Probably a bit late but having just fitted some the following might be useful for someone. I've found some bars have threaded plugs about a quarter of an inch in. These are usually meant for weights but serve equally well for mirrors. Where this plug isn't fitted the mirrors come with either a cage that crushes/expands as the bolt is screwed in, or a tapered plug that works against a coiled shim rawl plug styles. Either way they work but you might only get one easy shot to position them right. If you have to take them off the expanding plug can push inside. Either way just buy any that fits the bar diameter.
Andy
