How can I tell if the 9" on my 73 bronco is a big bearing or small bearing, and if it is a small bearing like I imagine, is it possible to put 31 spline axles in it or am I limited to 28 splines?

Thanks for any help

 

Look at the back side of the break backing plate. The backing plate bolts to the bearing cup. I've been told that if the bearing cup is a larger diameter then the axle tube. its a large bearing. If the cup is the same size as the tube its a small bearing. They sell bearing cups to change small bearings to large bearings, all you do is cut and weld. I've never heard of someone putting 31spl axles with small bearing cups, but anything is possible. The problem with the small bearing is not the axle its the bearings. The small bearing axle is more flexable and can take a better beating then the 28spl big bearing. 31 spline is going to always be better, but I wouldn't be discouraged by the small bearing unless your intention is big tires or hauling stuff.

 

Also the large bearing axels use 1/2 inch bolts for the bearing retainer and brake backing plate, the small bearing axel uses 7/16.
No you cannot get 31sp small bearing axels.
To change to large and do it right the housing should be put in a lathe and turned to cut the weld. Currie enterprizes sells the used ends for around $30 each I believe.
The ends they sell need to be turned also, as they are still welded to a short section of the old housing tube.
The small bearing axels tend to break between the bearing and the wheel flange. It happend to My bronco twice, once to me and once to the previous owner. Very scary and dangerous. When that happens you loose the wheel. If you strip the 28 splines you still keep your wheel not that big of a deal. I would only switch to the 31 if you are allready planning on changing out the carrier, for say maybe a LOCKER!
Your 31 year old small bearing axels on the other hand could be cracked and should be magnafluxed to check, or be replaced.
If you change ends you will need a set of big brakes.
There were 2 different width 11 inch drums used and you will need to know which sizes you will have before ordering your new axels. Might as well go with the 11x2.25 as they were used on 76&77 broncos and many year F150s, the junk yards are full of them. They are a good match to front discs.