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How to measure a cutlass bearing

Record the propeller-shaft diameter, housing bore and specified bearing length. Check the vessel specification as well as your measurements before ordering.

Identify the existing bearing

Start with the boatbuilder's drawing, a previous invoice or a part code that is already visible. Avoid removing a serviceable bearing solely to read its code. If the bearing hides the housing bore, use a reliable drawing or ask the yard to measure it when accessible.

Confirm what the bearing supports. A rudder skeg bearing may need a different design from a propeller-shaft cutlass bearing.

1. Measure the shaft

Use suitable measuring equipment on an unworn section of the shaft and check several positions. Compare your readings with the specified nominal shaft diameter. Wear, an unusual dimension or inconsistent readings need investigation.

A 1-inch shaft measures 25.4 mm, not 25 mm. Keep the original units and choose the designated nominal shaft size. Do not use the worn bearing bore as the shaft measurement.

2. Measure the housing bore

Measure the bare bearing seat when accessible. Check both ends and more than one direction for taper or ovality. Keep those readings; averaging them can hide damage or a poor fit. A bearing still in place prevents direct measurement of the bore behind it.

The nominal shaft size identifies the shaft the bearing is made to support. The finished bearing bore is larger to provide running clearance. Manufacturing tolerances describe the permitted range of bearing dimensions; they are not a range of compatible shaft sizes.

For a light interference fit, the bearing's outside diameter is slightly larger than the housing bore before fitting. Use the manufacturer's specified housing fit and clearance. A loose or oval housing needs assessment before you choose a replacement.

3. Confirm the bearing length

Check the specified bearing length and the space available in the housing. A common length-to-shaft ratio is not a substitute for the actual installation specification. All-rubber, flanged and other designs can have different proportions.

4. Check units and tolerances

Exalto supplies metric, imperial and mixed-unit bearings. In the metric/imperial range, shaft diameter and length are metric while the housing diameter is imperial. For example, MIPSF40 has a metric shaft size and an imperial housing size.

Phenolic water absorption and the effect of fitting are accounted for in Exalto's published tolerances. There is no single extra clearance to add across all sizes. Refer to Exalto's tolerances and clearances for the specified bearing.

5. Confirm the material and application

Exalto's standard brass, phenolic and GRP sleeve bearings all have a nitrile rubber lining. The names describe their outer shells. Choose a shell that suits the housing and the vessel's bearing specification.

  • Phenolic: A phenolic outer shell with a grooved nitrile rubber lining. The non-metallic shell avoids galvanic corrosion between the bearing shell and a metal housing. Exalto allows for phenolic water absorption in its manufacturing tolerances.
  • Brass: A brass outer shell with a grooved nitrile rubber lining. Exalto advises against fitting brass directly into steel or aluminium housings. A worn or oversized housing needs assessment; changing shell material does not correct its bore dimensions.
  • GRP: A glass-reinforced composite outer shell with a grooved nitrile rubber lining. Exalto identifies GRP as suitable for steel and aluminium stern tubes as well as compatible non-metallic housings. Follow the specified fit and retention method.
  • Rubber: An all-rubber bearing without a rigid outer shell. This design uses its own mounting arrangement. Choose it when the vessel specification calls for an all-rubber bearing, and confirm the dimensions and retention method before fitting.

See Exalto's shell and lining specifications for the manufacturer's guidance.

Send your measurements

Use our contact page to send the boat model, bearing location, shaft diameter, housing bore, length and any readable code. State the units for every measurement. Matching dimensions narrows the choice; material, retention and the application still need to agree.

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Check the product page for its current price and availability before arranging the work. Prices include GST and shipping within Australia is free.