JOHN L. - re: rear axle seal replacement. It's easy to replace the seals. It's probaby easiest to leave the rearend attached to the tractor frame but if you have the rearend out and/or plan to repaint it, just be sure to steady it securely. Remove the rear cover and drain the Hy-Tran. Then remove the rear tires/wheels. The C-clips can be pushed off with a large screw driver or two, may need to push both ends or they'll rotate around the axle shaft. Then just slide the axle out of the carrier. You can re-use the clips, they're really heavy-duty. Leave the axle carriers attached to the center casting. If you have a leak between the carriers and center casting, new seals are available. NOW would be the time to replace them. Use a large screw driver or seal removing tool to remove the old seals. They just pry out. One note, the 72 has plain bushings on the outer ends of the axle carriers. Make sure there's no excessive wear on those bushings, try to move the axle shaft up & down, a few thousandths movement is all there should be. Simply jacking the back of the tractor off the ground with the wheels/tires on while watching the axle shaft at the end of the axle carrier, and maybe holding your fingers there while jacking gives you a good idea of how much wear your bushings have. Worn bushings will accelerate seal wear and cause leaks. The later model Hydro tractors have needle bearings in that location and not only roll easier but don't wear. I put a set of axle carriers from a parts 123 on my 72 several years ago. My '72's bushing were really worn. But strangely, the 70's bushings were like new, in fact the factory seals didn't even leak after 48 yrs and wearing out 4-5 K161/181 Kohlers mowing.
Clean the recess where the old seal was good with shop rags & solvent like Brake-Clean. Check the axle shaft for wear from the lip of the old seal, and also any nicks or gouges. I coat the outside of the new seals with gasket sealer, do a search here for my posts on "Hylomar". Using a smooth flat board like a 1"X4" or 2"X4" a couple inches long and a small hammer, set the seal into the recess with the metal side facing "OUT", the rubber lip with the spring around it should go "IN", then hold the board against the seal and tap lightly until the seal starts going into the recess, then tap it a little harder until the seal is completely in. You may need to use a short length (6 inches) of a soft pine board about 3/4" square to seat the seal completely. Make sure the seal enters the recess evenly, meaning it doesn't enter crooked, more in one area than another. That will deform the seal and cause leaks. And always tap squarely, not at an angle on the metal part of the seal only.
Once the seal is in, put some grease, same stuff you use on the mower deck spindles and wipe some on the inside face of the lips of the seal. Then make sure the axle shaft is clean, spread a thin layer of grease on the outside of the axle shaft, especially over the splined end of the axle that enters the side gear in the differential. Make sure there's no sharp edges on the splines that can cut the seal lips. Carefully slide the axle into the new seal, bushing, and finally into the splined bore of the side gear. Never let the axle "hang" from the new seal & bushing. Always make sure you support the axle shaft as you slide it into place. Install the C-ring, make sure it's completely in the groove on the axle. The axle will be able to move in & out of the axle carrier a small amount, say 1/8th to 3/16th inch when installed. Repeat the steps on the other axle. Install the rear cover making sure the cover is smooth, clean, and FLAT, some more gasket sealer on the gasket helps prevent leaks. Then refill with Hy-Tran.
Page #70 of the 72/1X4/1X5 service manual shows pictures & disassembly, inspection, & assembly instructions.
http://ccmanuals.info/pdf/1x4-5%20Service%20Manual.pdf If your fussy about things, process could take an hour, and your first seal replacement might even take a bit longer, but the second one should not take over a half hour. Repetition always speeds things up, you know what tools to grab to do the task at hand. No special tools are needed to do the seal replacement other than what I've mentioned. A pair of needle nose pliers or small needle nose Vice Grips make removing & installing the C-clips easier.
My old 129 leaked so bad I's have streaks of oil running down the inside sidewalls of the rear tires, and needless to say, my external disk brakes didn't work real well. I had to park it over a shallow cookie baking tray. New axle seals & cork gasket dried up ALL the leaks. With the price of Hy-Tran now days it almost pays to dry up these leaks.
Can't help you on the NAPA part number for the outer seals. But my IH parts invoices all say, "Corn Head gearbox seal" on them when I buy those seals. The fact most IH corn heads use 2-3, maybe four seals per ROW unit is why the C/IH/NH dealers can sell those seals so cheap! Any other questions just ask. Ohh, and when you get the rear cover off, take a few seconds to enjoy looking at the insides of the rearend. IH was in a "Class of ONE" with their design and execution of best design & manufacturing of the CC rearends. All cast iron & steel components, forged steel gear blanks, insides & outsides of the cast iron housing all sealed to prevent rust, loose sand from contaminating the oil, leaks, and it just plain looks great!