It was made from silver steel, W1 I think. Yeah I hardened it, sharpened it and tempered it back to straw.
I was hoping to use it to get the tap out and produce a hole I could tap M4, any bigger than that and I'll be getting too close to the cylinder bore.
And yes it's aluminium, but I'm pretty sure it hit the tap. There's only a very small amount of tap left in there and it's been bashed about trying other methods so I doubt it's still in the same place relative to the hole position.
I knew it was a long shot with the side walls ~0.25mm but it was fun to try and next time I have a bit more meat to work with I'll definitely make another.