71 Body on 85 K20 Frame

[Chevummins]

Hello,

New here. I have a big project. I have a '71 C10 that is in fairly good condition. I had thought I would swap the body onto an also good condition '85 K20 frame and get a nice K20 truck powered by my cummins engine/drivetrain.

When I put the body on the frame the rear of the cab sits on a raised portion of the frame requiring a huge body lift and other adaptations. At the end of getting the body mounted I am sorely disappointed in how high the body sits.

I don't want to cut the nice body. I was considering cutting the frame and refabbing it. I also have 69 C20 frame and I started doing a straight axle 4x4 conversion to. Clearly the body would fit this frame fine, but the 4x4 conversion is also a large undertaking. Additionally, I don't feel like the C20 frame is very beefy to begin with. I see many boxing the frame etc... IF I am going to do all that frame work I could just mod the K20 frame to fit the '71 cab better. Has anyone done this? Any advice on my situation would be appreciated
[IMG][/IMG]

[Chevummins]

It doesn't appear that anyone has anything to add to my question. I laid up under the trucks and measured and pondered yesterday evening.

I think I prefer the 85 K20 frame to converting a factory '69 C20 frame to 4x4. I am considering extending the straight section of the frame underneath the cab and grafting the bends to where they occur right at the leaf spring hanger. Somewhere between 18 and 24 inches would be removed in the rear. Now I am pondering what to fab the straight section out of. Maybe 2 parallel 2" square tubes. Truss with something smaller like 1.5" square tube and then skin one side with .25" plate to make the outer edge smooth and flush with the rest of the frame. Kinda like this rough rendering

Guess the rough and tumble wheeler crews probably know all kinds of way to modify and graft frames.

[special-K]

I'm not sure what you have going on there, but I know these bodies fit on square body frames fairly well. The cab needs a 1" lift because the square body frame rises up that much more under the bed, so since the bed sits directly on the frame the cab has to come up. The only other issue is in front of the cab the frame is about 3" longer, so the core support has to be moved back. Same with the axle, a zero-rate can help there.

Hopefully someone with a truck they could get you pictures from will come along. Plenty have done this swap here and none have had to modify the later frame the way you describe.

[Chevummins]

Special-k, thank you kindly for your thorough answer. I have the cab resting very close to the frame. I am not surprised that someone has done this before, but I am not internet savvy enough to find it.

It would be great to just not modify the frame. Maybe some lift is preferable to not doing frame mods.

Again, thanks for the help.
Greg

[special-K]

On my friend's '68 K/20 sitting on what I would guess to be a late-70s frame, he didn't like the height and went to stock springs. It has 12.50/33s on 8.25 wide rims and looks like it has a a 2-3" lift. He relocated the front axle back using "Zero-rate leafs (that are blocks if you ask me) which adds one of those inches.

To move the core support back you need to either move the mounts on the frame, or he just drilled the frame and came up with his own cushions using generic stock. Try posting in farther down the menu below the parts boards in the 4wd and off road section. You will get better responses.

And welcome to the board! You'll get the help you need here.

[Chevummins]

As a matter of update to document the direction I ended up taking with this mod, I thought I would post a few pics and descriptions. Maybe it will help someone else down the road.
It finally dawned on me that the majority of the body mounts were on the 71 frame. All I really needed was the front suspension from the 85, and that would leave the core support mounts only to be modified. The cab and fenders can bolt as one piece and precisely identify where the mount locations need to be.
SO....
I cut both frame in half like so:
http://cubeupload.com/im/pypCrU.jpg

Then I Narrowed the height of the 71 (rear frame) a bit to nest perfectly inside the front 85 frame:
http://cubeupload.com/im/aANFcn.jpg

Once the 2 halves were slid together the pair looked like this:
http://cubeupload.com/im/VfN1XL.jpg

I also welded the 2 frames in short 1-1.5" beads around the V notches and the outer edges. All welds avoid being vertical and are stick welded using 7018 rods for strength. I am pretty confident that the frame won't fail in this area.

I drilled thru the frame and added 3/8 bolts that had a shoulder. Hammer them home to almost the point of stripping. I then cut off the excess bolt and welded the nuts in order to simulate rivets.
http://cubeupload.com/im/CCrXOi.jpg

I added the cab mounts back in their original holes and used hardware where the rivets were:
http://cubeupload.com/im/NgGPju.jpg

After a lot of prep work, we primed and painted the frame.
http://cubeupload.com/im/Ic1zcf.jpeg

I also welded the 2 frames in the V notches at each end. I used short 1-1.5" beads spaced out to avoid heat damage to the metal. I used 7018 stick welding for strength. I feel fairly confident that the frame will not fail in this area.

[72 tigger]

Looks like you did a nice job blending the (2) together- how much lower will the body be?

[Chevummins]

The body is probably about a 4 inch difference (rise in the frame). I also built the leaf spring hangers on the back leafs and I think they will ride lower also. The 85 rear leaf hangers were spaced a couple of inches below the bottom of the frame.

My thought is you can always add height, probably in a preferable way. Shackle flips, spring arch etc...but subtracting is harder. I am putting a cummins in it and may need a bit of a body lift to begin with. Hopefully only an inch or so...

Thanks for looking
Greg

[68 TT]

I did this swap a few years ago.

I added a section to the frame just in front of the arch at the back of the cab so the rear cab support could sit on the frame at a height that aligned with the bed. This is the limiting factory since the bed sits right on the frame.

I cut the length out of the now stretched frame behind the back of the cab as needed to set my wheel base.

After doing this I approached it differently with my 67 Suburban. I used all the square body front crossmembers and suspension parts and fit them to the 67 C10 chassis. Pretty much a bolt in with just the need to flatten out the parking brake cable boss in the side of the frame as it is right where the front leaf spring shackle mount needs to be.

Out back I used a set of leaf springs & hangers and crossembers off of a leaf spring optioned 69 Chevy C10 that I converted to trailing arms. The shape of the frames out back is different between trailing arm frames and factory 4x4 frames so all the parts need to be swapped out. I don't know if the GMC's that came stock with leaf out back use the same parts as the optional leaf spring equipped Chevy C10's or if they used the 4x4 shaped frame rails with the different crossmember and spring hangers.

The square body rear leaf spring forward mounts don't even come close to fitting a 67-72 chassis due to that rise in the frame. Way too much pinion angle along with about 6" of lift. Without the Chevy C10 optional leaf spring front spring hangers you would need to build some front spring buckets that placed the springs at a more desirable location but that would be pretty simple to make.

[Chevummins]

For even more fun... Here is the little 4bt that I plan to power this truck with.
https://youtu.be/XQ9uINUkj54

Next to conquer an NV4500 rebuild and then reassembly of the Gear Vendors Overdrive... NP205 Gosh the list is long!

[RPBurris]

I can post some pics up later. Mine is on a 78 chassis (as far as I can tell). 3" cab mounts front and rear using fabricated mounts. The bed aligns well with the cab.

[Chevummins]

Let's say it is only 3 inches. Then the stock 30" tires look like garbage. I think my 85 came with 35s. I don't want to run a tall truck. I want stock, small tires a 4bt and 30mpg. I think other trucks look fine with big gear, it's just not for me.