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Propeller Shaft Seals

There are two back to back shaft seals in the Bearing Carrier in the lower unit (Gear Housing). One seal is meant to keep the water out, the other is meant to keep the oil in. The Bearing Carrier must be removed to access these seals. The seals can be worn out over time and start to leak or, the prop shaft could be scored or damaged at the sealing surface. Also, the seals are often damage by fishing line. There is a large o-ring on the Bearing Carrier which must be replaced when reinstalling the Bearing Carrier. The Lower Unit Seal Kit comes with all the parts needed for separating the lower unit (Gear Case) from the upper unit (Drive Shaft Housing) and removing and installing the Bearing Carrier. There is a separate upper seal kit. The seal kits cost about $60 each.
There are several information pages and videos that apply to the replacement of the Prop Shaft Seals:
Bravo One Outdrive Service
  • Lower Unit Measurements
  • Remove Propeller Shaft
  • Remove Bearing Carrier
  • Propeller Shaft Seals

Torque down the Bearing Carrier Nut
Rolling Resistance of drive shaft only: 
3-5 inch pounds
Rolling Resistance of propeller shaft only (used bearings)
5-8 inch pounds
Overall Pre-load

Rolling resistance of drive shaft only
4 inch pounds
Rolling resistance of propeller shaft only (used bearings)
5-8 inch pounds
Overall Pre-load
9-12 inch pounds
I torque down the bearing carrier nut to get the exact same reading (within this range) I had before I took the unit apart.

My actual readings on this job:


 Rolling resistance of drive shaft only

 Overall Pre-load

 Port Drive

 4 inch pounds

 10 inch pounds

 Starboard Drive

 3 inch pounds

 9 inch pounds


"Hey! Your giving out bad info dude. Some guy told me shims and stuff .... You just slap that baby on there and torque it down"

OK brother. Just hold on a second. Let's take a look.

Mercruiser Service Bulletin No. 93-10 (1993)

A. Bravo Shimming Procedure for the Gear Case Bearing Carrier

Models Affected

All Bravo One Drive Units


The bearing carrier has been changed in length to allow for use of a load ring in place of the shims that are used in the older bravo units to set the bearing carrier preload in the lower gear housing.

This carrier (the new one) is identified with a C-3 that is cast into the carrier assembly directly behind the part number. Where as the old carrier was identified with a C-2 that was cast into the carrier in the same area. (The number is cast onto one of the ribs on the side of the carrier. You might need to remove the carrier to see it).

This carrier (the new one) will back fit in the earlier Bravo One assemblies with the following parts:

Parts Required:

a. (1) 67740 Load Ring
b. (1) 12-42256 Thrust Ring (.105) thick
c. (DO NOT USE) 12-812700 Thrust Ring (.200) Thick (This is the old thrust ring)
d. (1) 41641A6 Bearing Carrier Assembly (This is the new carrier)


When installing the 41641A6 bearing carrier with the load ring (that's the new one) the .105 thick thrust ring must be used. Shims are not required when using the load ring with the new carrier.

First install the load ring into the gear case then install the carrier along with the tab washer and the retainer nut then tighten the retainer nut against the carrier then proceed to tighten the retainer nut until the rolling preload is attained at the prop shaft.

Do Not use the .200 thousands thrust ring when using the load ring ("crush washer") or you will have end play on the prop shaft, also you will have poor thread engagement on the retainer nut to the gear housing.

If you are reusing the old carrier (C-2), thrust ring and shims, torque the retainer nut and MEASURE THE ROLLING RESISTANCE according to spec. You will need a new shim set to change the preload while using the old carrier (C-2). If the hard parts are to be replaced here, you will most likely need the carrier 41641A6 identified with the C-3 and the 12-42256 Thrust Ring and 67740 Load Ring.

Did I explain that OK? Old design uses shims of different thicknesses which you must select and new design uses a load ring ("crush washer"). Either way, you must measure and set the rolling resistance.

If you assemble the unit using the old carrier C-2 and old shim set then measure the rolling resistance and it is within tolerance, you are golden, good to go.

I recommend following the link below and printing out every service bulletin for your Mercruiser. Just go down the list and find the bulletins that apply.

Mercruiser Service Bulletins


Propeller Shaft Seals


Seal Kit for Mercruiser Bravo One
Lower Gear Housing
26-76868A2, GLM 87490


Hinge For Radar Arch
I made up this set of hinges for the little arch.
The Radome, GPS, TV Antenna and Anchor Light are mounted there.
I need to swing this down when towing so I don't snag any power lines on the way home.