I was running the 7x1 MR (5mm blank) motor this weekend with no fan and no problems. It was 95deg plus with a track temp around 140. I agree with most of the other guys...get a 5mm arm. They are way more efficeint than the 4mm's. The low turn 4mm arms can fade late in a race due to overheating.
What I run: Schumacher Mi5/Associated RC10R5.1/Associated RC12R5.2/Futaba/HobbyWing/Team EA Motorsports/BSR Racing
Where I run: Florida Indoor R/C Complex/Thunder Racing/Florida On Road State Series