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How to measure nut size for a marine shaft

How to measure nut size for a marine shaft

Posted by Chris on Nov 19th 2021

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When we are talking about nuts for propeller shafts, we usually don’t go by the thread size, instead, we list them by the shaft size. This works great when you have a propeller shaft that follows the SAE J755 standard (see chart below) and 99% of the shafting that we see in the US follows this standard. However, anybody with a lathe can make whichever threads they want, so occasionally we will run into something that does not match up to the SAE J755 standard. For instance, a 1” shaft should have ¾-10 threads, but maybe whoever made the shaft was not aware of this and made them to ⅞-9 (which is standard for a 1-1/4” shaft). When this occurs, the first step is to measure the threads and figure out what you have.

  • 1.Take a tape measure, or preferable calipers and measure one inch on one of the peaks of the threads (See Image 1).
  • 2.Count how many threads are in one inch. DO NOT COUNT THE FIRST THREAD. As you can see in image 1, after subtracting the thread where I started measuring, there are nine threads per inch.
  • 3.Measure the outside of the threads (See Image 2). It will usually measure slightly under. The shaft in the image is 1-1/4”, so the SAE J755 standard nut for it is ⅞-9. As you can see the threads are .873” instead of .875”, so they are .002” undersized.
  • 4.Consult the chart below (provided by Hydrasearch) to verify that you have something standard.

If your measurements are matching up to the SAE J755 chart and you still can’t get the nuts on, it is possible that the threads on your shaft are cut too shallow. To remedy this, you will either need to take the shaft into a machine shop and have them deepen the threads, or it’s time for a new shaft.

LEFT HAND THREAD: Left hand threaded nuts are very difficult to find, usually you will have to have made by a machine shop.

METRIC SHAFTS: If your boat was made in Europe or in some Asian countries, there is a decent chance that it has a metric shaft. Even though ISO shafting standards were created 30 or so years ago, they are not always followed. With anything metric, especially on older boats, your best bet is to call a prop shop and see which measurements they need whenever ordering nuts, shafts, or propellers.

If you have any questions, or need assistance, please don’t hesitate to send us an email or give us a call