The nuts only tighten the actual manifold down onto the gasket. That's the first part sealed just like the stock pipes. The sliding fit of the pipe onto the manifold also needs sealing, and this is where the sealer comes in. I use Permatex Hi-Temp. Comes in a red tube and smells pretty sharp. Make sure you degrease the manifolds and pipes before using the sealer or it won't seal properly.