I am trying to install a toro front wheel drive gearbox and have all the washers mixed up

Review diagram at;
https://www.partstree.com/models/41-20oe01-520-h-toro-garden-tractor-1990/wheels-and-tires-35/

Raise and secure front of mower off the ground.
Remove large wheel cover (if in place).
Remove small dust cap.
Remove cotter pin.
Remove castle nut.
Remove flat washer.

The wheel should now be loose and the outer roller bearing can be removed. Next slip the wheel off the spindle. Take care not to loose any parts from backside of the wheel.

Replace drive cable

I have a 22" Toro Recycler Lawn Mower with front wheel drive. After many years the original tires wore out, primarily the left front.
After I replaced the wheels the front wheels kept falling off. I learned that Toro uses an unusual two layer washer on the front wheels. You need a) either an exact replacement for this washer OR b) two separate washers. The hole of the washer is 3/8" Everbilt has the smaller flat washer PN 637-009 and the larger washer can be found in the Everbilt PN 281-293. The SMALLER washer must be placed closer to the frame, the larger washer is closer to the wheel. Once this was done it ran perfectly.
Later I found pieces of the original washers in the grass. Toro had the replacement washer available on their website, I was just in a hurry to get the mower back up and running. The Everbilt washers were purchased at Home Depot.

Got the same problem with mine, replaced the drive belt, removed the wheels and checked for rocks and pebbles, greased, checked also bottom of gearbox. grease was present no broken teeth, top pulley has excive side to side play. Have not disaasembled yet. Hope someone would have had a solution here.

The transmissions gear is bad need to replace

Many TORO mowers have a drive pinion that can be used on either side, however, internally, they ratchet one direction, drive the other direction. You may want to rotate (reverse) the installation of one gear and try it.

I’m happy to assist further over the phone at https://www.6ya.com/expert/rolandw_c013176d62ee17ae

most likley it is the gear box has failed but first take one of the wheels of to make sure that the plastic teeth on the weels didnt go bad (unlikley but wishfull thinking)

Problem is in the rear pinion gears on rear axle and the design of the axle itself. I have a 2005 model 20041 had the same problem. I found a Toro dealer that had the fix.The problem is creadted by the fact that there is too much "play" in the rear axle. You have to limit that play by installing shims, or washers First, but the 2 new rear pinion gears for the rear axle. While you're at it, you may as well buy 2 new rear wheels because the "grinding" has worn the sprockets on the reas r wheels.Next, get 2 "fender washers", 1/2" ID and about 1" OD. I found it easier to turn the mower over and set it on saw horses to work on it.( drain the gas and oil first). Remove both rear wheels, pinion gears and all the washers etc. (caution - keep all parts from one side together - do not pile all parts together - keep them seperated by wheel - the reason is I believe there is one more washer/shim on the right side than on the left). Now to put everything back together, find a spot on the axle where the washer will fit in between the standard stuff.Install new pinion gears and replace the rest of the washers/shims just as they came off. INstall new wheels and you'll be good as new!

Check the belt tension. There is a spring that holds the tension and connects the gearbox to the mower deck. If it has slipped off the belt will not be tightened properly.
Hope this helps.

If you take the wheels off and remove the belt from the small pulley on the the front axle underneath the pulley you should see a small transmission. You usually can split this unit in half . You will see two gears and sometimes one plastic and one metal. The plastic one is usually the culprit. Hope this will help you out. Donnyb60