Going For The 4•5•6 - Housing Research

Also during research I decided to take a chance and buy a rock guard spider for a GM 10 Bolt rear end from Purple Cranium. I had it shipped to The Roadie for test fitting as I was without Trailblazer at the time. What this did was confirm that our axle housings were completely different from the standard 10 bolt housings used by GM for years.

Here are some comparison pictures. Even though the sizing is close, the blots completely do not match up.

OK, even though that little exercise did not work out, I decided to march on by doing some research with a GM parts person, which led me to the part numbers of 2 different differential covers - one for the 8" and one for the 8.6". As their costs were quite small, I bought one of each to do a comparison.

On one side, I was still toying with the idea that it would be feasible to put 8.6" gears into my 8" housing. On the other side, this could also help in determining if the 2 axles units are interchangeable.

First off, here's a close up of the 8.6 cover:

As you can see, there should be no more questions of which rear end anybody has as there is an "8.6" stamped right into the cover. I would think this is not a new thing and that any Trailvoy, all the way back to when they started using the 8.6 rear ends, would have this.

Second, here are some side by side comparison pictures:

The 8" is on the bottom and on the left. My observations:

• The covers are the same in terms of shape, hole placement, shape and size of extrusion, pretty much everything
• The one big difference is the depth of the extrusion, its plain to see that the 8.6 is deeper
• The inside extrusion measurements (not depth) are the same for both - 8" vertical at the longest point, 7.5" horizontal at the longest point

Here is a picture of the 8.6 covering the 8 inch. The holes line up perfectly:

So what does all this mean?

It means that I need one more piece of critical information before I can declare that an 8.6" R/P can work in an I6 Trailvoy. And that is wether the 8.6" differential housing is longer/deeper in the front.

Common sense dictates that it is, to match the deeper cover and therefore provide more room for the larger ring gear.

However, one thing confuses me - if that's the case, then why are the cover's measurements the same in height? When you think about it, if you increase the diameter of the ring gear, would it not need more space, particularly at the very top and bottom of the cover extrusions, and not just in the middle? In other words, why is the vertical space not increased for an 8.6" ring gear?

The only answer would be that the cover is deeper to allow for more differential oil for better lubrication and not for ring gear size requirements.

Well, its not something I want to guess at so I am going to try to figure out the best points for measuring the front of the differential housing in order to compare with an 8.6". Ultimately this should lead to a final determination and direction I will be going in for the upgrade.