I have used a "glass drill" to drill hard metals, eg hardened stainless. These are used, obviously, to drill glass and ceramics and have a carbide tip with pointed geometry. Might be worth a try before you send the casting back.

Last used one to drill the hardened stainless jaws on a digital caliper to use it as a quill scale

Phil

Mehanite and similar is continously cast and cools more slowly and thereby avoids chilled areas.

Commercial castings are, to put it bluntly done in a hurry, I have seen just cast items still in their sand, kicked to break them open, thereby exposing red and often small parts to the atmosphere. That is when the chill set in! Best left in the sand longer.

If you are on your third cover it sounds like you have already sent two back so unlikely the third will be any better.

As the cover is quite small it should not be too difficult to anneal. If you have an open fire or woodburner then put it in there and then leave in the ashes overnight to cool slowly.

If you don't have a fire then a BBQ may work or failing that get some firebricks around it and heat with a propane torch until its glowing red and keep the heat there for 20mins or so and then allow to cool slowly.

I've saved a few small iron castings that way but if it does not work then do what you can with carbide tooling before rubbing on a sheet of abrasive on a flat surface and possibly resort to a solid carbide drill for the holes.

Another option is to get a bit of 1/8" steel or brass plate or cut a slice of a CI bar and just make a cover, mill a rectangular recess to make it look a bit nicer than a flat sheet and paint the recess.

I second jens Eirik's suggestion, I used cast iron from an old brake disk when I rebuilt the Stuart S6, ie., for the piston rings, and the eccentric for the valve gear. The valve chest cover could be resessed as per the origional one, and an S glued in, after painting, only you will know the difference. Ian S C