so I replaced the battery cables with 2/0 gauge welding wire soldered and heat shrinked with new ground wires. hook up 2 yellow top optima's and still slow crank, so I drop the batteries in my 6.0 start it up and run 2 jumper cables to the new battery cables and start the 6.0 and it still cranks slow! I cant figure it out? any ideas? was running a month ago I just and I suppose the cables went bad after almost 30years so this is why I made new ones

 

I still don't get why you changed the cables?

 

Next would be to get the starter tested at an auto parts store.

 

I tested the starter and I pulled a starter out of my 84 6.9 which had a rebuild about a year ago and I know its good and still nothing chwcked power at my starter switch and I found a bad connection fixed that and then batteries straight to the new cables. glad to say I got it working! just checked ever little wire and working on putting all brand new wires into this girl.
my old cables where shot rotted out from the inside so new cables and I wanted it to carry larger amp load and bigger batteries which is why I went 2/0 gauge cables

 

I've read your post 3 times, I still can't read what you are writing.

I get your cables were "rotted" but that's not a reason to replace them unless you just felt the urge to do it. I'm not following what you want us to help you with?

Did you have the same starting problem before you replaced the cables? What is the voltage at the batteries with the engine off? what is the voltage while idling? How new is your alternator? When was the last time you changed your voltage regulator? You might have ruled out the starter, but a year could be long enough for it to start going bad (but i don't think it's the starter)

 

I had some issues with the starter a while back, and it turned out to be bad connection between the starter and the block. I cleaned and smoothed the face of the starter and reattached it, and it improved greatly. There was alot of crud in there and since that's the ground path for it, it was reducing the current it could pass.

I don't remember exactly, but I may have removed the back half of the starter and cleaned bewteen the segments of the case as well (again, the case is the ground path)

Donno if that's your problem but it could help.