1989 MC18 HRC crank also has 14mm small ends and won't work with normal NSR pistons only NSR F3 pistons which are impossible to find or you can use Early NF5 RS pistons I think the part # is 13100-NF5-710.
I have the same crank in my MC18 but I use F3 cases as well. I use these pistons in my bike with no problems but they are of course single ring style.