1968 Pontiac Firebird Rear End Swap

Surprise, surprise. Just like everything else so far. The rear end work I had to do did not turn out as
easy as it should of. It started out as my desire to upgrade the existing 2:56 open rear end in my car
to something better. I was looking for something in the 3:23- 3:73 range but along came this 3:08 chev
posi rear end. Some things don't turn out quite as good as they seem and this rear was another one of
them. I have pretty much figured out that it came out of a 67 camaro. Once I had it home and had
started stripping off what was left of the old rusty brake assemblies, a friend noticed something I
hadn't very quickly. The posi rear had single leaf perches on it. I did not have the diff out of my
firebird yet to notice the difference. I had measured the diff length and the space between the
perches but had not noticed the depth of the perches. So here are the pics and descriptions of
the odyssey that was my rear end swap. It is still not done by the way, because while I did the
rear swap I was also doing a 4-wheel disc brake kit installation. After it was all installed on the
new rear end I discovered the kit I had was for a non-staggered shock application. Of course all
68 firebirds are staggered shocks so now I will be fabricating a shock relocation kit. And did I
mention that the new posi rear used different style U-joints than my old pontiac diff? aargh.

here is the all-important difference that I did not notice when I said yes to this posi rear.
The single leaf on the left and the much bigger multi leaf on the right. Very obvious when they
are sitting side-by-side right?

the first step was to measure the exact position and angles of the perches as they were positioned
on the original pontiac diff. Something I did not realize at the time but ended up turning out ok,
because I am so picky, was the important fact that 1st Gen firebird leaf springs and the perches,
are not sitting at exact right angles to the diff. There is a slight toe-in to the leaf springs and the
perches are special in that they also reflect this slight inward angle as well. Both left and right
perches are the same when sitting side by side on the floor but one must be positioned a certain
way on the drivers side and the other a certain way on the passengers. I actually did not know
that until after I had welded them into position on the new diff housing. That could have been
a real ugly fix.

So I measured the distances and angles very carefully as you see below.

then positioned the diff on axle stands sitting with perches dead level.

then carefully measure the angle the pinion u-joint saddle is at. I put a bar that matched the
u-joint cap size pretty close and then an angle indicator like you see below. I made a small pen
mark on the indicator and took pictures because it was actually several days between this step
and the re-welding of the perches on the new diff.

Then using a small grinder with a cutting disk and an air chisel, I removed the perches from
the diff's. On the old dif I was careful to remove the perches by cutting on housing side more
(to not damage the perch) and on the new diff I cut more on the perch side (to not damage
the housing). I was not to worried about scratching the housing on the pontiac 2:56 open diff.
I am pretty sure the market is not jumping all over those things. By the way removing these
was not easy. They were very well attached and had excellent welds on the sides. And that
center hole was welded and some more little tacks underneath along the tube to the bottom
of these perches. The chev rear perch was much easier to cut off for some reason. :-)

here is one the single leaf perches removed.

then get the new Diff axle tubes cleaned up

you can see the positions the old perches were on the tube. I tried to not cut into the tube at all.

then reset the pinion angle same as before .

I set the perches at the exact same position as the old rear. then tack weld,
then double-check all measurements.

you can actually see the way the perch sits at an angle in this shot. See how the bottom
of the perch sits out farther on one side than the other?

and the other side. see the little flaps on the bottom again? small on right side and bigger on left.

after double-checking everything. I welded them up solid the same as before. down the sides,
the middle hole and some tacks on bottom edge along the axle tube.

then after lots of wire and stripper wheel work on the diff housing I coated it with the Chassis Saver
urethane coating, then two coats of good quality semi-gloss spray paint. The Urethane coating is
super strong but not very UV light resistant and is supposed to fade to a grey colour if exposed to
sunlight if not top coated.

all coated and ready for re-assembly.

this page is getting a little long so I will put the rest of the axle re-assembly on a second one.

Click Here to go to Rear End Page 2.