I hate to say this, but the missing button may be the last thing you need to worry about. If it's broke off inside, you'll either have to take the spindle out, or if you get lucky like I did, I was able to drill a 1/8" hole thru the step pulley and knock out the broken piece with a rod. Either way, the button just doesn't pop in because the diameter of the knurled part won't pass without taking off the front spindle bearing cap. And make sure you read the directions on how to do it on the inside cover of the pulley guard so you don't damage the brass bearing spreader. Look for a parts list for your lathe (I'm sure there's one here somewhere) so you can see what's involved. There's also a weird spring that holds that button in and out tht is part of the problem if it is broken inside. There's a guy on Ebay that has a toolmaker working for him that made a few for his machine repair shop that I bought mine from. Like I said luckily I was able to do the repair without much trouble.