I am beginning to understand why there are stickers on truck frames say not to weld or drill on them. Its because the factory guys have seen what kind of mess people can cobble together when some one thinks they can weld. I went to look at a c65 that is a brush fire truck. They said it was a conservation truck before that and it had a tank and pump when they got it. It had been bright green like a fire truck to start with and now its red. The frame had been shortened and was plated from front spring hanger to rear spring hanger. Some one then took and welded some junk on the top and bottom were the frame was cut, Then so the bed would set flat more junk was welded on top of the frame. The bright red pant caught my eye so i put a bid on it.
An older medium truck is probably not a heat treated frame.
Just came home from a ford single axle that was a beverage truck with a 5th wheel. They added three feet of frame, moved the rear end and springs back, cut 12" of a different truck to add to it and installed the flat bed dump that was on it's frame. I cut and trued the mess, made a 3/4 plate and bracing for the end of the frame and trued up a hitch the customer took off a different truck and tacked it all up last week. Had my customer run it by a welding shop to weld it all up Friday/Saturday. Welder did a good job and customer was happy, now I am running the elect controller, trailer plugs, and air lines and glands to the hitch. I have also seen many frames patched and welded, this truck came from a auction and you could not see what was done from the outside but boy when you really climbed up under and looked you could see it. I have tried to stay away from trucks the last 16 yrs. rather than getting involved with jobs like this.
It isnt just the shadetree mechanic types. There is a dump box builder here that loves to cut and paste different sizes of frame together. Welding area is to be preheated on high tensile frames (110,000psi). Most class 8 today are 70,000 unless ordered different. Easiest way to tell... try to drill a hole.
I needed something to haul and dump several loads of tree branches and some dirt. I only have to go 5miles so i am going to use a 1961 ford F600 that i just got. It had a pusher axle made from a front axle that i torched off. I have been going over it with a fine tooth comb so it will be safe and i wont give the DOT guys writes cramp if i get stoped. The frame was lenghtened and sleeved but on the bottom flange they left a gap of about 1/2 inch and welded the ends to the sleeve. This was done on both sides each end of the new frame part. It has held up and as far as any one can remember it was done when it was new i got it from the grandson of the guy that got it new and he said he hauled 20,000 pounds of rock on it. I will never haul heavy loads on it but i am thinking about doing something with the gap.
A properly fitted and welded scarf or cleft joint is as strong as the original frame. That being said ,frames I did ,I typically put the joint in a spring hanger or someplace like that.