Ok, I was putting my engine back together and tried to adjust the valves to see if they were correct. I went to turn the crank shaft by using the front of the shaft to get to TDC. I cannot get it to turn at all. I don’t know if I’ve missed something when putting the motor back together or if the pistons have locked up. I removed the heads and was expecting to see damage but I could find any. What could be preventing me from turning the engine over? Sorry if this is obvious but I’m still learning.