Originally Posted by soma2x

cs1,
Not "the opposite". It's a matter of math and depends solely on the brake reach on the 27" wheeled bike. If the 27" bike uses 39mm-49mm short reach brakes, the Tektro R556 should work fine. The Tektros have a reach from 55mm-73mm. Using your own figures, lets assume that the 27" bike used the short reach brakes with pads dropped all the way down to 49mm. If you switch to 650b, you'll need an extra 23mm of reach. What's 23mm plus 49mm? 72mm. The Tektro R556s should have just enough room (up to 73mm). If the 27" bike used brakes with a reach longer than 49mm, then the R556s wouldn't work and you'd need a longer brake, most likely an old centerpull.
soma2x