Mikey_mick

I have been planning on putting a new 3.42 posi unit on my 77 cutlass. It was a 260 engine setup. I took the diff cover off today and saw that the pinion was stamped "15-41-gm-". Since 41/15 equals 2.73 i assume that means that it has a 2.73 gear ratio!!?? I wasn't expecting it to have a ratio that low. Does this mean I have the more desirable carrier that will accept the lower gears? I already ordered a posi unit but now I am wondering if I will need it.
What do you guys think?

midnightleadfoot

Many classic car and muscle car enthusiasts are looking for a differential that can be easily adapted for daily use, improved fuel economy or for racing. With the costs of Ford 9-inch third members and GM 12-bolts escalating, there are several low-buck alternatives still plentiful at your local salvage yard. The later model GM 8.5-inch 10-bolt differential is just such an option. The 8.5-inch 10-bolt was used in cars and trucks over a wide range of years. First appearing in the 1970 as a corporate replacement for the 10-bolt 8.2-inch or the 12-bolt 8.875-inch, the 8.5-inch 10-bolt was used in many different models by every GM division, with the exception of Cadillac, during the 1970s. Other than its abundant supply at your local salvage yard, another benefit of the 8.5-inch is that carriers and gears are easily interchangeable. Original gear ratios range from 2.41:1 all the way up to 4.10:1 in some pickups. All possible differential ratios above 2.73:1 can be swapped without having to change carriers (except for the truck differentials with 30-spline axles). The 8.5-inch will withstand some abuse; it shares the same pinion shaft diameter as the more expensive 12-bolts. Ten-bolt rears came in both Positraction and non-Positraction versions. Although it would be much cheaper to find one with Positraction already, many aftermarket suppliers can sell you carriers, axles and gears to convert a stock non-limited-slip axle to a Positraction.

midnightleadfoot

There are two gear thicknesses used in the 8.5-inch 10-bolt. All gears with ratios 2.73 or numerically higher (3-series) require the thinner-style carrier . The 2.56 gear requires the thicker flange carrier that also offset slightly more .
Ring gear spacers like this steel unit from Summit can be used to mount a 3-series gear on a 2.56-style carrier. The carrier has a thicker inside-diameter gear register that offers more surface area than other carriers, so Moore feels this is not a bad compromise. Summit offers this steel spacer with ARP bolts .