One of the most confusing parts of this project was researching the combination of parts needed to fabricate new, hybrid axles. On the surface, it sounds easy: find a tripot joint that mates to the spline pattern of the six speed transmission, find a CV joint that mates to the splines of the Fiero hub, and join them together with an axle rod. How hard could that be?
The trouble lies in the lack of any source with specific information on the bare axle rods: what diameter they are, how long they are, how many splines are at either end, what the spline diameters are, where the snap rings are located, etc...). None of the axle manufacturers provide a breakdown of these dimensions, nor of the internal components that would make mixing and matching a breeze.
In fact, there are at least 21 variables that come into play for each axle assembly:
1 & 2: the spline count and diameter of the tripot spindle;
3 & 4: the spline count and diameter of the tripot inner race;
5 & 6: the diameter and length of the tripot spider assembly bearing journals;
7 & 8: the diameters (ID and OD) of the tripot bearings;
9, 10, 11 & 12: the spline count and diameter of the axle rod at either end;
13 & 14: the length of the axle rod and location of snap ring grooves;
15 & 16: the spline count and diameter of the CV joint inner race;
17 & 18: the number and diameter of the CV joint ball bearings; and
19, 20 & 21: the spline count, diameter, and length of the CV joint spindle.
The breakthrough came when I learned the CV joints from a 2005-2007 Chevy Cobalt SS 2.0L (with the F35 five speed manual transmission RPO code MU3) would fit the stock Fiero wheel hub externally, and could be modified internally to fit on an axle rod from a 1985-1988 manual transmission Fiero.
Furthermore, the tripot joint from the same Cobalt axle would mate as-is to the F40 six speed transmission, and could be mated internally to a re-splined Fiero axle rod.
The Cobalt axle rod was however far too short (345 mm) to use on my project car, besides being relatively weak with a 24.5 mm minor diameter. The Fiero was chosen as the donor for the axle rods since the 27 mm diameter passenger-side axle is one of the industry's longest, uniform diameter axles, and therefore most easily customized with respect to length. In summary, here are the sources for each of the parts:
To get things rolling, I bought a pair of new Cobalt SS axles from NAPA, part number 94-9056 for $155 each, after taxes:
I first harvested the tripot joints and their innards from the Cobalt axle rods:
Then I attempted to harvest the Cobalt CV joints from the other end of the axles, without success. The trouble was that the snap ring for the CV joints is hidden. I tried using my 12 ton press, then went to a transmission shop and a machine shop without success. Clearly the snap ring grooves were cut too square... these were made not to come apart. Ultimately, the only way to remove the CV joints was to cut up the bearing cages so the balls could be popped out one by one until the spider assembly and axle could be pulled out:
Here are the chopped up cages and damaged balls:
The main bodies of the Cobalt CV joints were undamaged and new, so I salvaged them by rebuilding them with the cage and balls from a pair of manual Fiero CV joints, which are identical in size. The spiders (or inner races) must come from the Fiero CV joints regardless since only they have the correct splines (32 on 26.0 mm dia) to mate with the stock Fiero axle rod. Here are all the Fiero CV joint innards I used, though the balls and cage could have come from the Cobalt axle if they could have been removed successfully:
The hybrid CV joint was then built up using common tools and a lot of care:
The next step was to salvage the long axle rods from a pair of Fiero passenger-side axle assemblies:
Here's the CV joint end of the axle rod once it was cleaned up:
These manual transmission axle rods are 775 mm long and 27.0 mm (1.063") in diameter:
...there are 32 splines at both ends and the spline major diameter is 26.0 mm (1.024").
The hybrid CV joints slipped on these stock axle splines perfectly as planned, though the Cobalt tripot joints would not. Cutting the axles to the correct length and re-splining the cut down end to fit the inner splines of the Cobalt tripot joints was next.