Mercruiser Gear / Shimming Tool Help

Mercruiser Gear / Shimming Tool Help

 
 
By author Trish Dougherty

Mercruiser Gears – Installation and Backlash Help

Gears

For gears to function properly when installed into an outdrive gear housing, the mating teeth of one gear to the other must have the proper arc clearance between them according to manufacture installation specifications and tolerances.  If gear sets are installed with the incorrect backlash, several problems can develop – including overheating of gears, seized bearings and complete failure and of a gear set causing physical damage to the gear teeth. When installed with proper backlash (clearance), a Mercruiser outdrive gear set can function properly for the life of the stern drive; however, with incorrect clearance or no clearance, gear damage and failure can happen fairly quickly. For replacement gears (pricing and ratios), click here.

Mercruiser backlash

To measure the backlash or clearance between two meshing gears, one gear must be held in place by preventing rotation. This is typically done on the pinion gear, and while rotation is limited on the pinion, a measurement is taken on the other gears rational movement at the reference radius. The movement of the arc is converted to degrees (backlash is mostly usually in degrees) with a simple mathematical formula of: (inches of movement) x (57.296/radius of gear).

OEM-43-55778A3

There are several variations of tools that can measure backlash for gears on the Mercruiser, including an OEM version and several aftermarket tools. See the table below for manufacturer specifications for backlash based on 1963 – present models:

Model

Gear Ratio

Forward Gear Backlash

Reverse Gear Backlash

MC-1

14/28

.006” – .008”

.015 – .020”

1A/B/C/120/140/160/165

19/32

.006” – .008”

.006” – .008”

120/140/165/470/485/888/

898/225/228/233/250/260

20/33

.010” – .012”

.040” – .060”

120/140/165/470/485/888

898/225/228/233/250/260

17/28

.040” – .060”

1.02” – 1.52”

MR / Alpha 1 (1983 – 1990)

 

.017” – .028”

.028” – .052”

Alpha 1 Gen 2 (1991 to present)

 

.017” – .028”

.040” – .060”

Bravo 1

 

.012” – 0.15”

 

Bravo 2

 

.012” – 0.15”

 

Bravo 3

N/A

N/A

N/A

 

For 120, 140 and 165 models with the serial number 3780849 and below and 888 models with the serial number 3784374 and below use the table below:

Model

Gear Ratio

Forward Gear Backlash

Reverse Gear Backlash

120/140/160/165

17/28

.040” – .060”

.025” – .030”

888

20/33

.010” – .012”

.025” – .030”

 

Installation Tools

43-853642A2

When installing gears in the upper drive shaft housing of the MC-1, R, MR and Alpha 1 models, there are several tools available for shimming gear sets. We recommend using the GLM Marine drive gear shimming tool (OEM #91-60523, GLM #90170) – this measures the clearance on the pinion gear, and the driven gear backlash tool (OEM #91-60526T, GLM #90180). Both of these tools, along with many others are readily available through GLM distributors such as T.C. Electronics/Marine, located in Toronto, Canada (Order desk 416.751.7326).

 
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Emily H

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