help with my wilson welding finned backing plates

i bought these backing plates and drums second hand. every thing looks good but when you tighten down on the spindle nut the drum rubs the backing plate. do you have to machine the back of the buick drum with these plates like you do with the ford? or do i have something else going on?

 

Since all Buick drums are not exactly the same, it is common to take a little off the inside edge or lip of the drum to clear the backing plate. Not always, but sometimes. Nothing is wrong. Very common.

 

The answer to your question is yes.
this will help you http://www.jalopyjournal.com/forum/showthread.php?t=64647
if not, this will http://www.jalopyjournal.com/forum/showthread.php?t=166339