BE 35 Bonanza pilot reported partial power loss during cruise. Pilot returned to departure airport.

Narrative

During a maintenance test flight following a top-end overhaul (IO 520); we experienced a cascading engine failure. I was flying with one of our maintenance technicians to test the newly overhauled engine. After flying around our local airport; ZZZ1; we decided to fly up to ZZZ2 to test the airplane at altitude. Given the Bonanza's glide ratio; I always opt to fly higher to keep things safe. En route; everything was normal; and we enjoyed the engine's performance. The only anomaly was that cylinder 4 had higher EGT and CHT readings (the peak CHT was 412°C); but the tech assured me it was most likely a partially clogged injector and that it might just 'work itself out.' We landed at ZZZ2 without any issues and departed shortly after. Climb performance was excellent; but we noted slight vibration as we approached our cruising altitude of 5;500 MSL. 10 minutes later; at about 20 NM north of ZZZ; cylinder 4 suddenly spiked the EGT. I reduced power; and the cylinder failed. I immediately turned the airplane toward the runway XX threshold and instructed the tech to set up the tower frequency. I contacted ZZZ tower and informed them that we would be landing on runway XX with a partial engine failure. During the descent; cylinders 6 and 2 also appeared to fail (as per the JPI engine monitor).We landed without incident and taxied to the FBO; where we discovered that the number 4 cylinder had broken loose from the crankcase; snapping the through-bolts. As a result; the exhaust and intake manifolds failed on that side; causing cylinders 2 and 6 to fail. We towed the airplane to a friend's hangar for further investigation.Lesson learned. During maintenance test flights; remain over the departure airport and land every 30-45 minutes to inspect the repair. If we had; we would have noticed the impending failure in time.I teach in this aircraft and have extensive experience landing during simulated engine failures. This helped me ensure that we made it to the airport even if the engine had completely failed. That said; I'm sure my guardian angel was working overtime during the less ideal times during that flight. It was the perfect time to experience the failure as we were well within gliding range of ZZZ.