So now the C6 USA is running flawless everyday. I went to .3mm head shims. I have noticed it's tough to get the OS plug to seal properly as it seeps some around the plug but it damn sure runs well with it. Now i will buy the whole farm and switch to RB plugs cuz i don't like tightening it that hard. I will probably go down to .2mm shims since the murnan modified head button has 2 more small cuts in it to help with flame travel adding a small amount of volume. I'm used to running engines with raised compression with colder plugs. With the shimming being lower do i want the #6 or will the #7 work without being too cold? I live in Pa. and our tracks are close to sea level. Thanks.