Jon69

Since this is an active thread, I hope it is ok that I ask my question here.
I have a 1969 Cutlass S convertible with a rotten frame.
The original boxed frame has rusted out in several sections, so much so that most, if not all of the center section would need to be replaced or reinforced.

The Solution: Replace the frame.
The problem: Finding a factory boxed frame that is in reasonable shape, isn’t priced for more than I have, and close enough that I can actually transport it on what little budget I have is proving to be more than difficult.
I can get a Hardtop frame easily in the area.
As stated above the outer section is the same as the frames used on a convertible except they lack the boxing section pieces added to the inboard center sections.
The Question: What needs to be done to a hardtop frame (assuming it is in decent condition) to reinforce and stiffen it adequately to support a convertible instead?
I know it would have to be boxed, but is there any particular way this should be done in this application?
The transmission cross member is different on the convertible from the hardtop. Using a hardtop cross member would not be possible after the frame is boxed since these were made to attach to the lower section of the frame. Is there a big problem if the hardtop frame is boxed and some sort of bracket or tabs are added to the inboard boxing sections to support the transmission cross member?

joe_padavano

I moved this post to a new thread because it really is a different question. This question also comes up frequently.

First, many people think "boxing" the frame like the street rod guys do - just cut and weld flat plates to turn the C-channel into a box section. On the A-body cars it's quite a bit more complicated that that. As I noted, the outer part of the boxed convertible frame rails is basically the same as the channel section frame rails under a hardtop. The problem is that the pieces welded in to make it a box are fairly complex stampings that vary in height, depth, and cross section area. As you also noted, on the 68-72 cars the inner stamping also has a formed section for mounting the trans crossmember.

It is theoretically possible to cut the convertible-unique stampings from the convertible frame and weld them to a good hardtop frame. I doubt this is cost-effective, however, even with what A-body convertible frames are going for ($2800 or so at Carlisle - but hey, that's ASKING price... ). Without a frame jig, you also run the risk of warping the frame during the welding. Trying to fabricate the boxing sections from scratch is REALLY not cost effective, in my opinion, but I haven't tried it either. As a structural engineer, I can tell you that unless you duplicate the shapes of the factory parts, you won't get the same bending or torsional stiffness in the new frame. That will lead to doors that don't close, paint that cracks, and possibly more severe problems, particularly during an accident.

Depending on how bad your current frame is, it may be more cost effective to patch it locally. You might want to consider that.

Run to Rund

I have installedd the boxing sections from one frame onto another. It is time consuming but it works fine if you have the frame level, and weld different places (not putting all the weld in one area then moving on). To ensure that you press the boxes onto the frame the right amount (distance) and the same on both sides, install a tranny crossmember and you can even mock up with a block and tranny case to be sure the holes in the crossmember for the trans mount are centered-or use a tape measure! If your rusted out donor frame has good boxing sections, you can remove them with a slag grinder (cut through the welds) and put onto another frame.

Jon69

Joe
Thanks for the reply, and sorry I posted in the wrong thread.
I guess I will have to expand my quest.

Jon

joe_padavano

No problem, I just figured your question would get more visibility in a dedicated thread.