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73-87 Chassis Articles

Shortening a longbed frame

By: Leadfoot (Tim Sisco)

This is how I went about shortening the frame on my `82 longbed.

First off, remove back bumper and unhook tail and tag lights,then remove the 8 bolts holding bed to frame..

Then you will need to get it on a hard level surface and remove the fuel tank(s),driveshaft and carrier bearing,but leave rear tires installed.

Cut a piece of 4”X 4” to around 40” or so to place on top of the jack and support the frame.  This will keep the frame supported and level.  Place jack just in front of the back side of cab   about where the carrier bearing was.  Placing the 4”X 4” on the jack and running across the bottom of the frame rails jack the 4x4 up to just pre-load jack ONLY.

Now carefully measure back 6” from back of crossmember that supports rear of cab and run masking tape all the way around frame rail.  Check it with a square to make sure it is straight and square to the frame rail.  Now measure back 14” and mask around the frame again. 

These tape lines need to be exactly 14” apart ALL the way around.  This is where the difference is comparing a SWB to a LWB.   If your are cutting a torch, use the tape lines and mark it with a silver sharpie or chalk and remove tape.  If your gonna use a cutoff wheel, use the tape as a guide.

Remove all clips connecting to the frame that hold any wires or lines in the designaed cut area and pull the lines away from frame so you can safely cut the frame rail.  You don’t have to remove them, but you must get them out of the way.

Now you are ready to start cutting.   In planning your cut try to leave a little mettal that you can remove to make sure everything is square.  Fire up the torch or grinder and carefully cut that section of frame out.  Once it is out you can roll the rear section into place.  Care must me taken as to not kink brake lines or crimp wires. step back and admire your work, because now you are now committed to this project.

Check your fit and grind as needed to get things lined up and square.  Using a square across the top of the frame (from front to back, side to side, up and down) check to see everything is square and level.   Use the jack to to correct any issues. (the rear section rolling makes this lots easier)  Once everything looks correct, tack it in place.  Once everything is securely tacked together, re-check in all directions.  After verifying for correctness, weld it all around..

Leadfoot grinds the weld off of the top and outside of the frame until smooth, then using a patch cut from the section of discarded frame he caps it and then weld the patch thoroughly.

Alternatively we recommend cutting like this and splicing the sections and welding a plate over the topd as shown here here in this link: http://forum.73-87chevytrucks.com/smforum/index.php?topic=7334.0.

Moving to the rear of the frame, the rear crossmember must be relocated.  Using a torch heat the rivets glowing red hot and take a large chisel and 2lb. Hammer and knock the heads off the rivets. (these may also be removed with a cut-off wheel if a torch is not handy) then prevents frame damage from grinder or just cutting them off with the torch.   Once the rivets are out and the crossmember free, move it forward to the other set of holes in the frame.  They are about 6” forward of where it was previously located. Using Grade 8 bolts, bolt it back in (no drilling required,since the holes are already drilled in the frame).

Now mark the rear of the frame in the same fassion as described earlier and cut 6” off the back of the frame rails.

That basically covers the frame,all that’s left is the reassembly details.  Holes will have to drilled in the side of the frame to mount the gas tank brackets. You must use a tank from a shortbed truck to accomplish this as the longbed tank is too big to fit.  A short bed will fit as all the bolt holes line up to remount the bed. You also must use a shortbed driveshaft, or have one built.  Fuel may need to be modified.  Brake lines and rear wiring will either need to be shortened/ bent or replace with ones from a shortbed.

Reinstall bumper and hook up lights and your done..sit back and admire your work..