I posted this in the general forum yesterday and didn't get a single response. I know I should have posted here first, but for some reason, I felt like trying something different. Any help would be greatly appreciated. I just want to make the monster run so I can ride . I'm rebuilding a 2000 YZ426 that dropped a valve and destroyed the entire top end. Because I'm cheap, I picked up a head, cylinder/piston, and gasket kit off ebay for a steal (all in great shape, I might add). The cylinder and piston are the 444 big bore kit and the gaskets are specifically for the big bore. Along with the gaskets came another Cometic package that contained 2, .032 aluminum cylinder base shims. My question is about the shims. What is their purpose? Do I need them? Do I use one or both? Do I need gaskets to go between them, or would copper coat alone work? Do I even need the copper coat? This is the first time I've seen these and I don't want to do something wrong when putting my bike back together. Thanks in advance!!