Tried cheaping out & bought a 105 series 52 for my Ultegra frontend as all my investigations' online seemed that all 3 Shimano chainrings are interchangeable (& the 105 is half the price of the Ultegra). Yes the bolts line up & the cog works as a cog, although doesn't shift quite as well. Cosmetically though Shimano rather cleverly uses slightly different colors but most importantly, the 4 spiders do not sit flush to the crank, they step down (on 105) not on the Ultegra. Looks very crappy. Bottom line, spend the extra $50 fo the correct Ultegra cog - looks right, color matches, shifting seems better. I now have a 52 tooth 105 chainring sitting around!
Needed a shorter crank length to speed recovery from a bi-lateral knee replacement. This product is excellent in performance and price point. The shorter crank length enabled the rider to do full rotations sooner, and more comfortably. In addition moved from flat pedals to cleats much sooner than anticipated with 2 new knees.
Back to full training, for our 6-8 week adventures criss-crossing Europe.