Product review (1 stars): Buy this knob if you like wasting money.  Or just do what PUG LUVER did....
I was about to buy (3) of these replacement knobs as ours have cracked with basic use over 3 years.  After reading Pug Luver's review I tried her fix but I used 3/8" heavy flat washers instead because I did not have any 3/8" lock washers on hand.  It was a tight fit but I was able to get the washer on.  It doesn't go on all the way but it's enough to close the crack and provide a tight fit for the knob to fit over the valve stem.  The knobs all work like new...and they were pretty cracked.  I had even tried taking them apart and gluing them at one point in the past but the glue never held.  We'll see how long this fix works.  I should probably have tried reaming out the washer a bit just to get it on a little further but I'm lazy.  I'll keep everyone posting.  THANKS, PUG LUVER.  You made my day!!