It is a front-mounted snowblower, so the model number doesn't matter much, since this only happens with chain-drive snowblowers. His problem is that the keyed gear that the chain turns has slipped, and it is moving in and out. The cure is to remove the snowblower from the tractor and remove the 8" bolt that holds the rear cover on, which protects the chain. Then start dismantling the drive, starting with the chain. Remove the sprocket gear and ensure the key and keyway are not damaged. Then go to the front of the snowblower, and bolt the fan shear plate to the drive plate with a Grade 5 shear pin. Then return to the rear of the snowblower and start replacing the parts you removed to check for damage. You will need to align the upper sprocket with the lower sprocket. Next, loosen the adjustment bolts on each side that adjust chain tension, then reinstall the chain. If you don't mind working with greasy hands, soak the chain in a container of chain bar oil to lubricate it. Once the chain is back on the sprocket, adjust chain tension. I would use red Locktite on the set screws and make sure that they are tight.
If you continue to use grade 8 bolts for shear bolts on the fan, you are going to do damage to the snowblower and the fan. Don't forget to change the front gearbox oil at the end of every snowblower season. The 3/8" hex nut on the top with the pop-up pin in the center is a vent. When the snowblower gearbox gets warm, it vents the warm air inside the gearbox to the atmosphere. As it cools, it draws in moisture, and it condenses inside the gearbox and mixes with the gear oil. That is why it should be changed every year. To drain it, heat the Allen-head screw at the front of the gearbox until the Locktite bond breaks, then remove it. Then flip the snowblower over on its face and drain the gear oil into a pan. While the front allen screw is out, fill the gearbox till the gear oil runs out of the front screw hole. Best to do this when the weather is in the high 80s, not in the middle of winter.
