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post #3 of (permalink) Old 07-16-2003, 07:30 AM
Jim_Lou
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Re: Rusted Rear Frame/Shackle Mounts on YJ

The following is a writeup I did about a year ago on the subject:

For several weeks I have been restoring the frame from my '78 CJ7. The body was severely rusted and is being replaced. The frame was not nearly as bad.

There are two types of places where the frame had significant rust damage: Where body mount biscuits had trapped water between them and the frame, and where two pieces of flat steel had been welded together.

The body mount damage was worst at the grill mount, and at the two mounts on the crossmember behind the back axel. The frame at those places was rusted away so that the size of the original hole could not be determined. The rest of the body mounts are slightly damaged but do not require repair. I have no idea why those rusted out, while the others didn't.

The flat-metal damage was confined to several places in the rear of the frame. The worst of these was on the bottom of the horizontal section just forward of the rear crossmember, including where the rear crossmember is welded to the frame rails. There the inner and outer channel pieces overlap about an inch on the bottom.

Nearly as bad were the inside frame channels, just behind the back axel, where two stampings overlap and are welded together. Also requiring repair were the places where the crossmember behind the back axel attaches to the frame.

In all of these places the overlapped metal is welded together on one side, and water is free to enter on the other. In the case of the rear frame rails, water can actually puddle inside the frame and penetrate the joint until it evaporates.

One indicator of this kind of damage is bulging of the steel. Rust is bigger than steel, and as it forms, it forces the two panels apart. The only indication of damage in several places was the bulge, but when it was cut open, much of the steel had turned into rust.

That being the case, the proper method of repair is to completely remove the damaged panels and fit replacement pieces to be butt-welded. The butt-welding is very inportant. If you simply form a piece to go over the damage, and weld it in place, you're creating a new, even bigger place for rust to form. Because the water has entered from the inside of the frame tube, it will do so again. Worse, the existing rust holds water like a sponge, so there is a built-in supply ready to go to work on your repair panel.

If fitting the repair panel for a butt-weld is too difficult, at least cut out the damaged metal and trim up to the edge of the repair panel. That way the area that can trap water is minimized, and water that enters the joint will evaporate before it can cause much damage.

In the forward areas of the frame rails there was very slight damage, which must be because the inside chanel only goes into the outside channel about a quarter of an inch, so there isn't much area to trap and hold water.

One modification I'm going to make is to put drain holes at all of the low spots in the frame tubes. These are at the very back of the frame, at the front and rear bottom of the back axel arch, and at the rear of the front axel drop-down. All of those are high-stress areas, so I'm going to reinforce around the hole by seal-welding a washer-like piece of 10-gauge around the hole.

Attached is a picture of the rear frame rail. I had previously made a temporary repair, which is the outer tube. At the bottom of the picture you can see where the rust has progressively lifted the inner part of the channel. Throughout the frame, the inner channel is made of thinner sheet, so it is the one to bend. This is looking rearward after I cut it off with a bandsaw.

Another modification I plan is to make a large hole in the plate that closes off the front of each frame tube. That will allow air to flow more easily through the frame tubes and, I hope, dry the inside.
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