Learn the right way to check the fit of your propeller to ensure efficient power transfer.

Propellers, whether new or repaired, have to be properly fitted to the shaft to ensure that the taper transfers power effectively. Poor contact can cause the prop shaft to crack or snap, in addition to other issues, all related to the movement allowed when a propeller doesn’t fit well.

The machined tapers conform to an industry standard, making them alike in angle and length. But errors can creep in. A difficult removal can alter the female taper in the propeller hub; also, a replacement prop from a different machine shop will require checking.

Coat the shaft taper with a product called Prussian Blue indicator. Slide the prop onto the shaft and remove it. This will indicate the contact area between the tapered surfaces. If contact is poor — areas that remain blue — abrade the tapers together so their angles match perfectly. Start with a 240-grit lapping compound (applied to the shaft taper) and rotate the propeller left and right in a 30- to 40-degree arc. Rotate the prop 120 degrees on the shaft and continue lapping. Check the hub after cleaning to see whether more material must come off. If so, switch to 120-grit lapping compound to abrade faster. Once full contact is achieved, go back to 240 to remove the 120-grit finish. The abrasive paste I typically use is oil-based, which can be cleaned with diesel, kerosene or a spray lubricant.

When the tapered surfaces are finished,

install the propeller, check the fit without the key and mark the shaft at the forward side of the hub. Remove the propeller and fit the shaft key; it should be snug in the shaft keyway and slide easily into the prop hub, fitting without looseness.

Fit the propeller again and see whether it reaches the mark you made on the shaft. If it doesn’t reach the mark, the key is interfering and will have to be refitted. If a new key has been made, be sure that it has small chamfers on the bottom to match the shaft keyway so it sits all the way in.

The key can slide up the taper if the prop hub keyway grips it, causing a poor fit. Carefully file the key until the propeller fits all the way to the shaft marking, with no interference.

To assemble the hardware, tighten the full nut first to seat the propeller, then remove the nut. Tighten the half nut first, then the full nut against the half nut. Finish the job with the security cotter pin.