ORIGINAL: Tired Old Man

The outer bolts on a BME do not carry a load. If you try to torque them down to do so (there are 6 to choose from) you'll likely break the prop clamp waher. If you try to use the outer screws to retain the prop without the center nut the results are eventually disastrous.

The center nut works just fine on many engines, some much larger that a 115/116. The 289 Herbranson is one of them. The nut washer provides a clamping force similar to a vise.