[modeleng] Piloted "D" Bits?

I need to spotface/counterbore a seat for some bolts that cannot be done by 
commercial tools as they are too short to reach.  I have made and used "D" Bits 
before, but I have never heard of a piloted one.  Have any of you gentlemen 
ever made such a tool?  Any advice greatly appreciated.  TIA

Al Messer


      
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