when I first assembled my twin turbo stroker for dyno testing, i used one of the billet seal holders with double o-rings and the standard fel pro seal. It leaked on the dyno. I pulled the engine apart to check everything and replaced the billet holder with a stock one, using the viton nitro fel pro seal and I filled the sides with Ultra Grey. Then once in place, i filled the V area on the back side of the block with Ultra Grey. So far, it's not leaking , but it's been NA . I'll be adding the turbos soon. Dyno mockup is Richard Holdener style..