Also something else unique to some 406 heads was the use of a 1.65 exhaust valve stock from the factory. While still using the 2.02 intake valve. Most books, literature and websites list them as 1.55 which is incorrect. For years I found it odd many of the stock 406 heads I had only the larger ex valve. Till I learned they came that way stock. As the most common valve upgrades are both 2.09 and 1.65 like the CJ heads used. I had to LOL wondering why everyone only changed the ex valves. Duh!! Most books and websites also show the C3AE-C as 406 heads and having larger ex valves. Where the C2AE-B and C used them too. As for those C3AE-C 406 heads I've never seen a pair. Small chambers with valve spring seat cups and 1.65 ex valves. But I hear they are out there. I dunno but I'd hafta see it to believe it. There are C3AE-C 390 heads with larger chambers and NO seat cups. Smaller 1.55 ex valves too. Lots mistake these as true 406 heads which they are not. But see the number listed as a 406 head and say "well the book says so" so they must be. LOL.