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This is my first time using the forum. Last week, after doing several practice Instrument approaches each ending with missed approaches, with the two other partners in our 1979 M20J, we noticed the engine running rough when we were taxing back to the hanger. It didn’t seem too bad so we figured we would diagnose prior to the next flight. Also, we didn’t notice it when we were flying After a week of rainy weather, I pulled her out of the hanger, performed a preflight and fired her up. It was running super rough. When I throttled up the electronic rpm gauge would go crazy (running all the way up and then all the way down, even cutting out). Clearly I wasn’t going to fly it. When the mechanic on the field checked it out, he found the plugs were fine and so were the mags. What wasn’t fine was that 6 of the 8 bolts on cylinder two were missing and/or sheared off. Of the remaining, one was tight and the other finger loose. We bought the plane 18 months ago with an overhauled engine that only had 100 hours on it. We also had an annual done in August 2018 at a Mooney service center. The engine now has about 250 hours on it Any ideas how this happened? Did the overhaul guys mess up? Do the cylinder head bolts get checked on the annual? Is it possible that we have been doing something wrong with engine management that would have contributed to this? And most importantly, how do we make sure this doesn’t happen again? It could have been catastrophic if the remaking bolts would have left loose in flight.