main caps are metal straps/brackets on the bottom of the engine where the crankshaft is... and they have either 2 bolts per bracket or 4 bolts per bracket... for Hi-Po use the '4 bolt mains' as they are referred to are considered stronger and more desirable.
some of the guys in the Hi-Po shop could prolly explain it better, with pictures.
I'm sure Wrenchbender Ret. can straighten me out if I'm wrong...
-W
