According to my el cheapo caliper my old logo track bars measure 26.5 at the knurling. I installed them in a 1R stem and I also get that unsettling space at the back of the bars where the stem holds them. I haven't gotten them to slip once but I've only ridden the bike on the track five times so far but I'm no powerhouse so YMMV. At first I could detect a bit of creaking sound but I tightened the clamp bolt a bit more and that's gone away now. With all the bad press the 1R stem is getting on the list I feel nervous about the whole setup - sort of like a poodle on waxed linoleum. Get 1A stem and send me your 1R so you won't feel tempted to use it. I wouldn't want anything bad to happen to you.
Garth Libre in Miami Fl USA