WOBIf it is some sort of chilled iron from making the casting, it will likely be impossible to machine it with HSS tools. A plunge cut with a carbide end mill will make the initial hole and then a carbide boring bar will allow you to size the hole accurately. Reaming should not be necessary.
Interesting point. Once I screwed up a flywheel so I made a plug for it that was a press fit. This reminds me of that.I think I'd be inclined to reverse the flywheel in the chuck and face the other side to see if the 'plug' goes all the way through. It does look way to round to be a casting flaw.
That's interesting.I had the same question earlier this year. So this was the response from PM Research:
That is how the casting is made. The center core is placed in there to keep the flywheel from collapsing at the foundry. That is a good flywheel.