This adapter sleeve reduces the opening of your Rockler Pro Lift router lift (#52429 and #55803, sold separately), allowing you to mount a smaller 3-1/2'' diameter motor. If you are unsure of your router motor's diameter, check the manual, or measure the circumference with a tailor's tape. Measuring the circumference is easier and more accurate than measuring the diameter. The reducer has an expansion gap, so it will fit motors measuring up to 1/8'' larger or smaller in diameter than the listed spec. Accepts motors with 3-1/2'' diameter (11'' circumference), including: Bosch 1617Porter Cable 690 seriesPorter Cable 890 seriesDewalt 616 and 618Skil RT1322-00Others