OMC Parts Drawing *Cobra *Mercruiser *Alpha SEI Outdrives

Mercruiser Alpha One Gen 2 bearings

Tapered, roller and ball bearings for outdrive upper-lower units

Last updated on 10/26/2023

  • Include inside and outside race
  • Complete rebuild kits for all models
  • For upper units and lower gearcases
  • Manufactured by Timken USA

Ball Bearings for Outdrive upper-lower Units

Lower Unit

Description / OEM #


Price (Canadian)

ball bearings

21610 Bearing

P/N 21610 For lower drive shaft




ball bearings

21590 Mercruiser forward

P/N 21590 For forward gear, outside. (1.875 I.D./3.500 O.D.)

(1967 – 1995, S/N 2062141-0F679999)




$ 82.89

11420 ball bearings

11420 Roller bearing

P/N 11420 For forward gear (inside)



$ 24.89

21620 Lower unit ball bearing

21620 Ball bearing

P/N 21620 For reverse gear (1967 – present)



$ 56.89

11410 ball bearings

11410 Roller bearing

P/N 11410 Prop shaft (all years)




ball bearings

21820 Bearing

P/N 21820 Bearing





$ 61.89

Lower Rebuild Kits

All Kits Include Bearings, Seals and Shims


Price (Canadian)

Rebuild kit

25110 Rebuild kit

P/N 25110 Rebuild kit for MC-1 and 1 models, 1972 – 1983



$ 362.89


P/N 25120 Rebuild kit for Alpha 1 models (1984 – 1990)




$ 376.89

ball bearings

25130 Alpha 1 Gen 2 Gearcase seal bearing kit

P/N 25130 Parts kit for Alpha 1 Generation 2 models (1991 – 1995,
S/N 0D469859-0F679999)




$ 316.89

ball bearings, Seal Kits

25140 Gearcase seal bearing kit

P/N 25140 Complete kit for Generation 2 models 1996 – 1997, serial number 0F680000-0K999999




$ 337.89

ball bearings, Seal Kits

25150 Gearcase seal bearing kit

P/N 25150 Lower unit kit for Gen 2 models (1998 and up, S/N 0L100040 and up)



$ 328.89

Upper Gear Case


Price (Canadian)

21510 Tapered roller bearing assembly

P/N 21510 For top cover (1971 – 1995) (1-15/16″ O.D.)



$ 47.89

ball bearings

21511 Tapered roller bearing assembly

P/N 21511 For top cover (1996 and up) (1-25/32″ O.D.)



$ 15.89

ball bearings

21560 Roller bearing

#7 Roller bearing (1.47 Ratio)




ball bearings

21540 Roller bearing

#7 Roller bearing (1.62, 1.81, 1.94, 2.00, 2.40 Ratio)



21520 Mercruiser Roller bearing kit

P/N 21520 Double bearing with spacer. Compatible with all ratios except 1.32, all years up to 1997 (up to serial number 0K999999)



$ 375.89

ball bearings

21522 Bearing kit

P/N 21522 Same as above. For 1998 and up models only, serial number 0L100040 and up. Compatible with all ratios except 1.32.




$ 167.89

Shift Cables

21905 Gimbal

P/N 21905 Heavy duty gimbal bearing. Larger area between the shields allow for more grease filling.

30-862540A3, 30-60794A4



GLM Marine 21520 Bearing perload and gear set up installation

The latest Alpha One or gen. 2 outdrive units, beginning with serial number 0D492656 and above, no longer use the cone spacer between the bearings in the U-joint assembly to set the bearing pre-load and is covered in the following instructions. Also, the 0.D. of the new drive gear hub was increased by .0014 inch, which means that the bearing cones (old or new) now have a slight “interference fit” with the gear. It is important to note that these gear sets and bearing sets will back-fit to drive units that fall below the above listed serial number and that the new bearing pre-load procedure must also be used for those units. In those cases where the 21520 Bearing Set is used in conjunction with the original (slip fit) gear, the cone spacer must be used, and the old bearing pre-load procedure must be followed.


  1. Using an adequate tool, press the bearing cone (positioned as shown in Figure A) onto the pinion gear until it seats fully against the back side of the gear.
  2. Place the bearing cup onto the bearing cone. Note: Discard (Do Not Reuse) the small spacer from the old gear/bearing set. (Figure B)
  3. Place the large spacer onto the bearing cup.
  4. Place the second bearing cup onto the spacer. IMPORTANT: The spacer between the bearing cups must be free to move. Do not over-press the second bearing cone, as damage to one or both of the bearings could occur.
  5. If an over-pressed condition occurs (the spacer does not move freely), completely disassemble the bearings from the gear and start again.
  6. Press the bearing cone (oriented like Figure C) onto the pinion gear until the bearing rollers make light contact with the bearing cup.
  7. Temporarily install a hose damp on the bearing assembly to keep the bearing cups and spacer aligned with each other while accomplishing the next step. (Figure D) Assembling the U-joint I Gear Assembly
  8. Assemble the remaining components to the U-joint, (in the order shown in Figure E),then assemble the gear / bearing assembly, the washer and the nut (add Loctite to nut). Screw down the nut finger tight.
  9. Clamp GLM No. 90100 (U-joint retainer tool) in a vise and place the U-joint into it. (Figure F)
  10. Insert a suitable tool, such as a screwdriver, between the U-joint yolks (as shown in Figure F),to prevent the U-joint from rotating when screwing down the pinion nut.
  11. Screw the pinion nut down until the preload on the bearings starts to go up just slightly. Setting the Preload
  12. While holding the bearings, slowly rotate the pinion nut (Figure G). Check preload by rotating the pinion nut very slowly and while rotating, take a reading of the preload.
  13. If the preload is under the specifications 3 – 6 lbs. in. (0.7-1.1 N-M), torque the pinion nut slightly more as instructed in the previous step and recheck preload as outlined above. Continue this sequence until the proper preload is achieved.
  14. IMPORTANT: If while accomplishing the preceding procedure the preload goes over the specified limit, 6 lb. in. (0.7-1.1 N-M), the bearings must be totally separated from the gear and reassembled following the instructions on this sheet.
  15. Failure to follow these instructions will cause premature failure of the unit.
  16. New Bearings preload 2 – 6 lb. in.
  17. Used Bearings 2 – 4 lb. in.