Jimmie,
The casting has a tapered hole for a flat-head machine screw if I remember correctly.
Pressure and vibration caused this "wedge" to crack the casting at this point - again trying to remember.
So, any washer would work although a countersunk one that fits the screw would be best.
In reality, I think that a round-head screw and a regular washer should work if it is thick and strong enough to distribute the forces.
The ones I've seen them supply were machined flat and thicker than normal.
Ken,
V-1, 2, & 3
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