I'll throw in my 2 cents. Under sized cables will not work for 6 volts, as mentioned above, 0 or 00 cable size makes a big difference. Next, remove all cables and clean the contact points down to bare steel. If you use star washers, use them on the top side of the bolts, not between the cable and block.
Go have the battery load checked, have them do it twice with a five minute delay between checks.
Pull the starter apart and check the brushes for proper contact and spring tension.
As just mentioned above, a fresh engine needs drive time to loosen up the rings, so get it out and drive if you can, just don't shut it off till you back at home
Joe
