my catalogs list the CR80 from '86-'92 as taking the same piston, if I remember right they didnt change much on them from the mid '80's to the mid 90's...
if I remember right again I think the first year they liquid cooled them was in '84, but as far as telling you if the '84-'85 had the same piston as the '86, I couldnt say for positive sure...
looking at different part numbers though I see the CR80 takes the same piston from '86 to '95 with the '96 piston being a different part # and the '85 also being a different part #, I couldnt find a part # for the '84 piston though, but I would bet it would be different than the '86-'95...