I have been looking around the web at a number of harness installs and pretty much all of the installs involve drilling holes and using metal backing plates when attaching bolts to the car.

I was wondering why no one mentioned tapping threaded holes instead. That is how the original bolts are attached to the car - even the ones that must potentially support a lot of force - like the seat and seatbelt anchor points.

Obviously, not all parts of the frame are thick enough to support bolts, but many parts are. Why is this not a common approach?

 

Might want to ask in Competition...they would definately know.
Austin

 

Looking at any seat belt mounting points I have found that they are reinforced in comparison to the rest of the sheet metal on the frame. The point you want your harness mounted to is probably not going to be reinforced so you will need backing.

 

Not everyone is smart enough to figure out a good mounting spot from a bad mounting spot. It is much harder to pull the complete backing plate thru the unibody (spreads the force over a larger area) than to pull a nut and bolt thru the body. The rule makes everyone a smart belt installer.