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Help! 1966 427/425 Rear End Issue


New member
Feb 2, 2023
St. Louis
Looking at a 1966 427/425 - The rear end is a not original. The seller is telling us It is a 1966 3.55:1 posi 4speed 327v8 rear end. Stamp reads AN4166. Is this a major concern, being in a Big Block car? We thought the 4.11:1 gearing was preferred for the Big Block, the 3.36:1 ratio was less responsive, but not sure what the seller's information is reflecting. Can't find anything in the Black Book, but seller called it a 327 rear end.

Need your guys expertise - will is this be a big problem in the future? Or is this not a major concern?

Thanks so much for your help in advance!
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He told you right. AN is a 3.55 posi out of a 327 car. The same rear end for a 427 would be stamped AZ. My 69 coupe 427/390 had a 3.36 and it was plenty fast for around town and didn't spin the engine to death on the interstate. About 3000 rpm @70 mph. There were only 3310 4.11 rear end cars built for all of big block and small block production in 1966 compared to 5258 427/425 cars built, so having a 4.11 in a 425 horse car is not a foregone conclusion. Without factory documentation yours could have been a 4.11, 3.70, 3.55 or even one of 91 with a 4.56. That is based on my thoughts that the 3.55 rear might have been swapped in to get better highway drivability. Specs show that even the 3.08 and 3.36 were available with big blocks but I would think that would be with the Powerglide or M20 wide ratio trans like my 67 390 horse 427 or with a 300 horse 327.

If you buy it, spend some time driving it with the 3.55 and see how you like it. I think it will be an enjoyable car unless you are going to drag race it.

Thank you so much for your valuable insightful information. We appreciate you taking the time to help educate us on this issue. We are so thankful for having a forum of knowledgeable people which is a blessing to us.
Glad to help.


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