Having worked for many years in a machine shop, I can tell you that they don't (and really can't) check valves for stress cracks. What they can check if for bent or burnt valves and that's just done by experience. Bent will also show up when you face the valves and they are chucked and spun against a grinding wheel.
As far as recourse...my degree is in business management, I've run several auto related businesses, parts store, machine shop, three different garages, etc. If you were my customer, I would assess the damage, and if I felt (and believe me, I'm fair, but not all machine shops are) the shop had ANY involvement in the cause of this, I would repair the issue, and at least cut you a deal on the gaskets, etc. It's kind of hard to tell from here, but if the failure was totally my shops fault, I would take care of it.
There is an adage in business, and I'm sure that Diesel Care is quite aware of it thus the aggravation with our "list", and that is one happy customer will tell/recommend you to a friend, one unhappy customer will tell everyone who'll listen...that adage is very true and can/has caused many a business to lose customers. Now, with that being said, if you came in demanding I give you everything and then some, you're not going to get anywhere. So, if you find it is an issue, go in with an open mind and calm temper, tell them there is an issue you feel is possibly at least somewhat their fault. I'm kind of wondering about the shop you used because of the surfacing, etc. I would also recommend this to ANYONE going to have head machined, when you take them in, ask about minimum thickness and the setup of the precups...if they don't seem to know what you are talking about, I'd load up my heads and go elsewhere. ANYONE who's done diesel headwork has dealt with pre-cups and has the tools to cut them and their seats. We had a very fine diamond cutter that we used to resurface the combustion side face of the pre-cups.
Anyway, good luck with it all.
I know for years, even though I had worked in several garages with two of what I consider the best mechanics in the business, when I went to the machine shop I was somewhat intimidated. I finally found a machine shop I was comfortable with, and one day I was running a 5 bay garage and the parts store (I started as the manager of the parts store and immediately reduced their inventory from 500,000 to 180,000 and started almost doubling their profit as they had 200,000 in exhaust that was higher priced than what any muffler shop would install it for) so they put me in charge of the garage as well, which also doubled profits my first year. Anyway, the owner/operator of said machine shop called me one day when I was ready to quit but knew I had a family to feed, and asked me to come run the machine shop, so I did. He quickly found I could make him money working in the shop as well, so I did everything out there in between customers and in turn, we taught each other a ton, I taught him about business, he taught me about machine shop work. It was the hardest job I ever left because we were like family, we did many things outside the shop together, etc., but I had a state job offered to me to instruct in business and auto parts/mechanical that I just couldn't turn down.
