To ensure the best seal possible, your cylinder head mating surfaces must be perfectly flat—or, at least within the flatness specced by the manufacturer.
But testing your cylinder head surface isn’t as simple as just laying a flatedge across the head.
The gasket gurus at Fel-Pro put together a good video that shows you exactly how to test your cylinder head mating surfaces for straightness/flatness/smoothness. That way, you’ll know if you need to do any surface machining before you go to bolt everything back up.
The process isn’t difficult and only requires some basic tools and measurements. More importantly, it’s a must-do step to ensure a leak-free seal.
Check out this video to see how it’s done.