I'm thinking about SLP runners, they seem like the best deal for aftermarket runners. If I get these I'll want to siamese the plenum to match the runners. My question is how do you cut the gaskets to match the siamesed openings without creating vacuum leaks?

 

With scissors

Mr. gasket sells uncut gasket material in various thicknesses if you
can't trim the accel gaskets.

 

You probably won't want to use scizzors since the gaskets they come with have a metal layer inside. Some kind of sheet metal cutters will work better.

 

What's worse is that the OEM Gm gaskets and FelPro replacents don't even fit the stock runners. I punch them with a 1¼" trade size (1.660") conduit punch to get them to fit correctly.

 

Use Rol gaskets, they fit much better. And use a good pair of dikes (diagonal wire cutters) to cut the center portion of the gasket out for SLP runners