Sure-Lock Knobs To solve this problem with the TCF-Leo, Optec engineers designed a low profile replacement lock knob to replace the mirror support clutch covers. Made from solid 360 brass, the Sure-Lock knob screws directly onto the mirror support clutch in place of the taller black conical lock nuts. Featuring a large diameter knurled design, Celestron users will find the Sure-Lock replacement knobs fit tighter and hold the primary mirror position even more securely than the original equipment Mirror Support Clutches. Sold in sets of two, Optec’s Sure-Lock knobs are easily installed without tools. When removing the original black lock knobs, leave the grease in place on the captivated aluminum plunger knob that makes contact with the actual steel shaft. The conical shape inside each Sure-Lock knob will drive the plunger tightly against the shaft when tightened locking the mirror in place. A Word of Caution Note that the small stainless shafts will extend beyond the top of the Tru-Lock knobs. It is imperative the user exercise caution when the mirror is moved all the way back. During normal use at infinity focus, the mirror position is far enough forward to present only a short section of the stainless shaft. Be careful to avoid making contact with the shaft.