Also check to make sure you have the correct size pin/bolt. Grade would be most likely a 5 or less so I usually use the manufacturers parts for this. I got a blower and the shear bolt was the wrong size and was snapping all the time. One of the previous owners got the wrong size (part number), since replacing no more problems.
This is the large chain sprocket on my snow blower. Different brand than yours, but should be similar. You can see the quarter inch square key that fits in slot cut in impeller shaft and slot cut in sprocket hub. Then the set screw tightens down on it. My key is 1/4" X 1/4".
Mine also has a 1/4" bolt through the hub and the impeller shaft. Yours may not have that.
If your key is missing, it will need to be replaced.
I bought a 12" piece of 1/4" X 1/4" stock at local hardware store and cut a piece to length as I need one. I usually cut a piece to length, then chamfer (Bevel) all 4 edges on one end of it to make it a bit pointed.
I align the two slots, then push the new key in. Usually have to tap it in with light taps with a fiber glass or plastic hammer so as to not damage the end I am pounding on. Might need a temporary spacer or a pry bar behind the sprocket to keep it from being pushed too far back. Align the sprockets and chain and tighten the set screw.
Turn the snow blower impeller by hand a few revolutions to see if all is aligned as it should be.
If your key is still in place, then check to be sure sprockets are aligned, and set screw is tight.
If your chain is #40 chain, I would buy a 10 foot roll of new number 40 chain and cut out a new chain to replace the old one. I think I paid $22.00 for the last roll I bought. Nice to have some new chain on hand on a snowy Sunday afternoon when a chain breaks.
I do not have a chain break so I figure out the number of links I need, then grind off the rivet head on one side at that point and drive out the rivet.
Buy a couple new #40 master links also. Use one to attach the 2 ends of the chain to each other after getting it wrapped around the sprockets.
Do you still have the old broken chain to measure or count the links you need?
My chain has 32 inner links.
Yes hub can be tapped back or forward on shaft to align with other sprocket.
Hub and sprocket will be in alignment with the shaft if key is still in place.
Square cut in the sprocket hub and the square cut in the shaft it is on need to be lined up in order to get the key in place.
Yes set screw needs to be above the key so it can tighten down on the key.
Did the key shear off on yours? Or wiggle out and is missing?
You do need to have a key in there, not just the set screw. If you have just a bolt in there the bolt will bend or break off.
Just noticed you said your set screw was missing. They can be bought at a hardware store.
Usually 1/4". But they will need to know if it is course thread or fine tread.
May be able to slide hub and sprocket off the shaft and take it in with you.
See if you have a 1/4"-20 standard (Course) bolt around to use to see if threads match.
If the threads in the set screw hole look closer together than on the 1/4-20 bolt, then it is probably 1/4-28 (Fine) thread.
If the bolt screws in you should not leave that bolt in place as a set screw unless it is a hardened bolt. Set screws need to be hardened. And a little pointed.
On sprockets where the hub is covered, I like the set screws that stick out a bit with a square head on them.
See picture of my large sprocket. Square head bolt is the set screw. 1/4" key is in line with it.
Key and set screw are to keep the sprocket on the shaft and sprocket turning with the shaft. Not like setting the timing on a engine. No marks that have to line up on the 2 sprockets. Sprockets just have to line up with each other when spinning so the chain runs true.
This snowblower blows through chains more than snow I would have never bought the thing and went with something different if I knew, 4000 for equipment that is under designed the chain should be strong enough that the pins break, I've had mine for 4 seasons it works great till it blows the chain. I lube the chain and make sure it's tight and use the right grade shear pins but it doesn't matter. It should have a gearbox Instead of a chain or at least a #50 chain, that way you would just replace shear pins hopefully Massey comes out with a better design or a fix next time around.Im probably going to get #50 sprockets and upgrade the chain after 4 years of frustration I'm over it. If you have one be prepared to replace chains and always have a spare.