What kind of dope do you use

Permatex Ultra Copper Maximum Temperature RTV

Good for 700 degrees, works great!

http://www.permatex.com/products-2/...-temperature-rtv-silicone-gasket-maker-detail
http://www.permatex.com/products-2/...cone-gasket-maker-detail#sthash.E5hpr152.dpuf