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If it were my engine, I would scrap it and buy a big camIII engine of about 83 to 86 vintage. I have seen exactly what you describe many times on engines without piston cooling. Galling a piston or pulling a liner was quite common. The piston gets hot enough to expand and it sort of welds itself to the liner. It was very important to have a pyrometer on those engines. I would rather think that the engine was lugged. We used to run our non FFC 350's up to 2600 on upshifts on upgrades. I really doubt that the engine was run up to 4000 and I doubt that the tach even shows over 3000. We call them dry because they do not have the require a seal at the bottom like a Cummins and they don't get pinholes in liners like cummins did in those days. We have had several in yard mules, fork lifts and compressors and still have one left. I have worked on Detroits because I had to. I would be glad to discuss the matter over the phone with you to keep the posts from getting too long. i am not sure what you mean by abusive use, but the old cummins could not stand abuse. You might want to replace the small cam with a big cam emgine. My first cummins was a 1964 model and I have used nothing but cummins and I have watched them evolve. By the late 70's a pyrometer was no longer necessary and the engines had a much wider operating range.
CUMMINS SMALL CAM 350 DRIVERS
Drivers had to constantly keep their eyes glued to the pyrometer to watch combustion temperatures to prevent this from happening. They had FFC piston cooling, the big cam and improved turbos. By 79 cummins had improved their engines greatly. Find an old mechanic who has experience with these old engines to help you. If you have never worked on a cummins, it is really not something you can do your self. When liners are installed, the liner protrusion at the top must be shimmed correctly and the counter bore must be correct which takes special tools, but it can usually be done with out pulling the engine. Whoever, the knocking you are hearing is probably a liner that is loose.
![cummins small cam 350 cummins small cam 350](https://images.truckpartsinventory.com/parts/22216/1978-Cummins-350-Big-Cam-Engine-Assys-sa8ytP5acOrl_f.jpg)
The good news is that cummins parts are very cheap compared to Detroit and cylinder kits are sold individually.That is a cylinder, piston and rings. That is when combustion temperatures got past the melting point of the piston and the piston actually sticks to the sleeve. which means it did not have piston cooling. Cummins uses a wet sleeve compared to Detroit's dry sleeve.