I think I researched this very question a good while back and never could find a 5/8" hex plug with a gasket seat. However, on many engines they are exchangeable. The threads are the same, it is only the seat you are concerned with. Look closely at the cylinder head and see if there is a chamfer on the plug seat. If so, the 5/8" plug will work. I have been known to machine the chamfer myself to make the 5/8" plug work, but I can't recommend that; don't want someone ruining an expensive cylinder for the price of a new plug cap.

AV8TOR