For sake of completeness, below is the message I received, and my entire reply. I did fail to include G75 3.70 10-bolt ElCaminos in that reply though, so I have added that in [brackets] below. Thanks!
Quote:
Originally Posted by bbjohnny
Jeff, can you share your knowledge on Z 16 and 65 Chevelle Muncie tags? Z 16’s that I have seen all had 3857577 muncie tags. There is a guy selling a 3857578 tag that he claims is Z16 and if you Google that it seems to come up. Have you ever seen the 578 tag on any of your Z16s? Or do yours all have the 577 tag. Appreciate your help. Thank you.
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Great question, and one that I have already researched extensively.
All Z16's came with the 577 transmission. The 578 part number was initially listed in the AIM as for Z16 (which is why is shows up on many internet lists), but that designation was cancelled and changed to 577 on 11-18-64, which was long before any of the Z16 cars were built.
577 is just a regular 65 Chevelle M20 which has the internal speedo drive gear for a 3.31 or higher (numerically lower) rear axle gear. All Z16's had a 3.31 open axle, so the 577 M20 is what they all got.
The 578 p/n is not found in any parts books to definitively say what it was. But my belief is that it is a 65 Chevelle M20 with the internal speedo drive gear for a 3.55 or lower (numerically higher) rear axle gear.
While 3.55's and 3.73's were not available as RPO's in 65 Chevelle coupes, sedans and wagons, [a 3.70 was an available RPO in a 65 ElCamino with a 195HP or 220HP 283; and] a 3.73 was an available RPO in a 65 ElCamino with a 250HP or 300HP 327; and there was also a COPO number to get a 3.73 gear behind an L79; either of which would have necessitated that there be a different M20 transmission unit (with a different part number) that had the correct speedo drive gear inside of it for that axle gear. Again, I believe that was the 578 M20 transmission.
Based on the AIM notations and change dates, my belief is that the Z16 was originally intended to receive a 3.55 or 3.73 posi rear axle (which would have required the correct 578 M20 transmission); but due to inability to get any posi units at the time that those 200 axles were to be built, they downgraded to the 3.31 open gear that all Z16's actually came with. That change in turn resulted in cancelling the 578 M20 p/n designation for Z16 in the AIM, and reverting to the regular 577 M20 unit (which matches a 3.31 gear).
It is also notable that the Z16 page in the AIM where the rear axle part number is listed was redrawn and redesigned on 11-19-64, the day after the transmission p/n change, so that supports the conclusion that the two changes were interconnected.
So you can tell the guy that he still has a quite rare tag -- records indicate that there were only [80 M20 3.70 ElCaminos built (the other 16 were 3-speeds and powerglides), and] 225 M20 3.73 ElCaminos built [the other 68 were 3-speeds], plus however many of the COPO L79/M20 3.73 combos were built, which is probably only a handfull. But it is definitively NOT a Z16 transmission tag.
Jeff