We sought our information from the folks at Mr. Gasket's Lakewood division. There was an extended line (over 20 models) of SFI-approved, made in the USA bellhousings. These bellhousings are hydroformed using extensive tooling. Hydroforming the steel gives it the same thickness throughout the piece, regardless of any bend or radius. Normally when a stamping is made, the bends thin out
The standing joke is that they test bellhousings by throwing a hand grenade inside and seeing how many pieces come out. It's obviously not true, but it does illustrate how devastating a clutch or flywheel explosion can be. If you're not aware, those situations have been known to harm drivers and spectators, cutting the hands, arms, and legs of drivers and hitting spectators with shrapnel-like pieces. It's a relief to know that the bellhousing will contain the explosion and prevent you and those around you from what used to be your clutch or flywheel.
SFI bellhousings seen in the circle track industry have had a number of refinements added. One version that can help dirt racers has a smaller hole for the clutch fork and offers a tight-fitting rubber cover to keep it cleaner.
Mr. Gasket's Hays brand makes a series of bellhousings/adapters without certification from SFI. These are made the exact same way from the exact same materials (1/4-inch steel) but are not tested. Made for the aftermarket industry, they can be used for transmission swaps. They can make life easier when you're putting a Ford tranny on a Chevy motor or a GM tranny on a Ford motor. They fit Jerico, Muncie, Richmond, Tremec, and other styles. Because they are steel, you can safely weld on brackets you may need for the swap without damaging the adapter.
Hays also has a version that has the lower flange shaved off as close as it can be for optimum ground clearance. If your racing sanction doesn't require SFI-approved bellhousings, you can use these transmission adapters. Always check your rule book.