Frank Frame Talk - More Than Dimensions
 

I received another inquiry on using one year C-3 frame (75) on another year C-3 car (69) and I answered the question as follows. It is just one example of how the frame drawings do not give you the story of the differences in the frames. Dimensional differences are only part of the story. Here was my full reply for your scrutiny and push back.

"Well this is where the drawings for the frames will lead you to believe they are interchangeable, which they are not. I have laid this out for others before but let me show you why.

In 1968 they took the 1967 frame and modified it for the new body style.
In later 1968 they added reinforcing vertical gussets to the kick up area which were badly needed.
In 1969 they had what they wanted so they left it alone.
The late 1968-1972 cars all used the same frame with no modifications.
In 1973 they modified the front frame horns holes to accommodate the new front bumper.
In 1974 they attached a new rear bumper impact frame to the 1973 frame to handle the new rear bumper.
In 1975 they modified the rear of the frame significantly to accommodate rear impact bumper shocks.
In 1976 they squared off the 75 rear further and held that design for 1976-1980.

When you look at the drawings they show the dimensions but do not show you the differences in the frame design.
If you want to use the 1975 frame for your 1969 car you will have to cut and remove the rear section behind the kick up area as they are not at all compatible. In the attached pictures you will see a 1976 frame (almost identical to a 1975) sitting atop of a 1970 frame (again the same as 69) and you can see the differences in the rear sections. When you cut the 75 rear section to fit it with 1969 parts you will start the spending that you could avoid if you buy the correct frame.
https://cimg9.ibsrv.net/gimg/www.cor...0bb3e7dd28.jpg
https://cimg0.ibsrv.net/gimg/www.cor...eef412dccd.jpg
https://cimg1.ibsrv.net/gimg/www.cor...0f8ab1a52e.jpg
https://cimg3.ibsrv.net/gimg/www.cor...e4789d191a.jpg

Also, and you may know this already, the body mounts are different on the 1969 and 1975 frames. This can be accommodated but it is another modification that will cost to change.

And most important of all is frame condition. They all look great initially. Make sure you have the frame verified dimensionally and structurally before putting money into it. Measuring dimensions, squareness, and warp are all important when you are starting with a new frame of unknown origin. Tram gauges, gauge blocks, and a frame machine are all used on my frames to make sure they are true and square and not warped.

Whatever you decide to do let me wish you the best of success. The frame is the backbone of your car and often given the least attention. When a frame fails, and they do unfortunately, the car becomes unmanageable.

Best regards,

David Howard"

I hope some find this helpful. Just don't want anyone thinking a frame swap is "easy" to accomplish and without its own issues and expense; especially if you are paying someone else to do the work.

Have a great day.

David Howard
AllVettes4Me

Thank you for the informative post, great information sir. It's never easy do to things correctly. I appreciate your attention to the details and willingness to share.
Eddy

Nice write up David. I, like many on here, thought that the various year frames were interchangeable. I should have known that my 73 was unique as I thought someone had welded on the rear bar on mine as it was different from others that I had looked at.

Excellent write-up David!

Great post David!

Thank you for the kind comments. Not perfect but I enjoyed working on the frames I had in my possession and though I cannot fabricate myself I am happy to have been able to develop some improvements to the frames that have made their way onto other friends frames now.

The transmission cross member is the single nicest change I was able to improve upon for manual, four speed frames. This modification allows the owner to drop their manual transmission with ease and leave the engine in the car when this work is needed. I have seen it used on several cars now and it works like a dream. Shown here are a few pictures of my 1972 road racer frame that was modified using the bracket system which I utilized.
https://cimg9.ibsrv.net/gimg/www.cor...33a70fa486.jpg
https://cimg0.ibsrv.net/gimg/www.cor...f372e0dd55.jpg
https://cimg1.ibsrv.net/gimg/www.cor...0b72354ea8.jpg
https://cimg2.ibsrv.net/gimg/www.cor...340444076f.jpg
https://cimg4.ibsrv.net/gimg/www.cor...cc4a219aa5.jpg
https://cimg5.ibsrv.net/gimg/www.cor...3826f10b09.jpg

The gussets are another area that are worth replacing and reinforcing when working on your frames. Not shown here but I will get some pictures to show you which ones I am referring to for improving the frame.

New landing spots, called the body mounts, are also a great place to improve the strength and performance of your frame. Those #2 and #3 body mounts often have seized bolts in them and I have seen more rotten #4 body mounts than I ever thought would be present.

The front cross member is also an area that can be enhanced and improved with a simple, yet effective, addition. I have used a contoured plate shaped to match the contour of the front plate and I can use this spot as my lifting location without damaging the cross member. If my plate gets damaged I can remove it, repair it, put it back into service but honestly have not seen it get damaged yet, it is beefy.

I will try to get some pictures taken of the gussets, body mounts, manual Z-Bar brackets, and front lifting plate posted here soon for your consideration.

If you are going to take the time to restore your frame then consider these improvements and I know you will be happy with the results.

Best regards,

David Howard
AllVettes4Me

Wow. Great research and info. 👍

Hey David, have you tried cutting the cross member with the body still on the frame. I am at the point that I can do it, but I am still trying to decide on which way to cut it and keep it square and not cut my fiberglass floor. If I have to disconnect the body and lift one side at a time it's going to have to wait a long time

I was lucky enough for David to set me up with the shop that does his frames. Fortunately my 69 frame checked out dimensionally. I went all out with full seam welding and gussetting as advised in the Chevy Power Book. All 8 body mounts were replaced.
https://cimg9.ibsrv.net/gimg/www.cor...e48208f42.jpeg
https://cimg1.ibsrv.net/gimg/www.cor...08c390831.jpeg
https://cimg2.ibsrv.net/gimg/www.cor...800260961.jpeg
https://cimg3.ibsrv.net/gimg/www.cor...6543f2410.jpeg
I also have the front crossmember “cover plate” to hide all our dented fronts from jacking. This is a great mod. Best of all, however was making the transmission crossmember removeable in the same fashion David shows.

Here is a couple more pics of the crossmember mod and gussets.

https://cimg2.ibsrv.net/gimg/www.cor...76bc98d36.jpeg
https://cimg4.ibsrv.net/gimg/www.cor...bb65c6cb4.jpeg

No sir. A chop saw makes the cut square and body off is the only way to cut it and weld it safely. Sorry.

Thanks, I guess it'll have to wait til I do a body off a couple of years down the road. Bummer

Thank you Bill.

Fantastic information. 6am and I’ve already learned something today.

I did mine without lifting the body. You have to do it a bit differently with the body on. Instead of welding a flange I welded brackets to the removed piece of x-member. Not shown are the steel tubes I put inside the frame (car side) where the bolts go through to avoid collapsing the frame. Those I spot welded.
This all worked out great and is plenty strong. It was done 20 years ago and never had a problem.

https://cimg7.ibsrv.net/gimg/www.cor...13a9b235ad.jpg
https://cimg1.ibsrv.net/gimg/www.cor...9e8e86989f.jpg
https://cimg3.ibsrv.net/gimg/www.cor...1e053b341d.jpg
https://cimg5.ibsrv.net/gimg/www.cor...022add5c50.jpg

great information dave! I have a question about your first series of pic's. in this picture is the rear cross member frame dip (for the differential u-joint and drive shaft) off to the passenger side when compared to the centerline of the front crossmember or is that just camera angle?
thanks,,
Paul

https://cimg5.ibsrv.net/gimg/www.cor...4a99261860.png

My '72 was an old show car, and so I found the P/O must have redone all the welds on much of the frame...not a wreck involved, just nice clean seams....some years ago I removed the Muncie 4 spd and went to auto trans for the overdrive....Florida at 80+ needs an O/D.......so to remove the welded in cross member, I cut the thing with body on frame, about 3-5 inches from the frame rails, got welded on a couple of L angle iron, to fit over the stubs left on the chassis, drilled a 1/2 inch bolt hole going fore/aft to hold it in place, and added another L in lower back of cross support, just for extra help on the stubs.....

being lazy and only dropping the pipes once to get that cross support out, I cut the lower loops out and built up a structure across the top loop to keep it rigid....... dropping the pipes is easy pie now, and the cross member is even easier, 4 bolts and it's outta there, trans is mine.....

I moved the gauges for illustrative purposes when taking some of the picrures or else you cannot see them when they are all lined up straight. I did have one frame that had a twist in it from going into a ditch. Was off a little over an inch left to right and on the datum line plane. It went to the rack and was straightened back to correct and welds reinforced. Stayed straight as an arrow from that point on. This shop uses a compressed air rack that pulls and pushes the frame like it is soft metal, fun to watch and be a part of the process.

Something to look forward to then. I love lifting the bodies and working on the frames and the underside of the car, it is so easy to access and clean when the body is off. For my 74 Convertible the frame is in good shape so the cleaning will take place with the body on. Tempting to remove it, but not needed. With the gantry crane and lift strap system sold now my body lifting days are over now. It sure was fun though.
David

[QUOTE=zwede;1599060081]I did mine without lifting the body. You have to do it a bit differently with the body on. Instead of welding a flange I welded brackets to the removed piece of x-member. Not shown are the steel tubes I put inside the frame (car side) where the bolts go through to avoid collapsing the frame. Those I spot welded.
This all worked out great and is plenty strong. It was done 20 years ago and never had a problem.

Nicely done 👍