This is what happens when you trust eBay and the seller to send you the correct parts and not something that someone thought was close enough. Notice that the new guide measures 12.1mm, though seller said it was 12mm OD. See what an additional .1mm and a sharp edge can do to your heads? The correct guides are now loose in their housing which will likely cause catastrophic failure; destroying my engine. As far as I'm concerned, eBay/seller owes me some new heads...