And before anyone asks, it was new and annealed before fitting in case it was old stock. And the head sequentially tightened with a torque wrench to Triumph specs.
Okay, after the top-end work, did you run it for a few minutes, get it hot and then shut it down, let it cool then re-torque it again before riding it?
If you rode it much at all without doing a re-torque of the head bolts then it is very easy to blow a head gasket. When the engine heats up the gasket is squashed because the head expands against the top of the bolts holding it to the cylinder and puts immense pressure on the head gasket. When the engine cools and the head and gasket shrink again, the head bolt torque is no longer to spec, I have seen bolts on rebuilt engines become so loose they could be turned with your fingers.
So after a rebuild, or even a top-end job, what you did with the torque wrench during assembly means nothing at all once the engine goes through one heating and cooling cycle. If anyone blows a head gasket after a rebuild or top-end job, the reason why is almost always that they rode the bike after the engine was heated and cooled once. It literally only takes minutes of running to heat the engine enough to do this.
That is my two cents.
