Rear ford 9"

I just bought a rear ford 9" for my xj. How do I tell if it is the big bearing or the small bearing 9"? It has 31 spline axle shafts, does this mean it is the big bearing unit because I know they made a 28 and 31 spline 9". Also can I change it to the big bearing if I need to without switching housings?

I'm not of any direct help but I was given this site from a buddy on another forum. It's got tons of info on ford 9" and the 8.8.

http://www.rangerpowersports.com/

Small bearing will have 3/8" bolts on the housing ends.
Big bearing will have 1/2" bolts on the housing ends.

Okie Terry said:

Small bearing will have 3/8" bolts on the housing ends.
Big bearing will have 1/2" bolts on the housing ends.

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The large bearing housing will have 7/16" bolts (not 1/2") on the outer tube flange.

MaXJohnson said:

The large bearing housing will have 7/16" bolts (not 1/2") on the outer tube flange.

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Oops. It's the bolt holes that are 1/2" and 3/8".

MaXJohnson said:

The large bearing housing will have 7/16" bolts (not 1/2") on the outer tube flange.

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Hmmmm, I've had big bearing housings with both 3/8" and 1/2" T-bolt retaining flanges. I thought that the T-bolt size was a model year change around '80 or '81; with the earlier models having the larger 1/2" and the later the 3/8".
I wouldn't count on the flange bolt size as an indication, I'd measure the bearing race.

Pretty much any truck, FS Bronco or van housing will be guaranteed to be a big bearing. I'm pretty sure that any 31 spline housing will be big bearing (but that's a guess).

To answer your second question, yes you can change it to a big bearing housing, just buy the flanges(about $75 for the pair) take to a shop that does this type of work and pay them to weld them on straight, or you could do it yourself, it will be cheaper to buy a new housing, they go for about $30 around here for just the housing.

I checked. its the big bearing. thanks for the help