Tensioning the chain that drives the center drum on a MacDon CA20 or CA25 header adapter leaveas a lot to be desired. Tightening is just by holding the idler with a prybar and then tightening a bolt in a slot. However, once you reverse the feederhouse, the force on the chain inevidably drags the idler down in its slot, loosening the chain. After that it gets progressivly looser every time you have to reverse.
My brother designed a simple keeper plate to remedy this. This keeper plate replaces the washer under the nut, and has a spiral track in it. This keeps the idler locked wherever you set the idler tension. It's made out of 1/8" plate steel with a short piece of rod acting as a sub welded to it. We cut out the spiral with a CNC plasma table, but it is simple enough to cut by hand.
Here's a image from the parts breakdown that shows where the keeper is fitted.
The keeper is a pretty simple affair, made out of 1/8" steel plate, with a bit of a stub welded in that pushes down in the slot and keeps everything where it should be:
A PDF template you can print out and use to mark the steel to cut is here. Also available is a 2D CAD dxf file.
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