The 3 part crack check with spray cans is a cleaner like carb cleaner, penetrant, and developer. I used to use it a lot for quick check of mining parts and used Magnaflux brand if I remember. I did quick look on ebay and found penetrant
Dynaflux PHF Penetrant Crack Check Inspection Grade 9.4 Oz. | eBay , and developer:
Cantesco D101-A - Dye Penetrant Standard Developer, 12 oz Aerosol Can . The heads will have to be clean of grease, oil, carbon, and loose paint. Best to have pressure washed then spray with cleaner and wipe clean. Spray penetrant liberally and let sit for 10 minutes for penetrant to soak into cracks. Then wipe off with rags, do not spray cleaner on head or parts, spray on rag and wipe off penetrant. Then spray the developer on head to draw out penetrant if there is crack. Some welding supply stores carry the dye crack check spray.
That's what i have done in the past exactly with great results.
We used to use this system in the field a lot as a go/no go check.
A lot of times, you could see the crack with the naked eye and the penetrant would show the damage further. Use light coats after cleaning and drying thouroughly.
The dye i used was red and the developer was white.
I would grind or air arc the area at least 1/4'' past the crack with about a 20 to 25 degree angle, drill a small hole at the end of the 1/4'' beyond mark, and then preheat to a dull cherry color, and then arc weld with a 3/32'' Ni-Rod in slow steps with a lot of peaning between short welds, then post heat after completed to the dull red color and then submerge the part in sand overnight.
It usually turned out good unless the cast iron was of a really poor quality.
On these pieces, they would actually have voids and pockets with delamination, chunks of carbon, and other debris that made it ''pop'', and then you knew it was a crap shoot at that point.
I have used brass to weld cast iron with a torch and had good results, but to me, it's not the way to do it right..........
The most important thing to remember if you are welding cast iron is to make sure it's clean, go slow, and don't let it cool too fast.
I have done several exhaust manifolds, and a lot of castings for H.D. diesel engines like transmission cases and differential housings etc.
However, i have not done any heads with cracks between the valves, but i would like a shot at it some day.
Welding cast iron is a bit of an art in itself...........
I have welded cast aluminum with a tig and it's pretty easy, the prep is the same for cast iron, but it is less sensitive to pre and post heat from my experience.
The rented magnaflux option from O'Reilly's as previously mentioned looks like the way to go for you.
That's what i would do..........
