sometimes you have to flip the entire gear 180 degrees on the hub to get it to work right. i have put gears on before that were brand new and if i didnt put it on the hub with a certain side facing toward the hub, the chain would bind up a little and cause tight spots in it. Or if a gear has been ran for a while on one side and you try to put it back on but put it on the opposite side, it will bind the chain up 95% of the time.
Best thing to do is tell them to leave the nuts a little loose, just barely enough for the gear to move. Then roll the chain a few times to center the gear. Then when the two splits are vertical, meaning both splits are held by the chain snug the gear down. I usually do that when I change a gear anyway.
I would think if you bought the gears, hub and axle from the same manufacture, you could expect the gears to seat perfectly and go out and race.
I use to spend maybe an hour or so with each set to make them mount perfectly. Sometimes the holes need adjusted to make them fit right, sometimes the center needs opened up and sometimes there may be slight manufacturing burs where the two halves meet. If you don't take the time to make them exactly right then you get into the thing where the chain doesn't run perfect. You all know how it can get tighter then loose and it always makes you wonder if your chain is binding or something else is out of round. My experience is it's usually something else is out of round and the thing that's out of round it the gear because of how it's mounted to the hub. A quality half round file and a rat tail file should be required tools in everyone's tool box. They can fix a lot of stuff.
By the way IMHO, if the chain is good, the gears run true and the engine is mounted square to the axle, there's no reason for a chain guard unless your driver hits something or gets hit. My driver never hit anything and ya can't hit what ya can't catch. ...