Rust and the unit body Ford

A guide to common, yet hidden rust in 20+ year old salt belt cars

by Brent Peterson


After twenty years in a northern environment where road salt is the norm, steel can suffer greatly, particularly after the coatings designed to protect the metal begin to break down due to age. There are many common areas under the car that normally suffer at the hands of the demons of salt and rust. Most un-restored cars that have been driven in winter most of their life will likely have rust somewhere on the underside.

In my travels I have seen many older Mustangs, Torinos, Fairlanes and other unit body Ford automobiles that appeared in great to easily fixable condition from above, yet below were a total mess. It would seem that for many cars the body was repaired, but the underside was allowed to continue to rust. This guide is intended to point out these areas which in cases of severe damage could make a car unsafe to drive. While no car will have damage in all the areas described below, it is common to find at least surface rust in one of them.

Front Third

Each region of the underside of the car has its own places were rust can take hold and do its evil work unseen, the front third of the car contains most of these places. The most forward of these just rear of the lower control arms. The support for the lower control arm has a flange that extends along the bottom of the frame rail. This flange is a prime location for the development of a very destructive rust. The rust will eat its way clear into the frame rail if allowed to go unchecked. The next location is just to the rear of the first in fact they can become one rust hole. Rust will attack the frame rail where the steering box or idler arm is mounted from the inside of the frame rail. This takes us to the junction between the frame rails and the torque boxes, and into the middle third of the car.

Middle Third

The middle third is straight forward, there are two areas that are most prone to rust in this region. The first is the torque box and its junction to the front frame rails. The torque boxes are located just rearward of the front wheels under the front passenger's feet. These pieces are made of a heavy gage steel and were designed to make the unit body more rigid. The only other area prone to rust in the middle third are the floor pans themselves.

Rear Third

The rear third can hide its rust very well, and a casual inspection will not revel it unless the exact locations are checked. The first and most important are the rear frame rails behind the rear leaf spring shackle. Inside the frame rail there is an inner piece which the shackle is bolted to, it is the frame rail its self that normally will rust, weakening the leaf spring connection to the frame. In the trunk, the trunk floor can rust along its connection to the rear frame rails, and then creep along to the wheel wells. The flat regions of sheet metal, (underside of the trunk floor) between the gas tank and the rear frame rails, must also be checked for rust.


This section will deal with a quick overview of common body rust that can be repaired, at worst with new body panels. Underhood, the seams between the sheet metal parts should be checked, and under the hood lip. In the front wheel wells the rear part, inward of the plastic guard needs to be checked. The fenders will normally rust at the bottom between the front wheels and doors. The lower part of door skin and door drain holes areas require inspection. In the rear wheel wells, all the sheet metal should be checked inside and outside of the wheel well.

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