Prop shaft alignment

This morning I changed the rear diff' end prop U/J to cure some vibration at certain speeds.

As I was always told to, I marked the prop so I could ensure it went back on in the same orientation. Why, if you think about it new props are balanced before they go on the vehicle, if you replaced a prop you couldn't "know" the correct fitting orientation, so why have I been diligently marking every prop I have removed for the last 40 years?

'cos that's how your dad told you to do it, nuff said

You have been doing it wrong, that's why, you only need top mark a prop if you are splitting it, like when changing a uj for instance, you mark the prop and align it back up correctly, you don't need to align it to the vehicle.

nobber said:

You have been doing it wrong, that's why, you only need top mark a prop if you are splitting it, like when changing a uj for instance, you mark the prop and align it back up correctly, you don't need to align it to the vehicle.

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Yes I know about marking a dismantled prop that's obvious and makes sense but if you look on here or any motoring DIY type site they pretty much always say to mark the flanges so the prop goes back on in the same orientation.

I've been working on motors for over 40 years, cars, bikes, 4X4s, tractors, trucks pretty much anything on wheels and when ever this subject has cropped up everyone says the same, I just want to know why? On the face of it, it doesn't make sense, it would seem to have no value, so why does it seem to be such a universally held "good" thing to do.

I only mark if splitting a prop.

The prop fits 180 degrees 'out' on the diff/t-box flange, which makes no difference whatsoever to the balance, so the flanges/UJ's don't really need marking.