For some aircraft (F70/100, F50) ISA plus 35C, ie 50C for take-off. However, not so long ago the F100 I was flying could not depart when the OAT was greater than 40C as our performance tables didn't go above this. The tables now carried go to 50C.
However, if you operate outside the maximum allowable temperatures, you are on you own mate. You have just endangered your passengers, those on the ground, invalidated your insurance and quite rightly, put your licence in jeopardy. Just because the damn thing flies it doesn't mean that it's safe. Don't do it - you'll give us (pilots) a bad name for no good reason!