190XT Rear axle diameterj

Hello,
I hate to ask what might sound as stupid, but asking anyway. Does anyone know if all axle diameters are the same for a 190XT? I have a 1966 gas with what appears to be 3 1/4 inch, with 18.4x34 tires. i have a chance to replace with 18.4X38 and it appears axle is smaller in diameter on the this tractor. Both tractors are 190's with spin out wheels and the only difference is axle diameter? I think the other tractor might be newer, not sure. So if any one knows or where I could find out, please respond.
Thanks
Gary

I changed from 34 to 38 tires on my 190 gas .  The axel on the 190 was smaller than the 38 inch wedges so I  went a hydraulic cylinder rebuilder and bought a   piece of tube.  The inside dia is the same as the axel and outside the same as inside of the wedge.  I split the tube with a plasma  cutter so the key would fit .   You also have to add a strip of flat iron on to the key to  make up for the thickness of the tube.    Been on this tractor for 12 years and hasen't moved .

just make sure you keep the wedges with the right axels. The cast centers will interchange. You already have what you need. just put the wedges back on the tractor they came off of.and swap the centers