Joined: Feb 2012
Posts: 19,320
Likes: 5,427
From: Rochester, NY
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
Bosses with a removeable post (often found on Al or Carbon frames, less so on steel ones) have been made with a few different threadings. Generally larger in diameter and with a coarser pitch then the M6 threads that run inside the pivot portion of a canti mount. As to the need for a bottoming tap- I would think it is obvious as to is the hole a through one or with a bottom (as in blind).
Didn't the bases come with their posts? If so then can't you measure the post? Andy
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AndrewRStewart