Frame Material Cj5?

Hey there, I have couple questions about my stock cj 5 frame that maybe someone could answer. Is the frame made of a special hardened steel, and what’s the gauge of metal? It looks like it’s between an 1/8 and 3/16. Theres been a lot of talk here about not welding on the frame or heating for fear of making brittle spots?
I ask because im in the process of fabbing up a C CHANNEL frame out of 3/16 flat plate hot rolled to replace my crazy warped and rusted frame rails. Though I’m concerned the hot rolled 3/16 is not beefy enough to allow flex and keep its strength.
This is c Channel frame Im building not a 3/16 box jobbie. In part why I’m building a new frame is the rad crossmember gotta come out And moved forward under grill to get my engine swap forward different motor mounts as well a ps steering box is going in so major surgery to the front... it’s just better to build the frame from scratch.
Yes It would be easier to find a better stock frame, however in southern Ontario Canada the rust here is about the same as Michigan so finding anything useable is not likely. Thanks any feed back would be helpful. Jesse

 

AFAIK it's plain mild steel, nothing special. Exact alloy I do not know. Why can't you measure the thickness?

 

Hey, I did there’s sooooo much surface flak rust it’s hard to get a true reading with the calipers... I suspect from the factory it’s an 1/8. Thanks

 

The stock rails vary between 1/8” and 3/16” depending on how much it was stretched/compressed during its forming process. They are just a mild steel, nothing special, but you will notice that most cracks that happen on stock frames happen in the vicinity of factory welds. A weld not only causes the section of the surrounding metal to change in composition on a molecular level (before annealing, that they didn’t do AFAIK), but it’s also compounding in strength at the area where it, say, doubled in thickness. Think spring mounts. The section of frame where they are welded has become a solid immovable unit (at one with the spring mount), and the frame material immediately surrounding the mount has been stressed by the weld, possibly weakening it. As the rest of the frame is allowed to twist, and the welded area is strengthened and/or weakened.

This happens at any point in the frame that a bracket is welded to it, frame boxing is added, etc...

It is best to either build a C-frame and plan on frame flex, or use a boxed frame and keep the flexing to a minimum. Anything in between is a recipe for cracks.

If welding to the frame is unavoidable, weld in directions that don’t fight or resist frame flex. This is why fish mouths or Vs are recommended when adding frame plates. A frame will twist like a corkscrew, and a perpendicular weld will resist that twist. An angled weld will follow the direction of the twist.

 

11ga (.120in) is the thinnest i would go.
The 3a frame are 11ga. earlier cj2 frames were thinner. If you are boxing the frame 11ga .120in or 1/8 should be more than fine on a typical hunting and icecream getter.
If you're not sticking with stock engine or axles I would go heavier if the local prices aren't terrible go heavy simply because it will take longer to rust thru.

 

Excellent info! Thanks a lot for quick replies. I’ll continue on my 3/16 hot rolled c channel frame. Pics posted when done. J

 

I know my thoughts run contrary to some, but I like to all but eliminate frame flex. You should have a suspension that does the flexing and is made to do so. The frame has a very finite number of flex cycles in it before it starts to fail, along with extracting a price from everything attached to it. The MB/GPW/2A frames were the most flexible but you pay a dear price for that if that attribute is well utilized. Very few of those early Jeeps that were actually used as a Jeep will not have broken/patched frames.

If building from scratch, use a rectangular box section. If using a stock 3A/3B/CJ 5 frame, box it.

 

I second the no frame flex. I used 2x5x1/4" box tube on my CJ6, with additional crossmembers, gussets everywhere and such.

 

2x5x1/4! that frame is not going to flex one bit!

 

Which is worse, the heat stress or the stress riser that's caused by joining two members? perhaps it's just coincidental that welds are located at these joints? I thought the "classic" locations for cracks on these frames derived more from the mechanics of flex.

 

Hard to say without an experiment with one frame as it came off the line, and one annealed, then both frames treated the same for 50 years.