Whats a 16.5 shock?
I assume you are talking to rear shocks. There are no converters between shock sizes but most frames use the same shock size for all frame sizes. Following is an explanation how to measure the shock, the shock is measured by two properties , eye to eye length and stroke length. Eye to eye length is the distance between the middle of the two mounting holes on each side when the shock is fully extended. Stroke length is the length difference between a fully extended shock and a fully compressed shock. Usually there is no difference between shocks on a 16.5 inch frame and the one on a 19.5 inch frame (assuming its the same model of frame) but you need to check the proper shock size recommended by the manufacturer.
