Unfortunately the problem that many run into is that these aluminum heads grow more than iron heads with heat cycle, and the clearances need to be correct for oil control at running temperatures. Some of the guides are not finished to the proper sizes, and that causes one stem to be either tighter or looser than it should be, and it will use oil after the seal wears a small amount. A guide that is tight to start will scratch the stem if it does not seize the valve, and those scratches can wear on the seal over time, causing oil to seep by at a slightly greater rate. The bigger the valve stem, the bigger the problem. Also, some of these viton style seals are color coded for intake and exhaust usage, and if someone puts the wrong seal on the wrong valve, they wear quicker. There are seals that are made for .500 guides, .530 guides, and if you mix those up, the .530 seal will pop off a .500 guide. Of course they should have been correct from Edelbrock. I have found heads that had seven guides properly sized, and the eighth guide was not touched after installation other than deburr the end where it was driven in. Human error, someone just forgot to double check themself. Just saying. Joe-JDC