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Before pulling the remains of the old oil seal on the mower gearbox, I decided to open the gearbox and see the state of the gears. I opened it up and the gears are true hypoid, not just crown and pinion gears, and they were in beautiful shape. They did have a bit too much play and I noticed one of the tiny thrust bearings had disintegrated. It needed to be replaced or the gears would grind themselves to bits.

So I decided to disassemble the gear box to replace any bearings. These steps apply to most of the mower gearboxes - the same box was used on multiple models of decks.

(If you want to visualize these parts, see this file in the manuals thread
bolens_18421-0_deck.pdf)

1. Remove the universal from the drive shaft by driving out the roll pin and pulling it off. Remove the half-moon woodruff key - good luck.

2. Drain the oil from the gearbox. Put it in a pan and loosen the bolts holding it together. Remove the bolts and the back. The oil seal is not a critical thickness so if you tear it you can use RTV to replace it.

3. Remove the drive pulley. Loosen the two setscrews in the drive pulley - unscrew them almost all the way. PULL the drive pulley off the end. Pull, do not bang it off. Banging off the pulley can crush the tiny internal thrust bearings. The thrust bearing in mine was already gone, so I tapped off the pulley.

4. Remove the half-moon woodruff key used to drive the pulley. Good luck.


Here is the really tricky step. Read it through before you even THINK about removing the internal roll pins.


5. Remove the roll pin from the smaller gear on the vertical shaft. Do NOT try to drive the pin all the way through. It will NOT come loose because the gearbox does not have enough clearance. It will jam against the back of the gearbox, and you cannot even rotate the shaft to remove it. Instead, drive the rollpin about 1/4 inch, rotate the shaft until the pin faces outward, and try to pull it out with Vice Grips. If you cannot pull it out, then grind off the protruding roll pin, turn it back into the gearbox, and drive it another 1/4 inch. Rinse and repeat.

I tried driving it through and it stuck solidly in the gearbox. I twisted the shaft back and forth until the roll pin broke and I could rotate it outwards and pull it out with Vice Grips. It took about a couple of hours. Trust me that you never, ever, EVER want to do this.

End of really tricky steps.

6. Grind or file a slight bevel around the end of the shaft where the pulley was. You might need to push the shaft through and it may have a very slight flare preventing this. Mine did.

7. Pull or push the shaft out. Pulling is much preferred. I was unable to pull it out because the remains of the thrust bearing acted like a snap ring. Instead, I tapped it through the top. This will push the needle bearing out of the top. The top needle bearing housing is made of some weird super-hard adamantium metal, so it did not deform. You will probably have to wiggle off the thrust bearing and bearing washers as you go. Any oil seals will fall off in the process.

8. Inspect the needle bearings and thrust bearings. If one or more bearing pins fall out, the bearing is worn enough to need replacing. On my needle bearings two bearing pins fell out. I saved them but I doubt I can simply put them back. Nuts nut nuts.

9. Now you can push out the horizontal drive shaft. Push it gently through. Tapping may damage the thrust bearing but if it does not slide out gracefully then tapping it out is the only method. Inspect the thrust and needle bearings.

10. There is no need to drive out the rollpin on the larger gear.

On my gearbox the shafts and gears were fortunately in good shape. Now all I have to do is find the appropriate bearings.
 
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