I'm rebuilding a 94 12 valve cummins to put in a 99 V10 ford truck. the guy i bought the cummins from claimed it had around 300k on it. I tore it all apart. Still has factory hash marks on the cylinder. Its at the machine shop now getting cleaned. Do you guys recommend replacing the pistons in these? obviously if it has to be bored you must. But if it doesn't need bored then should i use factory pistons? I'll buy new bolts probably not studs though. The build will be completely factory. What about the tappets/lifters? My cummins manual claims the tappets need to go back in the same hole they came out of when you assemble the engine? Same with the pistons and connecting rods? I just want to make sure I'm not going over board buying a overhaul kit and spending extra money when its not necessary..... This is my first diesel rebuild so i want to make sure i do it right! The cummins manual just tells me to test tolerances. Basically what I'm getting from this book is if its within their specs. leave it alone. If its not broke don't fix it? Any help is greatly appreciated. Thanks

 

have the machine shop check the bore for tapper and check the pistons.. skirts norm. wont collapse on these pistons.. when I did mine I used arp head studs and arp rod bolts. if the pistons check out good use them. you wont gain anything by getting new ones. all tappets need to go back in the same whole so they wear on the same lobe.. you don't want to change wear pattern. pistons should be also but not as important.

 

put new rings in it and change all the seals and gaskets and put new factory size rod and crank bearings in and secure the kdp and paint it cat yellow and put it in the truck. i say cat yellow just to f with the flat bill yeti full of monster brochachos with their third gen rams with thier elephant ear 9 foot wide mirrors

 

Re-use the pistons. I would do rings, seals/gaskets, and studs (mains & head). Upgrade the exhaust valve springs. That way you are exhaust-brake ready, and you don't have to worry about popping a head gasket if you start modding later on.