looks like you've found the culpret Anlaf.
I'm thanking my lucky stars that I put yamabond all round my camchain tunnel .
I suspect that these head gaskets we are getting in the gasket sets are poor quality .
Before I put my head on I resurfaced it on some wet and dry on my window sill. Nothing elaborate but it highlighted a huge warping in the head between the two sides across the camchain tunnel.
The cam chain tunnel surface was at least 0.5mm below the rest of thre head.!
It took quite a lot of sanding to get it flat so I'm hoping it will hold when I start it up next week.
4th time lucky eh !?
Peanut, that is extraordinarily warped! Glad you caught and addressed this. Back in the day, we would occasionally find a warped surface, but never much over .005", even on 4-cyl heads. It seems that with age and mileage, these engines are displaying anomalies beyond the scope of traditional old-school wrenching.
The sealer dams/rings in 5Twins' headgasket pics remind me of the modified Fel-Pro headgaskets used successfully in the ill-fated Chevy Vega 4-cyl engine, whose non-deck unsupported aluminum cylinders would crush themselves trying to expand against a cast-iron head, only to shrink back about .020" when cool, allowing water incursion.
A note on lapping/resurfacing heads/cylinders: An old-school training regimen used on beginners and apprentices in the art of filing and sanding was to teach and challenge the student to be able to produce perfectly flat surfaces on wood and metal. Invariably, the student would produce a slightly convex surface, partly because of hand control, and partly because the abrasive has better 'purchase' on the outer edges of the workpiece, especially if the abrasive is slightly flexable, like sandpaper.
Resurfacing an XS650 head or cylinder on a perfectly flat plate with wet-or-dry will produce this convex edge, but no where near enough to worry about, so that's fine. In really critical situations, we would prefer the lap compound instead.
Of course, having a good machine shop with properly sharpened cutters take a skim off the surface is the best method.
Coupla final thoughts: Try assembling the head on the cylinder, with the alignment dowels in place, but without the headgasket, and see if there's any gap between the head and cylinder. And check for unblocked/clean camshaft region drainback openings that pass thru the head.