Drive Axles

When installing the 3800 SC Series II engine in the Fiero, the drive axles were a major concern of mine throughout the project.  When making the engine mounts I was concerned about how much misalignment they would be able to handle, both horizontally and vertically; then along with the right length, the splines on the axle shaft would have to take the Pontiac Fiero stub shaft on one end with the regular Chevrolet Grand Prix GTP tripot on the other end going into the transmission.  Before we get too carried away here, a word of warning when working on axles, they contain a LOT of grease that makes a nasty, sticky, disgusting mess  … everywhere!  I was expecting that it would cost around $500 to get custom axles made, so I decided to get stuck in.  I got great advice from my buddy Rick and will pass it on, clear a space on the workbench, cover it with newspaper and get PLENTY of paper towels ready, and then get some more.

First thing’s first, let’s identify the parts and define the terminology, at least the terminology that I’m going to use …


Apparently, the Driver’s Side (DS) Fiero automatic axle is often plug and play on 3800 engine swaps that use the 4T-65HD transmission, so I offered it up after cleaning and removing the boot so that it doesn’t get in the way when doing the trial fit.  The rollers of the Fiero automatic axle are a perfect fit in the GTP DS tripot, however my transmission must be $mounted closer to the Passenger Side (PS) than other swaps making the Fiero auto axle too short …

FreeAxle4.jpg  LoadedAxle3.jpg

The first picture on the left above shows the no load condition where the axle rollers are at their outer most point, the second picture was taken with the suspension jacked up so that the axle is about level, i.e. the rollers at their maximum travel into the tripot.  The rollers want to be at least 3/4″ further into the tripot for me to be comfortable with it.  Notice that the rollers don’t travel very far between the two conditions, I read somewhere that the engineers generally design for about 2″ of plunge … I suspect that is for the tripots.

After finding that the Grand Prix GTP axles I had were not suitable and struggling to find any information on axle shaft lengths and interchangeability that could give me enough confidence to make a purchase, I decided to follow the lead of Dennis LaGrua (a member of Pennock’s Fiero Forum) and see if I could find a place that would custom build some axles for me.  I did some Googling and called a few local places but it didn’t look good, some didn’t do any custom work, some referred me to someone else and some didn’t even bother returning my calls!  Then I found New Tech CV Rebuilders and talked to Siva, I was amazed when he told me it would be no problem for him to make up some axles for me and only wanted $100 each!

So I went through all my measurements (see below), and cross referenced my numbers with my friend Rick’s measurements since he is using the same transmission mated to a SBC V8 in his Fiero.  At first it was very confusing because while my transmission is located 1/4″ off centre making the PS axle shaft 1/2″ shorter than the DS, Rick’s V8 engine pushed his transmission over more towards the DS.  Even accounting for that, the total combined length of both my axles was an inch longer than Rick’s total length – they should be the same.  With some more head scratching, yet more calculations and further measurements, we determined that Rick’s transmission is sat lower in the cradle than mine by at least an inch.  My calculations suggest that for a 10″ long axle shaft, with axles at about 30°, reducing the height of the transmission by 1″ would translate to the axle shafts being 1/2″ shorter on each side – bingo!

So I gave the compressed lengths I needed and the GM tripots that I had to Siva at New Tech CV Rebuilders and he put together the axles for me using large diameter axle shafts from a GM application.  When I first installed the axles and checked that they rotated freely throughout the suspension travel, I found that the DS axle locked up and would not rotate when the A arm was jacked up.  After a lengthy inspection I found that the top of the ball joint on the A arm was sticking up too far out of the knuckle and rubbing against the outer stub shaft of the axle!


There is a key way cut into the ball joint shaft so when the bolt is in place, the shaft sits lower in the knuckle and easily clears the stub shaft.

This was an exciting phase of the project.  With the axles in place and ready, it felt close to finally getting the Fiero off the axle stands and having it move under it’s own steam – the first time in the 3 years I had owned it!  In the 4 years of driving once it was completed, I did not have a single issue or concern with the axles.

Here is the information and measurements I collected while researching, I know it wants tidying up but the overall message here is that if you get custom axles, you don’t need any of it.