After Croix, I wanted to be 100% sure of the state of the 350 so I decided to remove the head and get a good look at top of the pistons. You may remember that the left sparkplug was a bit light, and the left was a nice chocolat brown (caramel). Well, the short of it is that the left piston had cracked quite severly but thankfully no bits had cracked off. The bore is not touched, but I will need a new piston.
The cause is obviously some heavy detonation, which could be (probably is) either a very lean carburation or some airleak. No reason to suspect an airleak, although when I put it back together, I will ensure that the reed rubbers are well sealed, as the new rubbers are not that great a fit around the pipe joining the two sets together, I don't know if this would cause just one side to go, but it's a plan.
The base gasket looked a bit tired aswell, (could it be leaking!) so this will get changed aswell.