Felpro 1026's are designed with a pre flattened copper ring and are made for aluminum heads. 1000's have a steel ring and are for iron heads. The bore size on the 1000's is 4.020. The bore size on the 1026's is 4.090. Others may have other opinions, and may have used other brands of gaskets with aluminum heads with good results, but this is what Felpro says for their gaskets. I use the 1026's with Champion GN1's and really like them. I used Cometics before and my coolant kept disapearing, probably because my block was not smooth enough on that build.