SLT-6 is 0.060 Over - Runs Hot...Why?????

Boring it .060" is no problem for a Slant, long periods of sitting can cause sludge to settle in the bottom of the water jacket. Hot tanking can often remove the sludge, but many times it doesn't, which diminishes water capacity inside the water jacket. Also, double check timing and carb jetting, both can affect cooling.