Otari MX-5050 BIII MR-1L Capstan Motor Bearings Replaced / 629Z Bearings Modified

A problem, bearing noise from the Otari MR-1L capstan motor in my Otari MX-5050 BIII-2. Noise is not too bad, but it won’t get better. Cleaning and repacking with grease helped but still have a slight noise. Yes, the front bushing was oiled properly with Otari capstan oil. It is the rear ball bearings that are noisy. A bearing stethoscope confirms it.

More problems, Otari used custom NSK 629 Z bearings with 629 X marked on inner race indicating custom inner race ID ~ .003” larger than standard bearing. Bigger problem, the last place that had them for $88 each, Precision Motor Works, sold the last two a month ago.

Athan advertises MX-5050 capstan motor rebuild for $385 but they show a BII motor that takes off the shelf bearings. They never answered my inquiry if they had custom bearings for the MR-1L motor and would do the rebuild for $385.

The plan. Get two NSK 629 Z bearings from MRO, $14.77 each. Cobble together a jig to hold the bearings inner races from spinning and open them up .003.” I’ll use my 56-year-old cast iron Rockwell drill press and hope for the best.





I had some 2.25” wide ¼” aluminum I cut into sections ~4 & 5/8" long. I drilled two ¼” holes near each end. I drilled a 3/8” hole in center. I drilled a base plate to allow for the exposed bolt heads so it sits flat on press. I used a clamp to hold the OD of the two bearings together, so races were centered. A drill bit was used to position them in center of jig. A bit of tape around top of drill bit helps center them in the jig. Two shouldered 2” ¼” USS bolts hold it all together very securely.

629 ZZ bearings have metal shields with no rubber seals. We tape them on each side to keep debris out. Worst case if something gets in, clean and repack. The tape also helps the smooth aluminum to hold the inner race.




I used small ¼” Dremel sanding drums. The grit od is ~ .358." I knocked them down a bit with file tapering them so they would go into race bore. If too tight the sanding drum will just spin on the rubber mounting. I ensured I ran the sanding drums through both bearings. I used four to open the ID. I would remove bearings and measure them as I went because I don't have a small dial bore gauge.




Once close to size I followed up with a .354"/9mm 240 grit Silicon Carbide ball hone with some lapping compound to polish and put some crosshatch into inner race ID.



Oh joy...



So far so good. No grit in bearings, tape did its job. Old 629 X bearings at right. Let's hope the enlarged ID isn't lopsided and outer race jams in the bore.



I haven't found any pics of these motors disassembled so these are for Otari folks who may wish to attempt something this crazy.




This is why you need slip fit bearings. You cannot open up ID of stator coil plate to slide bearings through.




Inner bearing installed from rear and retained with large snap ring. Use good snap ring pilers. If you slip and scratch rear housing bore, the bearing OD can hang up. You d not want to have s tuck bearing. Insert capstan motor shaft. A chunk of wood with hole drilled in it secured in vice helps with disassembly and assembly.




The finger style tension washers must go convex to convex. The ID is too large to rest against inner race and they would push on shield of bearing if reversed.



Install rear bearing, pushing down to compress tension washers and attach small snap ring on motor shaft. Again, use good snap ring pliers.



I don't ever want to try this again. I got lucky. Took a few hours. Bearings fit properly and I have a quiet motor!