When you bend a 6 inch shank like that you have too much speed. When you break a carbide burr like that it’s either turning too fast or it’s too big to fit where you are trying to make it fit and it’s chattering or both.
I think I have 3 burrs that are shorter than 6 inches. Even with an air grinder, those short shank burrs are pretty much worthless. Your hands and the grinder is too close to the work. It makes it harder to see what you are doing.
You have to get some way to slow the grinder down. To clean up the guide area (which if I’m porting I do it and charge like hell for it because that big guide hanging in the port is a flow killer among other things) you’ll need smaller diameter burrs with 6 inch shanks so you can get in there and see what you are doing.
3/8 diameter is about the biggest burr you can get in there and not fight it. 1/4 inch is probably better.
It takes a HUGE amount of time to narrow the guide boss. You need to use a small burr to start and then get bigger.
I should have taken some pictures of my burrs tonight. You’d be surprised how few I use.