I regularly fly a Cessna U206G with the IO520.  We recently replaced the engine with a factury reman.  Since the install the number three cylinder runs an avergae of 30 degrees hotter than the other five.  As a result, I tend to run the engine rich to keep the CHT of the No. #3 cylinder under 400 degrees. If I lean to best power the No.3 CHT goes up to 423 +/-, with the cowl flaps wide open.  The No. #1 also gets up to 400 and the others tend to average in the high 380's, with the exception of No. #6, which always runs cooler than the rest.

My question is - how hot is too hot?