Measuring for a cutlass bearing
Three numbers identify any cutlass bearing: shaft diameter, outside diameter and length. Taking them correctly is ten minutes with a vernier caliper — taking them wrongly is how the wrong bearing arrives at the slipway.
The three measurements
Shaft diameter (= bearing bore)
Measure the shaft itself with a vernier caliper in three places along where the bearing runs, and use the LARGEST reading. Shafts wear where the old bearing sat, so a single reading in the worn zone under-sizes the bearing.
Outside diameter (= housing bore)
Measure the strut or stern tube bore at both ends and average them. This is the bearing's OD — the press fit is already built into the published size. If the two ends differ noticeably, the housing may be worn; tell us.
Length
Match the original bearing's length — standard bearings run four times the shaft diameter (1" × 4", 25 × 100 mm), but never fit a longer bearing than the housing was designed for.
Never measure the old bearing's bore. Worn rubber has grown the bore, and rubber deflects under caliper jaws and reads falsely even when new. The shaft and the housing are the reference surfaces — the bearing is just what fits between them.
The imperial vs metric trap
Imperial bearings step in eighths of an inch; metric bearings step in 5 mm increments. The traps live in the near-misses: 1″ (25.4 mm) is not 25 mm, and 1-1/2″ (38.1 mm) is not 40 mm — half a millimetre is the difference between correct running clearance and a bearing that grabs the shaft or rattles.
As a rule, older Australian, UK and US-built boats run imperial sterngear, while European and Asian builds run metric — but decades of refits mean mixed systems are everywhere, which is why Exalto makes a whole series with metric bores in imperial housings (the MI codes). Measure with a vernier and let the numbers decide; never assume from the boat's origin.
- Caliper reads 25.3–25.5 mm → 1" imperial shaft
- Caliper reads 24.9–25.1 mm → 25 mm metric shaft
- Caliper reads 38.0–38.2 mm → 1-1/2" imperial shaft
- Caliper reads 39.9–40.1 mm → 40 mm metric shaft
Don't add your own allowances
Every published Exalto size already includes the engineering: the bore carries the designed running clearance for the nominal shaft, and the OD carries the correct light interference for a standard (H7) housing bore. Order the nominal size that matches your measurements and the fits take care of themselves.
One nuance worth knowing: phenolic shells absorb a little water and swell, so Exalto machines their bores slightly larger than the equivalent brass bearing — the dry fit you check on the bench tightens to the designed clearance once the bearing has been wet a day or two. It's normal, not a fault.
Measuring FAQs
Can't I just read the size off the old bearing?
If the shell still carries a legible code, that's a great starting point — send it to us and we'll decode it. But don't measure the old bearing's bore: worn rubber has changed size, and rubber deflects under a caliper anyway, giving false readings. The shaft and the housing are the truth — the bearing is just what fits between them.
My measurements fall between standard sizes — what now?
First suspect the units: 25.4 mm is a 1" imperial shaft, not a 25 mm metric one, and 38.1 mm is 1-1/2". If the numbers genuinely sit between standards, the housing may be worn oversize or the shaft turned undersize — both solvable with an Exalto special machined to your dimensions. Send the exact figures and we'll sort it.
Should the new bearing be a tight fit on the shaft?
No — a cutlass bearing runs on designed clearance, a few thousandths of an inch, so water can form its lubricating film. The published bore sizes already include that clearance, just as the OD already includes the press-fit interference for the housing. Order the nominal size matching your shaft and housing measurements; don't add allowances.
What if I can't measure the housing bore with the old bearing in place?
Measure what you can — shaft diameter and bearing length — and read the code off the old shell if visible. Failing that, the housing bore can be measured during removal: most jobs replace like-for-like, so a clear photo of the old bearing's end (showing shell thickness) plus shaft size usually pins the OD down. We'd rather check twice than supply the wrong part.
Measured up? We'll confirm the part.
Send your three measurements — or the code off the old shell, or even a photo with a tape across it — and we'll identify the exact Exalto bearing.
Three measurements identify any cutlass bearing: shaft diameter, outside diameter and length. Send what you have — even a photo of the old bearing — and we'll confirm the right Exalto part.
(03) 5973 6444