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 |
Want to save and print this chart for your shop? Click below.
It would be great if that chart was available to be easily saved and/or printed.
Ask and ye shall receive Dana. I added a downloadable PDF to the bottom of the story. Enjoy!
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.
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,
Just for entertainment the writers of the blog, please consider maybe including information on the Borg Warner 7.75” ring and pinion this was a nice product used in GM products in the 80’s and 90’s.
Is the 14 bolt a good rear end for a K1500 4 wd and can I change the To 3.73-1 to match my front diff.
Will my 2000 Chevy 2500 full float, open differential carrier with 4.11 gears accept a Detroit locker ? Or do I need to find a different carrier?
I have a 2968 Chevy C10 pickup truck 12 bolt rear end how do I know if it has a 3 or 4 series carrier
It has 373 gear
I have a posi assembly with the casting numbers 410409N and a GM2 on it. My question is will it probably fit a 65 el Camino. It has the 10 bolt type with the dif cover that has the eye lid shape. I think it is 8.2 but not really savy on the differential aspect of things.