Wayne Chong Sang, Contributor

It is important that restoration be carried out in accordance with very specific methods/techniques in order to preserve the structural integrity of today's 'unibody' vehicles.

In a unibody vehicle, the body is made up of several load-bearing components, referred to as structural members which, together with some of the external components, provide the structural strength of the vehicle. Unlike older vehicles (pre- to mid-'70s), the structural members of these newer unibody or unitised vehicles are made up of stronger, lighter, higher strength steel. Identifying the high strength steel components in a modern vehicle could prove very time-consuming; this information would be available from manufacturers and in some after-market publications. One easy-to-remember and good rule of thumb is to approach these cars, considering all thin-gauge (20-gauge or less) panels and structural members as being made of high-strength steel.

Heat sensitivity of high-strength steel (HSS) is a critical consideration some can withstand temperatures of 650 degrees Celsius for up to three minutes without weakening. Most have temperature limits as low as 370 degrees. All heat-sensitive high-strength steel will be weakened if heated excessively, essentially converting it to mild-strength steel.

What this means in the construction of a unibody vehicle is that a weld or section of a component designed with yield strength of 65,000 psi, after being subjected to temperatures above these limits, would be weaker and collapse much sooner than anticipated. High-strength steels are also brittle; unlike mild steel, HSS tends to break or crack if bent and straightened. Most of these types of steel cannot withstand the high temperatures of oxyacetylene welding; the recommended method would be with a MIG welder.

The first choice of replacing structural members should be at factory seams/joints; this should be done whenever possible or practical. Should deviation from this be necessary the manufacturers would normally have recommended 'sectioning' points. It is important to adhere to manufacturers' recommendations so as not to further compromise the integrity of a structural component by cutting through reinforcements.

Sectioning would be the method most commonly used when using 'used or second-hand' assemblies. There are specific areas on a unibody car that special sectioning techniques must be adhered to, they are the closed/box sections and the open-surface/single layer panels.

The closed/box sections include the front rail (chassis leg) A pillar, B pillar, rocker panel and the rear rail (rear chassis leg). The open surface/single layer panels include the floor pan, trunk floor, quarter panels; quarter panels are included as an open surface component because sectioning is recommended only in areas where the single layer panel areas of a quarter panels are accessible.

When sectioning structural panels, the three basic types of joints to be used are: the butt joint with inserts; the offset butt joint without inserts, and overlap joints. There are different variation and combinations of the three basic joint types, which would depend on the area and application of the structural panel being sectioned.