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Discussion Starter · #1 ·
What is scoop on '65 12 bolt rearends? Did all 327 cars have 12 bolts, or only the ones with 300 and 350 horsepower engines? I'd appreciate any info on this.
 

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all 327 cars had a factory 12 bolt. others were optional. it goes as follows-
250hp- 3.07
300,350,z16 3.31
if it had factory air,a 250 was upped to a 3.31, just as a 283's 3.08 was upped to a 3.36 10 bolt.
 

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I had a 350hp parts car that had a factory 10-bolt posi with 3.07 gears. That must have been a special order since the 3.31 gear was standard with the 350hp cars. I think it got a 10-bolt since that 3.07 gear does not seem to be very common (maybe not even possible?) in 12-bolts. My point is, even weird combos were possible.

The 12-bolt rears seem to be somewhat uncommon even in very hi-perf applications in very early 1965 production. In later production the 12-bolts were more common in hi-perf applications while the 10 bolts (typically non-posi) showed up in the mom & pop cars as BAD has said. But the 12-bolts in and of themselves weren't an option, the use of a 10- or 12-bolt was dictated by other option choices. Certain gears and HP output of the engine seems to dictate whether the car got a 10- or 12-bolt.

js65cv nailed the standard ratio-by-engines data.

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js65cv-
Hey Man thanks!
You just confirmed what I had been thinking. When I finally broke down and scrubbed the rear on my 65 to find the code I couldn't really tell what code it was, it was something like the 2.73 code or the 3.36 code or something(like GB and CB, im goin off memory too late to check the reference) I had thought it was closer to the 3.36 code by the looks of it.
Anyway, I have a 283 w/ air with a 3.36 geared pegleg 10 bolt. Cool. Finally some peace of mind!


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Steve
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[This message has been edited by Steve D 64 SS (edited 05-02-2001).]
 

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Discussion Starter · #7 ·
Thanks for your replies, this info confirms the options on a '65. I know of a '65 parts car that has a 250 HP 327 with a 12 bolt. It has factory air so I assume that it will have 3.31 gears.
Another question......did all '65 327 engines have "double hump" heads? I know that earlier 62-64 250 HP 327s used the smaller "power pack" heads from the 220 HP 283 engines.
 

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You can get a pretty good idea of the gear ratio that should have come with the car by reading by reading the restoration packet from GM. In general it depended on engine, transmission, AC, and model (elky or wagon often got higher ratios). Also higher "special-purpose/mountain" gear ratios could be had as an option (G-something). You can usually infer 10- versus 12-bolt by the small numerical differences, e.g. 3.07,3.31=12 while 3.08,3.36=10. It is not surprising to hear the factory / dealer didn't always follow exact procedures.

Steve, my 65 with L6-230 and AC has CB=3.36 too (and is consistent with GM info). I'm kind of curious about this AC thing. Was the idea of the higher gear to compensate for the power lost to the compressor, or to raise the cruising RPM so the compressor works better?
 

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i would assume that an air car had a higher rear to make up for lost power.
remember,the rear was matched up to the motor and trans. as rich said, the 307 was uncommon. i can only assume that maybe in the car he mentioned that maybe the motor or rear was changed at one time. my father owns a car with a 3.07. it was standard with a 4 speed 327 250 hp. the other one mentioned was the 2.73 which was standard with a 327, 250hp powerglide. both 12 bolts. i guess the only option was posi or not.
i hope i havent confused things more
 

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The Z-16's had a full size rear that was cut down to fit in the Chevelle and used full size brakes.

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Since there seems to be some interest, here is the complete breakdown for rear axels in 65 Chevelles w/ 327s, per the restoration packet:
250 hp: all models with manual trans, elkys w/ glide got 3.07; all models except elky with glide got 2.73.
300 hp: manual trans got 3.31, glide got 3.07.
350 hp: manual trans got 3.31, no glides.
The optional 3.73 ratio was only available with elkys.
That's what the book says, I'm sure there are exceptions. Also, the assembly manual indicates that 250 hp w/ glide was the only case that got upgraded when AC was ordered.

[This message has been edited by Carl Brune (edited 05-04-2001).]
 
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