How can I measure to see what size main and rod bearings I need, I know to use plasti guage but from there how do I decide
That depends on if they are direct fit bearings on your motor. Then there are punch marks somewhere that tell you what size bearing was used.
Proper way is to mic the journals for size. But in many cases you can still read print on back of old bearings. Originally there would have been (std) printed on back of bearings if they are standard or original size. If crank was turned say ten thousands, bearing would have (.010) printed on back for undersize crank and so on for different sizes.
