Sundowner 23 pilot reported loss of engine RPM and engine roughness in the traffic pattern. The pilot immediately landed and turned the aircraft over to Maintenance.

Narrative

While trying to do some planned traffic pattern air work; after taking off I noticed the engine dropped 200 RPM (uncommanded) with some light engine roughness. I immediately turn left for traffic pattern and landed safely without a problem. The engine troubleshooting started immediately after safely landing with magneto check which was OK and engine oil temperature and pressure in the green arc. CHT during the short flight was at 1;400 Degrees F for #3 cylinder while the CHTs for all cylinders was 340-360 Degrees F. Carb Heat checked ok too. I took airplane back to my hangar and started for some more troubleshooting the following days. A pilot friend recommended me to run a static RPM which I did and noticed I was only getting 2200 RPM of the recommended 2250-2350 RPM per the airplane's TCDS (Type Certificate Data Sheet). A borescope process and compression checked of all cylinders followed afterwards and found cylinder #3 with its exhaust valve which appeared it had a hole on its face and its compression test was at 0/80 PSI.Removed this cylinder and found the exhaust valve with a hole through the base of the valve. I took several pictures before removing the exhaust valve and talked to Lycoming Technical Representative and supplied them with a broad information of my operation procedures and engine parameter operation information along with not less than 30 pictures of the burnt valve in place inside the cylinder. Once I removed the valve several pictures were sent out to Lycoming. I further stated to Lycoming Technical Rep that I never operate my engine LOP (Lean of Peak) but always ROP (Rich of Peak).I suspect that the failure of this valve with a burnt through hole was due to probably an incorrect mix of the alloying chemical elements (Chromium; nickel; manganese; silicon; vanadium; tungsten etc.) during its melting process along with an incorrect heat treatment afterwards of the finished valve; at the end of the manufacturing process; for annealing; tempering or stress relief and a deficient quality control of their manufacturing processes rather than an operator misuse.I am filing this safety report because it may be that potentially there could be some more valves like mine manufactured from the same batch and date out in the field installed in some other Lycoming engines. I have shown pictures of this failed valve to my AP mechanic and some other experienced AP mechanics and pilots and their opinions have been that they never seen anything this bad before in their lifetime profession. My engine is a Lycoming O360-A4K; 180 HP whose failed cylinder/valve was installed new almost 6 years ago; which was bought new from a Lycoming distributor; installed by a certified AP mechanic and failed after only 183 hours in service which is supposed to last 2;000 hours!