Q: I have a 1970 El Camino with a 12-bolt differential and 4.11:1 ring and pinion. I’m not sure if it’s the original gear ratio, but I want to change to 3.42:1. I see this ratio is only available for a 3-series carrier, but 4.11:1 is available for 3- or 4-series carriers. How do I identify which carrier I currently have?
A: GM uses different carrier breaks (carrier deck heights) to accommodate different ring and pinion gear ratios. Pinion gear diameter decreases as the numeric gear ratio increases because there are fewer teeth on the pinion gear.
Likewise, ring gear thickness must increase to properly mate to the smaller diameter pinion. After a point, the increased rotating mass of the thicker ring gear begins to negatively affect performance.
To counteract this, different carrier deck heights are used to reduce gear weight. Some gear manufacturers offer two different series options in the same gear ratio; these may be referred to as “thick gears” or “thin gears.”
To determine what series carrier you have, place the carrier on a flat surface like a shop table with the ring gear mounting surface up, then measure from the mounting surface to the table. Use the chart below as a guide to measure your carrier.
NOTE: Dana differentials utilize carrier breaks as well.
Contact Summit Racing if you need assistance selecting the proper gear series.
GM Differential Carrier Measurements
|GM 12-Bolt Car||2.76:1 and down, 2-series, 1.410" deck height||2.76-3.73:1, 3-series, 1.825" deck height||3.90:1 and up, 4-series, 2.188" deck height|
|GM 12-Bolt Truck||2.73:1 and down, 2-series, 1.420" deck height||2.76-3.42:1, 3-series, 1.700" deck height||3.73:1 and up, 4-series, 1.943" deck height|
|GM 10.5" 14-Bolt||4.10:1 and down 4.165" deck height||4.56:1 and up 4.520" deck height|
|GM 7.5"||3.08:1 and down, 2-series, 1.660" deck height||3.23:1 and up, 3-series, 1.950" deck height|
|GM 8.2"||2.73:1 and down, 2-series, 1.615" deck height||3.08:1 and up, 3-series, 2.000" deck height|
|GM 8.5"||2.56:1 and down, 2-series, 1.530" deck height||2.73:1 and up, 3-series, 1.720" deck height|
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