So you want to put a 2nd Gen F-Body Rear End in your 1st Gen Firebird?

Article: 9 Author: Steve Lothridge


Since I just finished doing a 2nd Gen to First Gen Firebird Rear End Swap, I decided I should write down everything I encountered and post it for anyone who might do the same someday. At the same time that I put the new Rear End in the car, I also put in new leaf springs and bushings so I'm going to include the good stuff from that where appropriate. I've tried to put this together in such a way that you get a lot of detail but it's not confusing. Hopefully I've succeeded. Some of you may ask "Why would you want to put a 2nd Gen Rear in a 1st Gen Car?" Well, they're more readily available, which also means cheaper. If you're looking for a 3.23-3.55 posi for a 1st Gen Firbird, good luck, there just aren't that many. On top of that, the 8.5 inch rear is much stronger than the 8.2 inch. Of course, there are some mods you'll have to do so that's something you have to weigh out to see whether it's worth it to you. Maybe this write up will also be useful to those trying to decide what to do.


Now for some short disclaimers. The car I was putting the Rear into was a 1969 Firebird which originally had a 2.56 open Rear with multi-leaf springs. The new Rear was a PX code 3.23 Posi with a casting date of C187, from a 1977 car. I know there are some minor difference between the early 2nd Gen cars (1970-1975 or 76) and the later ones (1976 or 77-1981) with respect to U-Joints and Brake parts so I can't be sure that everything that worked on mine will work on yours.

Here's The Good Stuff

Overall Width

I had read many times that the 2nd Gen Rears were anywhere from 1-2 inches wider overall than the 1st Gen Rears. Well, I took many, many measurements to be sure I wasn't fooling myself and found that the 2nd Gen Rear is only 1/4 inch wider overall (1/8 per side) than the 1st Gen. This measurement was taken on the outside of the ends of the axles where the lugs are pressed in.

Multi vs Mono Leaf

As I understand it 1967 models will have mono-leaf springs. 1968-1981 are all multi-leaf. I don't know too much about mono to multi leaf conversion, I'm going to assume that you are swapping into a 68-69 car. If you want to put one in a 67 car, I would guess you'll either have to use your original mono-leaf mounts/perches or convert to multi-leaf. Let's just say that you will have to work out all those further complications.

Spring Mounts (Or Perches)

There are some very important things to remember here. First, if you haven't figured it out yet, the 2nd Gen mounts are much further out (about 2 3/4 inches) from the center than the 1st Gen mounts, you will have to cut them off and weld different ones on.

Why not just re-weld the ones you cut off, but in the new location? Well, the first gen mounts have a built in angle such that the springs are further apart at the rear of the car (by the shackles) than at the front. The 2nd Gen does NOT have this angle. I didn't know this and simply cut and moved the mounts on the 2nd Gen Rear. It turned out there was enough play between the spring and the mount that I could still cock them a little and they would fit but I would recommend to anyone doing this to either cut and use your old 1st Gen mounts or get a set from another 1st Gen Rear end.

Another good reason to do this is that the 2nd Gen mount is about 1/2 inch "taller" than the 1st gen which lowers the spring (and the car) off the axle by that extra 1/2 inch.

The last thing you should take care about is the pinion angle. This is the angle between the Rear End pinion and the drive shaft. If you screw up the mounting of the perches you will mess this up. I took measurements of the angle between the pinion and the spring mounts on both Rears and found them to be the same, so I just cut off one perch and moved it to it's new location (you should be using a new perch here), made sure it was level with the other one, and re-welded it.

A different (or better) way would be to place the Rear under the car and put the car's weight on the springs. Then set the angle to what you want it and tack weld the spring mount. Remove the rear and finish the welding. I couldn't do it this way because I didn't have any welding tools and the car was in my garage. By the way, this didn't cost me anything because I did the cutting and welding at a friends house, but it did take a few hours. MAKE SURE you have your measurements correct, you don't want to have to do this twice!

Axle Removal

You will have to remove the axles and break backing plates to be able to cut the mounts very well. Be sure to understand how to do this before you start, it's pretty easy.


The brakes parts are pretty much the same for both. I bought all the new brakes parts by asking for the 1977 parts, used the 1977 backing plates and drums. As far as brake lines, if you are getting new ones, buy the ones for the original 1st gen year. This way you don't have to re-route the line coming back from the front of the car. On mine there was a bracket that mounts to the top rear end cover bolt. This holds the fitting at the end of the flexible hose that the two brake lines thread into. Make sure you get this off the old rear and put it on the new one BEFORE you get the rear up under the car or it might be hard to put on. Plus you'd hate to have to remove a cover bolt after you have your gasket sealed well.

Seals and Bearings

Seals are cheap, so replace them. Don't buy them from the repro companies, they charge a lot. Get a decent brand from a local parts store, they should only be about $1-$2 a piece. As far as bearing, they are more difficult to replace and more expensive but you should at least check the old ones for wear and replace if necessary.

Axle to U-Joint Distance

By my measurements the 2nd gen rear is longer from the axle to the u-joint mount by 1 inch. Unless you have a lot of play up at the transmission end, you will need to have your drive shaft shortened or fabricate a new one. While you're at it, why not replace the u-joints, especially if they're still the original 30 year old ones. My cost was $65 for cut, balance, and install 2 U-Joints. They wanted about $125-$150 to fabricate a new shaft from scratch.

While you have the drive shaft out, why not replace the output shaft seal on the trans too, again very cheap. Oh, and I was able to get my driveshaft out without removing either exhaust pipe but you might not be so lucky, be prepared to have to unbolt one at the manifold or header in order to get the drive shaft out.


Make sure when you get the Rear, you get the hardware to strap the U-Joint to the Rear End. The hardware was different on the two Rears for my swap. It may be the same for you but it's better to have it to be sure. Also, depending on the donor car you may need to get a "Big/Little" U-Joint to adapt pontiac type to Chevy type.


If you changed ratios you'll need to recalibrate you're speedo, I know you already know this but for some good info about doing this with Auto Trans try going to the Q&A section, Archive #5, and look at Topic #1162. By the way, I haven't done this yet. It's weird to cruise to work at 90 MPH...

Gas Tank

Probably a good idea to remove the gas tank as a precaution. Definitely do this if you are also replacing the springs as you will probably need to use a torch or grinder to remove some bolts! While you have it out, why not clean it out and get it painted nice. I found all sorts of junk in mine...yuk!


As far as leaf spring replacement goes, just a few comments. If they've been on for 30 years you WILL need to cut or torch the bolts off. (Remove the gas tank!) Even then the rubber bushing are a pain to get out, be patient. You will probably also have to scrape the rust out of the body slot before you will fit the new bushings in.

If you use the aftermarket 5-Leaf Springs, they will probably raise your car up 1-2 inches. You should think about getting shock extenders for the top of the shocks so that can take the extra travel.

Parts To Buy

I'd recommend having the following new parts ready when you start:

  Bearings (If You Choose)
  Brake Lines
  Brake Parts
  New Posi Tag
  Gear Oil & Limited Slip Additive
  Cover Gasket
  Housing to Break Backing Plate Bolts/Nuts (if you break 'em)

  U-Bolts & Nuts (May Break 'em)
  T-Bolts & Nuts (May Break 'em)
  Upper & Lower Pads
  Trans Output Shaft Seal

  Front Eye Bushings (If they don't come with the springs)
  Front Bracket Bolts and Clip Nuts
     AMES carries these, others only have the clip nuts
     If these haven't been replaced very recently,
     you will probably break the clip nuts
  Front Eye Bushing Bolt/Nut
  Rear Shackle Bushings

  Tank Straps
  Attaching Bolt Kit
  Tank to Body "Anti-Squeak" Kit

Hope this turns out to be helpful to someone.
Good Luck
And if anyone else does this swap and has some feedback, let me know.