I'm having trouble bucking the rivets on the skin side of the upper engine mount bracket flange in my RV-8. Does anyone have a suggestion on what kind of bucking bar or technique I should use on these rivets? The shop heads of the previously driven rivets are making it hard to get and keep the bucking bar in position. I drove two rivets (that will be drilled out) already and you can see how bad the shop heads are because of this.