Most valve guides are cast iron, unless you go to the trouble and expense of installing bronze ones. It's pretty easy to get one a little out of alignment as it's driven into the head and cause a hairline crack as it tries to align itself with the hole bored into the head. I usually store guides in a freezer overnight and heat the cylinder head a little before driving the guides into place, or if I want a really snug fit I'll bore the holes in the head a little undersize and cold-soak the guides in a bath of dry ice and alcohol before driving them into place. Depending on how good your relationship is with your machine shop, I'd suggest asking them to share in the expense of correcting their mistake, maybe buying a gasket set, etc. in addition to replacing the broken guide. How long ago was the machine work done?
Jerry

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