I also have a brother pc 420. Although I would never let my hubby take any machine of mine apart, I am often brave enough to do it myself!
Last time that I had a major thread jam and then couldn't get it to run after I cleared the fabric out.... the problem turned out to be a small 1 inch hunk of thread that was caught in the hook mechanism of the thread cutter. The hook is off to the left of the bobbin shuttle, and reaches across to grab the threads when you use the automatic thread cutter. That little piece of thread kept the hook thingy from retracting fully and I guess it effected the way the bobbin shuttle is supposed to operate. Perhaps this is the metal bar that you encountered.