Front Lower Unirail Section Procedure for the General Motors 200 Series Van

Introduction

The first and second Research Bulletins published by Tech-Cor in 1990 focused on repair procedures for the outer Sheet Molded Compound (SMC) panels and also the rear structure on the new General Motors 200 series van.

This Research Bulletin will deal with repairs to the front structure of this vehicle platform. The 200 series van incorporates a complex front unirail that is a major load bearing member of the vehicle structure, Photograph 1.

Since it is not practical or necessary to replace the entire rail if only the front portion is damaged, a section procedure is a viable alternative, Photograph 2 and 3.

The section is performed using an offset splice with a 1/4 inch overlap, at a location where there is an internal reinforcement that aids in locating and fit-up procedures for the replacement components. For this reason it is recommended that a rough cut be performed on the damaged rail ahead of the section splice area, so that final trimming to the exact dimensions can be accomplished in a simplified manner.

Since this particular lower rail is serviced as separate components that are welded together to form the complete assembly, the section procedure is accomplished by installing one component at a time, verifying measurements and welding the individual members together to form the completed unit.

This Research Bulletin deals with repairs to the left front portion of the structure, and all dimensions specified pertain to the left side only. If this procedure is performed on the right side of the structure, the locations of holes used for measuring purposes may differ. Therefore, all measurements must be verified prior to performing any rail cuts.

Repair Information for the GM 200 Series Van

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 must 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 so that proper dimensions are attained. This must be done prior to cutting and removal of any primary structural member.

Tech-Cor does not recommend a "butt joint" or a "butt joint with insert" for this section repair. Because of several advantages of welding and the application of corrosion protection, we recommend a lap joint with a final overlap of 1/16" minimum to 1/4" maximum with a continuous edge weld around the rail at the section joint. This overlap provides necessary measurement tolerances during the cutting and fit-up of both the replacement and original structure.

It is very important that the replacement structure be positioned over the original structure. This will allow the application of the corrosion protection to be more effective since the open portion of the joint will face the open end of the rail. To achieve this overlap, carefully split the 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.

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 for the GM 200 Series Van

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.

Damaged Rail Procedure: (Engine Compartment Side)

• Measure the inner portion of the damaged rail (engine compartment side) from the inner edges of the two large holes stamped in the inner rail. These two holes measure 509 millimeters apart.

• Scribe and cut a line 180 millimeters ahead of the rear hole, measuring towards the front of the vehicle, Photograph 4.
  

• Care must be taken when performing this cut to avoid damaging the internal reinforcement.

(Wheelhouse Side)

• Measure outer portion of the lower damaged rail, (wheelhouse side) from the holes that measure 163 millimeters between the inner edges. The locating hole towards the front of the vehicle is threaded.

• Scribe and cut a line 25 millimeters ahead of the rear hole, measuring towards the front of the vehicle, Photograph 5 and 6.
  
  

• Drill out spot welds that secure the reinforcement to the damaged rail.

• Care must also be taken not to cut into the internal reinforcement when cutting the outer portion of the rail.

Replacement Rail Procedure:

(Engine Compartment Side)

• Measure inner portion of replacement rail section using the large stamped holes as a guide.

• Scribe and cut a line 174 millimeters ahead of the rear hole measuring towards the front of the rail, Photograph 2 and 3.

• Cut replacement section at the scribe line. The inner portion of the replacement rail does not contain the internal reinforcement.

(Wheelhouse Side)

• Measure outer portion of replacement rail section using the threaded hole as a guide.

• Scribe and cut a line 19 millimeters ahead of the rear hole measuring towards the front of the rail, Photograph 2 and 3. This will allow enough additional material for the appropriate overlap at the section joint.
  
  

• Drill out spot welds that attach the reinforcement to the replacement rail.

• Cut replacement section at scribe line.

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 wire 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.

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.