Ashcroft pinion washer question
 

I’m putting in new Ashcroft ring and pinions but they have a M12x30 bolt and washer.

my question is the washer just an off the shelf Home Depot washer I can get or a specific critical thickness washer.
I ask because all I can seem to find is the washer for about $11.00 each and if a .11¢ washer from the local hardware store will work I’m a cheap bastard.
thanks,
dcphotos