow access to apply anti-corrosion material including primer and top coats. (The section joint for the left side rail on a manual transmission equipped vehicle should be located forward of the transmission mounting bracket.) It is important that the replacement rail section be placed inside the original rail portion remaining on the vehicle, as this will aid the internal corrosion protection operation.

The new rail section is attached with a lap weld joint, using a 1/4 inch (6 mm) overlap. The lower horizontal seam and the vertical (side) seam of the rail are welded from the inside of the rail, while the upper horizontal seam is welded from the outside. Welding the rail in this manner allows the component to be sectioned without removing the entire drive line and only a limited amount of adjacent components for access during cutting and welding operations.

Section Joint Location

Locate the 1/4 inch diameter (6 mm) reference hole on the top of the rail approximately 21 1/4 inches (540 mm) from the front edge of the rail, above the rear half of the transmission mounting bracket. Refer to Photograph 1

The section joint location is approximately 1 1/2 inches (38 mm) forward of the reference hole. After returning the unibody to its proper dimensions, and removal of the damaged components has been completed, the existing lower rail can be marked for cutting. When the Shock Housing Panel is being replaced, the following steps can be omitted.

A vertical cut is made in the Shock Housing Panel to create a window located 23 1/2 inches (597 mm) rearward of the front edge of the rail, approximately 4 inches (102 mm) rearward of the leading edge of the shock housing panel. Remove the spot welds that attach this portion of the Shock Housing Panel to the upper and lower flanges of the lower side rail. There are approximately two spot welds per flange in the window area.

The window is then "opened" to allow room for cutting the rail and welding the inside of the section lap joint.

Procedure

Both the damaged rail and the replacement rail are cut straight through. The damaged rail is cut 1 5/8 inches (41 mm) forward of the leading edge of the reference hole. The replacement rail is cut 1 3/8 inches (35 mm) forward of the leading edge of the reference hole (Photograph 3) to allow for the 1/4 inch (6 mm) overlap at the section joint. With the replacement rail clamped in place, the measurement from the reference hole's front edge, to the front edge of the rail is 21 1/8 inches (537 mm). Refer to Photograph 3.

As with any structural component replacement procedure, verify all dimensions, secure all loose components and review the welding information section before proceeding to any welding operations.

The replacement section of the unirail is placed inside the original frame rail for ease in welding and corrosion protection operations. After confirming the dimensions, continuous lap weld the three sides in 1/2 to 3/4 inch increments allowing for adequate cooling between welds of the rail section joint. Welding the bottom and side of the section joint seam is performed from inside the channel (Photograph 4), while the top joint is welded from outside the rail (from inside the engine compartment).

Clean the section joint welds and apply corrosion protection material to the lap joint inside the rail and caulk the underside of the top seam, before the window on the Shock Housing panel is "closed" and rewelded. Install the remaining body components to complete the repair duplicating the factory weld locations using the MIG plug weld method.

After all welding and cleaning operations are complete, the outside of the section joint seam on the bottom and side of the rail should be caulked and properly protected against corrosion.

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 pre-existing 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 (WAS-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.