Kempe, O rings seldom swell significantly unless some obnoxious chemical has been put on them, so I'm not surprised it turned out to be all right under the crud. I'm not sure the wire brush was the best tool for cleaning out O ring grooves, and I'm sure it isn't the right tool when one of the parts is aluminium (as I think the drum drive reduction gear housing is). However it sounds like you have overcome the problem and all should now be well.
