Q: Truck won't start after replacing head gasket

Hi, the way to tell if it was assembled incorrectly is to do a compression test on the engine and see if you have compression on all cylinders. You may have timed the engine incorrectly and this would show up as having low compression. This engine may have been damaged because it was timed incorrectly, an example of what might have happened include, bent valves and other possible engine damage. I recommend you have a mechanic, like one from YourMechanic, diagnose why your vehicle is not starting to avoid any further complications.