VPI MK-4 platter problem

You should also replace the entire plinth. The Mark 3 and Mark 4 plinths are different than the Jr. On the 3's and 4's you get an acrylic/steel sandwitch that is much heavier and better made than the Jr's. It will naturally allow the Mark 4 bearing cup to fit perfectly as well!

Sounds like you only got half of the required bearing assembly. You obviously need the bearing well from the Mk-IV bearing, which will accept the larger diameter spindle. You should be able to order this directly from VPI, although you may have to buy the complete bearing assembly.
I'd call them instead of e-mailing. Then you can talk to a tech that should know exactly how to resolve your problem, give you prices, etc.

Good Luck!

my mark III platter was not able to interchange with the stock jr well. think you'll need a mk 4 bearing cup. kurt