The 8 1/4 thread

some more info i found

8.25 Tech Info

We change rear end gear ratios most often to change the performance characteristics of our cars - faster quarter mile times, or maybe even better gas mileage. But, what exactly changes inside the differential to make that happen?

Of course, changing the number of teeth on the ring & pinion gears will give you different ratios. Both pinion gears in the pic are for an 8.25 axle. The larger diameter gear is a 2.45, and the smaller diameter gear is a 3.21. The 3.21 gear set has a thicker ring gear and smaller pinion tooth count to archive that ratio. The 2.45 has a thinner ring gear and larger pinion tooth count. It is important to note that each of these gear sets must use a different carrier to achieve the change in ratio.

Sometimes manufactures use different carriers or cases to accommodate larger or smaller pinion gears, called a "case break". The mounting flange or surface that the ring gear rides on is located closer to the center of the differential for a smaller pinion like the 3.21, or it can be located outward for a larger pinion like the 2.45. Sometimes this can cause additional cost to a gear change, but can also gives you an opportunity to add a sure grip.

Two differential carriers were used in most 8.25 axles. Ratios smaller than 2.71 used one carrier and anything above 2.45 used a different carrier. Most consider the 2.71 and up carrier to be a “standard” 8.25 carrier, and the 2.45 / 2.26 to be a less desirable “special” carrier. Notice the carrier used with the 2.45 gear has the ring gear located about 3/8 inch further to the outside, away from the differential centerline, to accommodate the larger pinion gear. If the 2.45 gears had been designed for the standard differential, the ring gear would have been so thin that there would have been no room for the attaching bolts to thread into it.

It is possible to use a 2.71 or larger gear with the “special” carrier with the use of a ring gear spacer. This could lead to a reduced strength of the assembly. And, some tracks (at least used to) prohibit the use of spacers. It is not possible to use 2.26 or 2.45 gears with a standard carrier.