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
Size is only half the match. Note the shell of the old bearing, or the material the boatbuilder specified: phenolic, brass and GRP shells all carry a nitrile rubber lining, while all-rubber bearings have no rigid shell and mount differently. If the housing is steel or aluminium, avoid a brass shell; Exalto advises against that pairing.
Also record what the bearing supports. A propeller-shaft bearing and a rudder bearing in a skeg can share dimensions but not an application.
Find your bearing size
With all three dimensions in hand, look them up in the cutlass bearing size chart, which lists every size we carry in its original units. If nothing matches, or you would like a second opinion, 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.
Once you have the right bearing, the replacement guide covers removing the old one and fitting the new one.