Measuring your Saginaw 800 Gearbox - 3" or 3.25" box?

 
3" Gearbox measurement

3" Gearbox measurement

Customers often end up with a gearbox that they don't know the vehicle it came from.  Then, they want to know "can it be upgraded to a fast ratio?" One way to answer that question is to know what size gearbox it is.  

Saginaw 800s come in two sizes; 3" and 3.25". When talking about 1960s through 1980s GM automobile boxes, most B-bodies (full size GM cars) came with the 3.25" box. A-body and F-bodies came with the 3" gearbox. The 3" can be upgraded to the 12.7:1 fast ratio, the 3.25" cannot. (The 3" box can be a drop-in replacement for the 3.25" and that's how full sizes vehicles are upgraded to fast ratio.) 

To check your gearbox size, measure the round aluminum endcap (which faces the front of the car) as shown. 

3.25" gearbox measurement

3.25" gearbox measurement

There are other ways to tell what gearbox you have, but sometimes you still need to know the size. Other ways to identify your gearbox will be covered in a future post.

 

 

Marty Preuss