does anyone know the part number for a stock base gasket for 1996 form 3 600. *thanks hope you can help.

Go on this site and click on Ski-Doo * http://www.hlsm.com/ *It's an online microfiche index, they got parts number for just about everything.

they give you part numbers but dont tell you if its the 4mm 5mm or 6mm gasket that come with it from the factory

I hope you saved part of the original base gasket, because you will have to take a micrometer and measure it's thickness. It could have used any of the gaskets depending on tolerances during assembly at the factory.