Hi Burton,

you are quite correct in the fact that there is a octolink BB with the name BB-CS20,I am aware of this(although I think this BB had a 73mm shell),there is also is an older BB with the identical name(BB-CS20) which is square taper,this BB has been discontinued for a long time now,mine was found on a 1980's MTB,(although I don't know if this was original)

This one had the fixed & adjustable cups the other way round to normal with the adjustable on the drive side(unlike the later octo' which had them the normal way round),I dont know why Shimano used the same name/code for 2 totally different BB's(perhaps as they had stopped making the former some time before)

Anyway the solution was simple enough,although the CS20 was 122.5mm in spindle length,because of its fatter drive side tapers the crank arm sat further out than it would on a standard BB of the same measurement,so I invested in a 127mm UN53 BB & low & behold a perfect fit & decent chainline,problem solved.