Camshaft sprocket

I bought a 1991 b2600i with a cracked head. After getting a new head I put everything together and turned the motor over by hand and the nipple on the camshaft broke off. I then noticed that the timing chain colored link didn't line up with the timing mark on the cam sprocket. So I pulled the motor out, had the crank turned, new main and rod bearings. Took the timing chain cover off, got a new timing chain. Put every thing back together tonight and started to turn the motor over and the timing chain is trying to twist the cam nipple off again. The cam doesn't even move when I try to turn the motor. I can't figure out what I'm doing wrong, please help!!

Maybe one of your cam caps is installed backwards. Maybe one of the cam caps got put on the wrong journal and it is pinching the cam. I would loosen all of the cam caps and see if the cam starts to rotate when you turn the crank bolt. The cam caps are not interchangeable from one journal to another. Maybe when the head got cracked, maybe due to heat, the cam got warped so it isn't rotating true anymore. You might need to take it to a machine shop and have it tested for runout. What did you torque the cam caps to? Maybe they are overtightened.

Well I started this project about a year and a half ago that's when I had all of the head work done to it and after I started having problem I gave up on it for a while and now I back to working on it. So I don't really remember but I thought that the rocker arms are what held the cam down not individual cam caps but I could be wrong. I will go to my dad's house and look at it tomorrow. The cam should be fine as it is new (not the original one from the truck when the head cracked).

Should I have taken the head and had it machined to fit the new cam??

Good question but I don't think that is where your problem is now.

Have you tried to rotate the engine backward?

Before you do anything else, since you provided new information. I would try this. Loosen the rocker arm shafts so that all of rocker arms are free of the valves and so all of the valves are fully closed. My thinking on this point is that your cam timing is off and the engine is holding a partially open valve against a piston, essentially locking it from rotation. With the valves closed and out of the equation, you should be able to rotate both crank and cam. At this point, I'm going to guess that your cam chain timing is off, even though you did some repair work on it. The good news is, if this is the case, that you didn't try to use the starter; that would have bent valves.