leaking HX manifold main 24 he

[phil hair]

hi, i have a main 24 he with a heat X manifold that has had small leak on it for a while, the inside of the box was all furred up. ive cleaned it up and replaced the manifold, but for the life of me, i can not get it to seal.
What can i use to make it seal? something like plumbers mait? would hate to use silicone.

thanks.........

 

[gmartine]

High temp silicone liquid gasket for up to 350 deg C should do.