CAST IRON THREAD PLUGS
I make my own large threaded repair bushing from tapered pipe thread oil galley plugs, with an internal hex. The material of these plugs is hard enough to tap clean threads into, and very similar to your cast iron block or head. They are available in many sizes and the internal hex works well as a drill guide with a perfect center. These work well for jobs like stripped Chevy starter bolt holes, that someone else failed to repair. On these, I have made a drill guide that bolts to the block existing holes and allows me to drill the hole out large enough to find the original center.

Keith Hennen
Hennen Engine Parts & Machine
Green Isle, MN
September, 2016