There is some confusion about what exactly is similar and different between GM 10-bolt 8.5″, 8.6″, and 8.625″ rear axles. First and foremost, 8.6″ is simply another name for (and an easier way of denoting) 8.625″. This means that both 8.6″ and 8.625″ mean the same thing, however people in the industry have found it easier to refer to an 8.625″ axle as an 8.6″ axle so 8.6″ is the term most often used (even though it really means 8.625).
With that said, now comes the fun part explaining how to distinguish these two axles apart, as well as identify which parts are and aren’t interchangeable between them.
The 8.5″ axle was used on vehicles up to 1999. The 8.6″ was used on vehicles 1999 and up. Since 1999 was the transition year, some vehicles received the 8.5″ while others received the 8.6″. The easiest way to tell the difference is by looking at the brakes. All 8.6″ axles have disc brakes and 8.5″ have drums.
Both axles use the same ring and pinion gears so that means a ring and pinion set from a present-day 8.6″ will install into a 1980’s 8.5″ and vice-versa.
The main difference between the 8.5″ and the 8.6″ is with the carrier bearings.
The factory 8.5″ carrier when installed in a factory 8.5″ axle uses Timken bearing part # LM501349 and race part # LM501314. The inner diameter of the bearing is about 1.625″ and the outer diameter of the race is about 2.890″.
The factory 8.6″ carrier when installed in a factory 8.6″ axle uses Timken bearing part # LM603049 and race part # LM603012. The inner diameter of the bearing is about 1.780″ and the outer diameter of the race is about 3.060″.
Nowadays, most locker manufacturers produce parts only for the 8.6″ and include properly sized bearings to retrofit them into the older 8.5″ axles. Therefore, the inner diameter of the bearings for these parts will be about 1.780″ and the outer diameter of the race will be about 2.890″. This same logic also applies to differential carriers, but it is important to note that the parts are BACKWARD compatible only. That’s to say that the newer 8.6″ differential cases when used with the appropriate sized bearings and races can install into older 8.5″ axles. As of this writing, there are no bearing and race combinations that allow an 8.5″ carrier or locker to install into an 8.6″ axle. When retrofitting an 8.6″ carrier (or a locker or a spool or any device that replaces the carrier) into an 8.5″ axle, it uses Timken bearing part # LM102949 and race part # LM102911.
Side Gears, Spider Gears, and Internal Parts
Some general guidelines to follow (which seem to be pretty consistent in this industry) when ordering differential internal kits go like this…
- We have never seen a 1999-newer truck or SUV with the new body style not take an 8.6″ internal kit.
- 1999-2000 vehicles with the old body style came with either an 8.5″ 30-spline or an 8.6″ 30-spline.
- The majority of 1999 old body style trucks and SUVs use an 8.5″ internal kit.
- The majority of 2000 old body style trucks and SUVs use an 8.6″ internal kit.
- 1981-87 GM 10-bolt front and rear solid axles used 28-spline shafts and need an 8.5″ 28-spline internal kit
- One very grey area is the Blazer and Suburban front straight axle models built between 1987-92.
- 1987-92 front straight axles came with either 28-splines or 30-splines.
Please keep in mind that these are general guidelines and are in no way definite for any vehicle. To be absolutely certain about which axle is in your vehicle, we recommend contacting a GM dealership parts department with your vehicle’s VIN, or opening the axle and measuring the carrier bearings and counting the axle splines.
Factory Carriers and Lockers
The 8.6″ axle was available with three different carriers: the “Large Window”, the “Large/Small Window”, and the G80 Gov Lock.
The G80 Gov Lock is notoriously known as the “Gov-Bomb” or the G80 “Ticking Timebomb”. This unit has proven itself time and again to be incapable of handling excess torque loads from even the stock engine with stock sized wheels and tires*. If you plan to drive off-road or install larger wheels and tires, then it’s HIGHLY recommended you replace this unit with either a full locker (ARB, Detroit, Eaton), a spool, or an open carrier with a lunchbox locker (Powertrax Lock Right). Some drivers have gone as far to say that an open carrier is better than a G80 Gov Lock because it’s nearly impossible to explode the gears in an open carrier by over torquing them.
This information is current as of March 2, 2012.
*This isn’t our opinion. This is what our customers have reported to us in addition to many documented cases about G80 failures which can be found online.