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Hi,
At the time, GM was trying to phase in metric fasteners. Seems like they had limited success. I have found mostly metric fasteners on my 82, but it seems S.A.E. fasteners sneak in on certain assemblies. Maybe the Vettes were unique considering the C3 frame dates back to 63. I doubt any attempt was made to change the fasteners on an existing assembly, but so much stuff was added to the C3 through the years (particularly the 82) that perhaps the metric fasteners came in with the newer stuff? It can be a pain when you are trying to do a job and have to keep switching socket sizes. Just my 2 cents...
Tom
Apparently, y'all weren't around in the '70s. We were all told that metric was wonderful and the Next Big Thing.
Here's the bottom line: Whatever was "legacy" -- like engine blocks -- was SAE. Whatever was new -- like the Crossfire stuff -- was metric. In other words, they didn't change their tooling, they just did the new stuff in metric.
It's easier nowadays that everything in cars is metric. And before anyone starts bitching -- remember, it's a necessity so the US automakers can compete in a global marketplace. Not to mention sharing platforms with their overseas divisions....