So you want to put a 2nd Gen F-Body Rear End in your 1st Gen Firebird?
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
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.
you have your measurements correct, you don't want to
have to do this twice!
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
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...
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
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:
FOR THE REAR END
Bearings (If You Choose)
New Posi Tag
Gear Oil & Limited Slip Additive
Housing to Break Backing Plate Bolts/Nuts (if you break 'em)
FOR THE SWAP
U-Bolts & Nuts (May Break 'em)
T-Bolts & Nuts (May Break 'em)
Upper & Lower Pads
Trans Output Shaft Seal
FOR NEW SPRINGS
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
Attaching Bolt Kit
Tank to Body "Anti-Squeak" Kit
Hope this turns out to be helpful to someone.
And if anyone else does this swap and has some feedback,
let me know.