I see what you mean now.A few of the more expensive engines dont use a full bodied cast flywheel.They use a solid steel center and then pour the cast around it.Huskys do it this way,and you found out stihls.I had an olympic that was the same way.When i turned it it looked like yours but when i screwed my hub on it covered that area up so nothing could come apart.I sent ya a pm with some info.I see youve turned down the top of the cylinder round where the fins where.Is this for the water cooling jacket to fit.I woudnt use it like that unless you could somhow tighten a flat piece of aluminum up against it that was large enough to cover at least a half inch over the whole center sectiion.