IF YOU SEARCH THROUGH dunkenb POSTS YOU WILL FIND A THREAD ABOUT FITTING A NEW MC21 CRANK INTO A R5K MC 18 BOTTOM END. SO FAR IT WAS A SUCCESS. MY BIGGEST ISSUE WITH MY 18 IS IF AND WHEN THE CRANK GOES. I DON'T WORRY ABOUT IT NOW, THERE WILL BE A SOLUTION OUT THERE WHEN THAT TIME COMES ALONG.
GOOD LUCK AND ENJOY YOUR 18.