Tapping cast iron

Tapping is fine but use a coarse thread. Whitworth was invented for use with Cast Iron. These days UNC or Metric coarse. Use a washer under the bolt head as well. Like with machining Cast Iron no lubricant.
 
I just had to drill and tap a bunch of small blind holes in a casting. I was deathly afraid of breaking a drill or tap off in the casting, so I used name brand American made drills and taps with tap magic in every hole and I didn't have any issues.
 
I'd be inclined to drill the cast iron and tap the steel because the steel part is [presumably] easier to replace if the threads strip out or you break a tap.
 
Cast iron will hold up as long or longer then steel when tapped , if it strips out a nutsert is an easy repair , just drill and tap the insert thread screw in and push the little pins in after screwing it in. Heli coil is another option. I've used both in all types of metal even spark plug holes.
 
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