working on the Mills High-top slot machine

 the machine was paying-and-playing just fine, but then it started only giving 2 coins when 2 cherries showed.  What was up with that?  

turns out the screw was missing, allowing these plates to slide around and misaligning the payout arms

the screw needed is 10-24 1/4" or 3/8"

the top two plates there don't have threads, and the third plate does, but the bottom (4th level) doesn't have any hole at all, so that's why you see that screw dirty only at the bottom.

The screw popped out again after a few hundred pulls, but turns out I had forgotten to include a lock washer, so got some of these

I also built a spreadsheet that simulates the slot machine being pulled 500 times.

Here are 5 players sitting down at the Mills High-Top with 100 nickels, feeding one in 500 times, and how their purse changes:

After 40 players playing 500 times (or until they lose their money) each pull here is the profit of the house

The players won a total of 701 nickels above and beyond their initial 100 nickel purse.  That is split amongst 8 players.   8 of the 40 players actually had a profit after 500 pulls, and you'll find them celebrating at the bar.  Only 2 of the 40 players earned more than 100 nickels in profit.

But the house won 1956 nickels in profit.  It doesn't pay to gamble, kids.  The house always wins.  No matter how many times I run the simulation, it will always even out to approximately the same kinds of numbers, and variations even out as we expand the trials.