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Old 14 August 2008, 12:22   #1
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Tighting the nut on a prop

Just wondering does the nut that hold on the prop have to be very tight. If I twist the nut very tight then I cannot get the split pin in.

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Old 14 August 2008, 12:35   #2
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Just wondering does the nut that hold on the prop have to be very tight. If I twist the nut very tight then I cannot get the split pin in.



TSM


I just tighten mine up and then back it off to get the pin to line up, I do it with the engine out of gear so I only tighten to the point where I can no longer hold the prop still, this seems to be fine for mine.
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Old 14 August 2008, 12:43   #3
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Thanks chris

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Old 14 August 2008, 13:00   #4
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To do it properly you should tighten the nut to the recommended torque. If it then needs to be turned to fit the split pin, you have a few options. Over tighten it, under tighten it, shim it using thin washer(s) or remove some material from the face of the nut. If you don't have machining facilities and you choose the latter method, you can do a good, even job of facing the nut by holding it firmly and rubbing its face on a piece of course abrasive paper, about 60 to 120(ish) grit will be ok, supported on a flat surface. Move the nut in small circles for a while then change your grip to rotate the nut a couple of flats and repeat this procedure a few times until satisfactory.
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Old 14 August 2008, 13:04   #5
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not particularly tight! IIRC mine is speced at something like 30Nm for the prop nut. in reality I don't have a torque wrench that goes that low, and I'm not sure how you torque it correctly and line up the pin with the castellated nut. More important to get the pin in (without overtightening so you can never get it off again) than to get the nut tight IMHO,
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Old 14 August 2008, 15:24   #6
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Quote:
Originally Posted by Chris View Post
I just tighten mine up and then back it off to get the pin to line up, I do it with the engine out of gear so I only tighten to the point where I can no longer hold the prop still, this seems to be fine for mine.
Been doing it this way too for 30 plus years with zero problems from 5hp to 300hp outboards.
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Old 15 August 2008, 09:25   #7
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When I got my current engine there was almost 1/8" travel of the prop up & down the shaft because of a homemade washer between nut and prop. On replacing said homemade bit with a pukka Yam splined washer (and therefore doubling the value of the engine) I found everything lined up nicely, and it was fitted as per Chris' description.

Bottom line is, the prop is sitting on a splined shaft, so the rotation force is taken care of. When in forward (when the real work is being done) the thrust washer is doing all the work of transferring the force created by the prop to the engine leg & on to the hull. IF it's loose, all that means is that when you engage reverse, the prop slides down the shaft until it hits the back washer & vice versa when you engage FWD. May cause something to fail prematurely if you do a LOT of manoevering or use reverse as a brake. The nut won't rattle off because of the split pin.

I'll have to second Chris & Rigaud on this one.
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Old 15 August 2008, 10:03   #8
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I'm astonished!

The whole of your engine power is being applied through the connection between the prop and it's shaft, is it not worth getting its fitment correct? Also, the propeller is centered by being seated firmly onto the taper of the shaft.

I realise the description I gave of facing the nut may seem awkward but it only takes a few minutes and then it'll be correct while you use the same prop. To put it in perspective; if the pitch of the thread is about 1mm, which it won't be far from, and the nut is wholly out of alignment, 1/12th of a turn, then only 1/12th of a mm needs to be removed from the face of the nut to make it correct. For those who still can't use millimeters that's about 4thou. A human hair is typically 3-5thou thick

Isn't it worth 5 minute's work to know you've got it correct?
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Old 15 August 2008, 10:51   #9
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Yeah, but the nut plays absolutely no part in transmitting the forward thrust, and the splines take care of the rotation and concentricity of the prop relative to the shaft.

Like I say, mine had 1/8" play along the shaft when I got it. I replaced the dodgy washer 'tween nut & prop, and noticed absolutely no difference in vibration or handling when I fitted it properly, other than it now doesn't now go "klunk" when I engage reverse as the prop slides back along the shaft.

I agree a sliding prop is far from ideal, but as the nut has no part in the forward motion, 1/12 turn back will not be significant. I bet the prop will shift 4 thou or so forward when you're at WOT anyway as everything compresses.


Tapered shaft? Is that a volvo thing? I have only ever encountered splines (or shear pins on small engines) and thrust washers. Theory is the shaft just transmits the rotation, the gearcase takes the forward thrust so the gears don't end up being thrust in or out of mesh as the shaft pushes it's way forward.
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Old 15 August 2008, 12:10   #10
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So you don't reckon a loose prop will be subject to various driving and water induced loads which will cause fretting of the spline surfaces and the seating taper and any knocking or impact load caused by the moving propeller will bear on the bearings and their seatings within the gearcase causing consequent wear and out of balance forces?

If you've not noticed a taper on your propeller shaft, you could do with visiting Specsavers.

The propeller blades produce the diving force and accept other external forces which are transmitted to the hub through the rubber damper within the prop. The prop hub then transmits the total of these loads to the propeller shaft. In forward drive, most of this is a forward propulsion force which is passed to the propeller shaft via the taper at the front of the propeller or its thrust washer. The forward bearing within the gearcase carries this force and transmits it into the gearcase.
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