prop shaft threads

[Dennis Lindauer]

Upon removing the prop on our 28 O'Day, it was discovered that the threads on both the shaft and the nuts are in real bad shape. I'm considering filing down the threads, and retapping using the next smaller die. I'd be going from a 3/4 X 10 to a 5/8 X 11 on the thread sizes, making up for the smaller size by using a washer. Any thoughts from anybody on this?

 

[david lewis]

How bad are the threads? Please be advised that the top portion of an external thread provides almost none of the strength. It is common practice (refer to a machinists handbook or mil-s-7742) to truncate the threads to make tapping a hole easier since the tips of the threads will not actually be contacting the root of the threads on the tap (under a microscope it will look like the tips of the threads are flat instead of pointed as you might expect). This drastically reduces the torque required to tap a hole. On an external thread the tips can also be truncated with only about a 5% loss in strength. Before you do anything try chasing the external thread with a die of the current size. measure the OD with a micrometer, if you dont have access to a micrometer then try a vernier or dial caliper. Even if you have to buy a micrometer it is cheaper than a new shaft. Anyway if the outside diameter of the thread is within spec for the thread size get new nuts and you should be in business. Worst case scenario is a new shaft, if your stuffing box and cutlass bearing are in bad shape you might want to just do the whole deal all at onceas always good luck on your projectdave

 

[Russ Marcks]

I agree

I agree with Dave. The weakest point of that shaft is at the root of the thread. I would be cautious about cutting new and smaller threads. If you have a friend who is a good machinist or an engineer, this is quite a simple calculation to determine the loss in strength of the shaft when cutting it down. I would try to repair the shaft threads and get a new nut.Russ