Originally Posted by Staples

This is a donut style gasket. If you want a leak-proof design, this is the route to go. Unfortunately, I can only think of one company that makes aftermarket test pipes where the piping protrudes the flange to allow this type of gasket to be used, Motordyne ART pipes (newer version).

I've used RTV sealant by itself, with gaskets, I've tried Z1 metal gaskets (which are still on the car). They all have leaked on me, on the passenger side where the manifold bolts to the upper flange on the test pipe. This is on two different sets of aftermarket test pipes - as I thought it could be the way the pipes were produced and not allowing them to sit flat on the flanges to bolt down, causing them to leak.

The Z1 metal gaskets held up the best and the only way to actually tell it's leaking is by listening through a stethoscope, pretty minor. You can't see much black soot around the flanges as you could with the other gaskets and just using RTV sealant, so from the naked eye it looks fine.

I finally got a hold of a set of newer ART pipes that will go on the car once the weather gets warmer.