Driveline / Q&A / Tech

Mailbag: How to Identify GM Differential Carriers

(Illustration/Summit Racing)

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.

(Illustration/Summit Racing)

Contact Summit Racing if you need assistance selecting the proper gear series.

GM Differential Carrier Measurements

GM 12-Bolt Car2.76:1 and down, 2-series, 1.410" deck height2.76-3.73:1, 3-series, 1.825" deck height3.90:1 and up, 4-series, 2.188" deck height
GM 12-Bolt Truck2.73:1 and down, 2-series, 1.420" deck height2.76-3.42:1, 3-series, 1.700" deck height3.73:1 and up, 4-series, 1.943" deck height
GM 10.5" 14-Bolt4.10:1 and down 4.165" deck height4.56:1 and up 4.520" deck height
GM 7.5"3.08:1 and down, 2-series, 1.660" deck height3.23:1 and up, 3-series, 1.950" deck height
GM 8.2"2.73:1 and down, 2-series, 1.615" deck height3.08:1 and up, 3-series, 2.000" deck height
GM 8.5"2.56:1 and down, 2-series, 1.530" deck height2.73:1 and up, 3-series, 1.720" deck height

Want to save and print this chart for your shop? Click below.

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  1. It would be great if that chart was available to be easily saved and/or printed.

  2. Daniel Wilson says:

    Wow… I had no idea about the complexities and differences between the different GM differentials.
    There’s always the option of simplicity combined with strength with the Ford differential featuring the familiar 9” ring gear.

  3. Eliza Martin says:

    it’s the 12-bolt that appears to get every one of the awards. Which is all well and good, since when contrasting the 10 and 12-bolt units, it’s very the more strong of the two. That is typically demonstrated when utilized in elite,

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