Stock ring gear bolts are hardened to approximately US grade-8 and seem to be hardened all the way through, which makes them brittle. US-made replacements are not over-hardened and are much tougher, which works better for shock-loads like when 300 horses are routed through a differential. The serrations under stock boltheads work, or don't work, as Marlin mentioned. US bolt mfgs provide a washer so there's a compatible surface under the bolt head. I don't suggest mix 'n matching high-stress parts such as this. Star washers are mostly to combat high-frequency vibrations, which is not what we encouinter here. Stick with whats in the provided kits. I advocate torquing to ZF's spec and using US made bolts with aircraft-grade 032-thick stainless steel safety wire.