I believe the C version of the part was the original. See the link.I don’t think we have enough data to know for sure if the -D rings are any better, but yes the original 5/6 synchro rings have been superseded by an updated part. MU7Z-7107-C has been superseded by MU7Z-7107-D
https://www.bronco6g.com/forum/thre...aping-noise-grinding-sound.33282/post-1715013
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