I have run the GTB 6.5 in a Bk2, a B4 and an Mf2. The only thermal I have had to date was running a short high bite track over geared by ~4 teeth on my pinion gear after 9 minutes of continuous running. This would have been taken care of by cutting a small vent int eh body to allow some air in. I have never run the fan. I think it is only needed in carpet touring car racing.
"I have no idea what you're talking about, so here's a bunny with a pancake on it's head."
In mind and body, we're all heading towards old age. It is slower to go by bike.