Looking at a 68 Firebird; appears to be original 400, 4 speed car, it's rust free, and the price is right. However, still trying to figure out exactly what she's got. Have posted a lot of info on the car on the ramblings and chatter page, but figured this was a tech question, so here goes -
I got a lot of numbers off of the 4 speed tranny, but not sure if I have the right ones; here's what I've got - this from a tag on the tranny: 978766WO. Other numbers - cast into the tranny - 392566 and 3088536. Any clues here? Was I looking in the wrong spot for numbers?
Rearend numbers - 9795107 also CFD also 35. Clueless on this. The r/e is posi (both wheels turn same direction when jacked up off the ground), and has a tag, "Use Limited Slip Fluid" or something like that, but don't know if the tag is aftermarket; it might have come with the chrome r/e cover that the car has on it now.
on the axle tube, just under the brake line, about 6" toward the driver's side of the gear housing, is a faintly stamped two letter code starting with X or Z, and a second letter. This will tell you what the ORIGINAL gears were. To find out for sure you'll have to remove the cover.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
Hey, I got the codes off the rearend - it reads 136 ZH. I can't find "ZH" in the code section. I'm certain it says ZH because I scraped the paint down to the bare metal. The 136 could be a 138 maybe, but I doubt it. Any ideas here?
Also, I think it's at least a 4.11 gear, but probably numerically higher. That's a seat of the pants guess, but I had a '66 Vette with a 4 speed and 4.11's, so it was an educated seat of the pants guess. Of course, it's quite likely that the gears have been changed. Looks like prior owner was using the car to drag race or street race.
If it was from a GTO you should be able to see the eylets for the control arms still on the center section, though some birds got universal centers with eylets... i think only in '67 though.
Correct about the eyelets in '67 Firebirds. The rear ends were taken from the Tempest parts bins. Just another shortcut to get the '67's out in a hurry.
2012 Mustang Boss 302 #1918, Competition Orange. FGF replacement 2006 Mustang V6 Pony, Vista Blue. Factory ordered. 2019 BMW X3 (Titled to the wife, but I'm always driving it for her. So I'm claiming it) Old projects, gone but not forgotten: 1967 FB 400, original CA car. After 22 years of work, trashed by the guy who was supposed to paint it. I had to sell it. 1980 Turbo Trans Am 1970 Mustang fastback, 351C 4Bbl, auto 1988 Mustang GT, 5 speed 1983 F-150 4x4, built 302 1994 Chevy K2500 HD 4x4, 454 TBI
Don't know about this being a GTO rearend. I've seen a Chevelle r/e in a Camaro before - quite easy to tell that this has been done - eyelets are obvious, and, unless it's been narrowed, they're too **** wide. Will look again today, but certainly don't think there are any eyelets on this one, doesn't appear to be narrowed, and leaf spring perches look to be factory welds...
You don't need to take the diff cover off to determine the gear ratio. If you've already lifted the back of the car off the ground and both wheels turn together (because of the limited slip) just count how many driveshaft rotations it takes to make the wheels spin one full rotation. It can be hard to judge portions of a rotation on the driveshaft but if it's a 4.11 it will be pretty easy to tell because the next nearest ratios are a ways away (3.90 and 4.33(?)).
If you're down where gear ratios are bunched closer together (3.23 3.36 etc) you can get a more accurate read by spinning the tires 10 rotations then dividing the number of driveshaft rotations by 10. You don't even need to estimate the last partial driveshaft rotation because you'll have the first two digits of your ratio just by counting complete driveshaft rotations. For example, 37 and a bit more driveshaft rotations can only be a 3.73.
So I pulled the cover off the rear diff - here's what I found - 4.33 gears (39 teeth on the ring and 9 on the pinion) - which validates my seat of the pants guess that it was numerically higher than a 4.11.
Now I'm wondering if this was the factory ratio and maybe I'm not reading the code correctly off the rear axle tube. I was pretty sure that it was a ZH code, but the stamping fades out at the top of the Z and H, so maybe it's a ZR instead - that shows up as a 4.33 HD 4 pinion for a '68 Firebird...
If my PHS documentation would show up, maybe it would solve the mystery.
Look on the front pinion side of the rear end & look for an "N" cast into the housing, this will validate it as being a nodular center section. These were used only on 3.55, 3.90, & 4.33 rear ends only. Could answer more questions about the rear end!
