A crop harvesting header includes a cutter bar arranged across a forward edge of a header frame structure, a draper or auger transport arrangement for transporting the cut crop for further processing, and a reel with a reel drive mechanism for driving rotation of the reel including a variable speed drive motor and a drive transmission having two different drive ratios for high and low speed reel operation. The transmission is mounted in a housing with a readily removable cover part allowing access to a chain and sprocket drive system where the chain can be loosened to allow it to be transferred from one set of cooperating sprockets to another. The reel shaft is movable to release the chain tension and adjusted by a cam and ratchet arrangement.