If it's a base plate see Table 14-2 of the 13th Ed. AISC manual for minimum recommended washer thicknesses to use
with oversized anchor rod holes.
There's a rule of thumb for plate washers. The thickness of the washers be a minimum of one-third the diameter of the anchor rod, and that the length and width of the washers equal the hole diameter plus one inch.
Chapter 9 of Guide to Design Criteria for Bolted and
Riveted Joints by Kulak, FIsher, & Struik - I believe it's a free download from AISC has guidance, which says in part:
"DESIGN RECOMMENDATIONS FOR OVERSIZE AND SLOTTED
HOLES
Hardened washers are to be inserted under both the head and the nut if oversize or
slotted holes are placed in the outside plies of a connection. A490 bolts with
diameters greater than 1 in. should have at least 5/16-in. thickness material under
both the head and the nut in order to bridge over a slotted or oversize hole. (Use of
multiple washers to make up the thickness will not be satisfactory.) If this
additional material is hardened, no washers will be necessary. However, if ordinary
structural steel plate is used, standard hardened washers should be added under
both the nut and bolt head."
I used to align motors and pumps etc 5 to 1000 HP or so.
Even with OEM sized foot holes there was danger of multiple standard washers "coning" and moving the motor back to a prefered (by the washer, not me) position. I'd torque a few bolts with various thickness or stacks of washers to establish a thickness that ensures even temporary coning can not happen.
