I would suggest going on Youtube and checking out Gary Farro(spelling) ZF transmission rebuild. The guy has MANY videos of rebuilding the trans and he is so knowledgable. I contacted him personally and he responded quickly. There is a place called Eriksons in CT. that he buys all parts from. They supposedly have everything. Your mechanic seems very honest, but that transmission isn't that rare. Good luck.

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Dave S.
Northeast
2003 4.6is
2005 E53 4.4
2003 E39 Msport(a REAL ONE!)
1959 Fiat 600