how big can a 200s block be bored to? 66mm?

 

66mm safely. Many go 66.5 with little problems. I usually go to 67mm, which is very risky due to the heat and the paper thin cylinder wall you have that point, but those engines always have oil coolers added to them and they are rebuilt often, so it's not been that big of a deal. But when rebuilding those, I almost always go back with another culinder and bore up again because the walls of the original 67mm would be warped. It DOES get that thin when boring to that size. If you want to play safe, stick with a 66 or 66.5