"Green" (brand) ball bearing conversions were originated by...The Green Ball Bearing Co. (now part of Bearing Technologies, Avon, Ohio).

MP has sold them, with the catalog proviso: "for drag racing use only", for quite a while. They definitely make life easier if you're making frequent "pig" swaps; however, being ball (vs. huge OEM Timken tapered rollers) they, of course, have a small fraction of the load carrying capacity. I have never personally seen a set last more than 10K, the stockers can go 250K.

I was originally worried that they would fail catastrophically, but they do seem to give plenty of warning - "snow tire" type noise. Still, they are a much weaker design -- this is intuitively obvious to even the most casual observer.

I'd personally not even do even one high-speed road-course lap, or one Auto-X lap (with sticky tires) with 'em.

Rick