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

Customers often end up with a gearbox - but they are unsure of 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 early B-bodies (full size GM cars) came with the 3.25" box, later models went with the 3". 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". That's how full sizes vehicles are upgraded to 12.7:1.) 

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

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.

3" Gearbox measurement

3.25" gearbox measurement