Rear frame crossmember & sill replacement keeping body on frame ?

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I have collected parts to replace my very tweaked rear crossmember, including the angle brackets and frame supports as well as the rear sill which has been the worts piece of sheetmetal since I got this truck 19 years ago. (dang I feel old). This will also involve re-doing my hacky exhaust "extension" that was a short term fix to cut down on exhaust fumes in the back where my kids ride.

All of the "in progress" pictures of rear crossmember replacements I have seen are from "frame offs." Has anyone done this with body on truck? Any tips? Really just looking to avoid getting into and realizing something that gets me "stuck."

Pics cause everyone likes pics.

IMG_8756.jpeg


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I assume you're bolting the new crossmember & angle brackets into the frame? I think riveting would be difficult with the body on. (For me it would be difficult even with the body off!)
There are some good posts on here on how to remove the rivets. (Grind off head flush, drill a small hole in rivet shaft, then use a punch & hammer to knock rivet out. This worked well for me.) The sill plate can be done with the body on. I made my own sill but the process is near the same with the reproduction sills. I used a 110 volt Miller MIG with gas (no flux core) and it was pretty straight forward. 4.5" grinder with metal cut-off wheel is your friend. I purchased the left 1/4 panel (complete) & right 1/4 panel (lower patch piece) from CCOT. They fit very well.

rear sill.jpg


rear sill a.jpg


rear sill b.jpg


rear sill c.jpg
 
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I assume you're bolting the new crossmember & angle brackets into the frame? I think riveting would be difficult with the body on. (For me it would be difficult even with the body off!)
There are some good posts on here on how to remove the rivets. (Grind off head flush, drill a small hole in rivet shaft, then use a punch & hammer to knock rivet out. This worked well for me.) The sill plate can be done with the body on. I made my own sill but the process is near the same with the reproduction sills. I used a 110 volt Miller MIG with gas (no flux core) and it was pretty straight forward. 4.5" grinder with metal cut-off wheel is your friend. I purchased the left 1/4 panel (complete) & right 1/4 panel (lower patch piece) from CCOT. They fit very well.
Yes, I am planning to use rounded head socket cap screws and also weld where practical. I have good welding equipment and plenty of experience so not concerned about that part but was more concerned about having enough space to get to the rivets for removal. I may also box the rear section of the frame. Good to know that it is practical to replace the sill on the truck as well, thanks for the pictures!
 
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I replaced mine with the frame on. You can get to all the rivets. Cut off wheel and an air chisel worked the best. Just bolted everything back into place with no rivets or welds (I don’t know how to weld 🤣)

You may have to get creative with the hardware you use so that you can fit it in between the body and the frame .
 

fjc-man

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I drilled the rivet centers out in 4 size steps, then chiseled the heads off. If you do the cross member with the old sill out of your way, that will make it way easier.
 
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The frame crossmember is twisted but look solid , why not try to unbend it
Fair question! It’s bent it multiple places as are the brackets it is riveted too. New was not that expensive, rather just start freshy fresh and not spend time trying to bend and potentially still end up R&R
Pre-79 didn’t have rear body mounts bolted to the bumper, did they? Do they have rear mounts?
Yes there are two mounts from sill to rear frame crossmember on this ‘74
I drilled the rivet centers out in 4 size steps, then chiseled the heads off. If you do the cross member with the old sill out of your way, that will make it way easier.
Since I am also replacing all the bracketry and leaf spring mounts I think this reduces the number of rivets to remove? I hope! I foresee getting impatient and plasma cutting the heads off and air chiseling the remainder but good to know drilling out the center is practical.
Old sill is probably salvageable but since new sill is on the way might as well replace and speed up the crossmember work. Good to know!
 
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Be persistent, and you’ll get there. Hint: it may take awhile.
I am the type that I hesitate to tear it apart, but once I do I will kill myself to get it done ASAP! Prepared for this to take a little while
Mine is a 1977 and it has four body mounts that bolt through the rear bumper/crossmember :)

You can totally do it! I'm right up the road if you ever need help.

Matthew
I noticed after I posted that there are 4!
Thanks for reaching out!
That tailpipe should really be posted over here... https://forum.ih8mud.com/threads/random-redneckness.1163759/
Haha tru tru. I am from Georgia. It's part of what happens when you build a rock crawler with exhaust tucked up high out of the way but then have kids and stop rock crawling and need to no longer smell fumes in a hurry!
 

jim land

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I have change the crossmember and braces on my 42 with the body in place and not cutting one single rivet , because i also change the frame ends , the body acting like a jig to place the parts already assemble with cap head srew

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I like it @jim land !
I hadn't thought of doing it this way, but I bought new frame ends so I could do it the same way and then replace the sill after doing the frame work. Should be a lot quicker than cutting rivets!
 
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Hey @jim land did you use hex drive socket head cap screws or torx drive? I found both in M8, and while I prefer torx, the hex head are 140k PSI strength vs. 110k for the torx drive.

In addition to the screws did you add some welds to strengthen the crossmember and brackets?
 

jim land

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Hey @jim land did you use hex drive socket head cap screws or torx drive? I found both in M8, and while I prefer torx, the hex head are 140k PSI strength vs. 110k for the torx drive.

In addition to the screws did you add some welds to strengthen the crossmember and brackets?

hex head cap and gold zinc oval lock nuts , (better than common nylon nuts ) , no weld and no signe of weakness since 2018 , its my daily and i pull a trailer often.

After the fitting , cutting and welding was all done , all the bare steel parts was painted separately and assemble before the final frame weld

20180507_140217.jpg


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Awesome, thanks @jim land !
I have similar socket head cap screw and a distorted thread locknut + washer. I like your hitch reinforcement. I don't pull a trailer much but will use a hitch hauler and bike rack often. =
 

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