This is one of my favorite topics!

To install C-band LNB on the offset dish you have to have LNB-holder properly adopted. On this picture d all needed dimensions for 1.2m and 1.8m Channel Master dishes (yellow digits for 1.2m and blue digits for 1.8m). Some time ago I've designed universal holder which fits to any CM offset dish (0.9m, 1m, 1.2m, 1.8m and 2.4m) and can be used for Ku and C band LNBs, only the clamp has to be changed from 40mm to 63.5mC-m band LNB on the offset dish should have conical feedhorn.