How to measure a cutlass bearing

Don't try to pull the old bearing just to read it

Cutlass bearings are bonded into the stern tube with a press fit and (often) marine adhesive. Removing one without specialist pulling tools usually damages the bearing beyond repair — and even if you do get it out, the part code stamped on the side is almost always destroyed by the extraction process. Don't pull a bearing just to read a number.

Instead, measure your propeller shaft and stern tube housing directly. If the boat is on the hard with the shaft removed, you can measure the bearing in situ through both ends of the stern tube. The numbers you record become your I/D, O/D and length — match them to a row on our catalog and you have your replacement.

1. Measure the propeller shaft (I/D)

The Inside Diameter of the bearing must match the diameter of your propeller shaft. With callipers:

• Measure the shaft in three different places along its length, well clear of any wear marks at the bearing-contact zone.
• Take the largest reading — that's the working diameter you need to fit.
• If your shaft is metric (e.g. 25 mm, 30 mm), use the Metric or Metric / Imperial filter on our catalog page. If it's imperial (e.g. 1", 2½"), use the Imperial filter.

Don't add clearance yourself. The published Exalto I/D tolerance already builds in the working clearance the bearing needs to rotate freely on a shaft of that nominal size.

2. Measure the stern tube housing (O/D)

The Outside Diameter must match the bore of the stern tube the bearing sits inside. With internal callipers or a bore gauge:

• Measure the inside of the stern tube at the seat where the existing bearing is — or where a new one will sit.
• Take readings at both ends of the seat and use the average.
• If the tube is metric internally, you want a Metric (M_SF) bearing. If it's imperial, you want an Imperial (I_SF) bearing. Boats built for the European market sometimes mix the two — a metric shaft in an imperial-bore stern tube — in which case the Metric / Imperial (MI_SF) range is what you need.

Cutlass bearings rely on a slight interference fit on the O/D to stay in place. Don't try to size the bearing yourself for press-fit clearance — Exalto's published O/D tolerance already accounts for that.

3. Measure the seat length

Length is the easy one. Measure how deep the bearing seat in your stern tube is, end to end. Your replacement bearing's overall length should match — never longer (it won't fit), and never significantly shorter (it'll lose support). The Length column on our catalog shows the full length of every Exalto bearing in the same units as its diameters.

4. Understand the tolerance ranges

Every bearing on this site shows a range for both I/D and O/D, e.g. I/D 2.514 – 2.518". This means the actual bearing you receive will measure somewhere inside that range — that's the manufacturer's quality-control spec.

Your shaft size should fall below the lower end of the I/D range (so the shaft has clearance to rotate freely), and your housing should be above the upper end of the O/D range (so there's interference to hold the bearing in place).

Phenolic swell factor: phenolic bearings swell slightly when submerged. Exalto's published I/D tolerances already account for this — the range given is the dry-fit size; the wet-running clearance will be 0.005" to 0.010" less.

5. Metric, imperial, or mixed?

Exalto bearings come in three measurement systems. The letter prefix on the part code tells you which:

• M_SF (e.g. MPSF50) — Fully metric. I/D and O/D are both in millimetres.
• MI_SF (e.g. MIPSF40) — Metric shaft, imperial housing. Used on boats with a metric propeller shaft fitted into an imperial-bore stern tube (or vice versa).
• I_SF (e.g. IPSF20) — Fully imperial. Both I/D and O/D in inches.

Our catalog has a tab filter so you only see the system that matches your boat.

6. Which material?

Exalto make their cutlass bearings in four materials, all manufactured in the UK. Each has a distinct application:

• Phenolic — the modern default. Low friction, long life in clean saltwater. Excellent vibration damping. Our biggest-selling range.
• Rubber — nitrile rubber with longitudinal water grooves. Best choice for sandy, silty or debris-laden water because the grooves let particles pass through instead of grinding the shaft. Slightly higher friction than phenolic.
• Brass-shelled — phenolic liner bonded inside a naval brass shell. Used where the stern tube housing is slightly oversize or worn and needs the mechanical grip of a metal shell. Heavier and more expensive, but very long-lived.
• GRP (glass-reinforced polyester) — direct bond into fibreglass stern tubes. Common on production fibreglass boats where the stern tube is moulded GRP. Don't use in metal stern tubes.

Still unsure?

Email us your boat model, propeller shaft diameter, and stern tube housing bore. We'll reply within one business day with a specific Exalto part code recommendation. Use the Contact page.