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Why replace coupler with prop shaft?


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I assume there's a reason, but I haven't found where anyone has explained it. I don't buy that they are a "matched set" - they're machined independently, not fitted together. The shaft has the same torque forces on it at both ends and we don't give a second thought to slapping on a new prop which has the same tapered fit. So why should the coupler be replaced when a new shaft is installed? Seriously would like to know why it's recommended. 

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I have only used the OEM when I have replaced the shafts that I have. They are, I assume trued before they are sold as separate items that are probably just chucked in with a shaft order. BICBW.  I have not been told that the original ones that I have reinstalled have experienced any vibrations or other adverse wear. IIWM, I would use what I had. 

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1 hour ago, wdr said:

I have only used the OEM when I have replaced the shafts that I have. They are, I assume trued before they are sold as separate items that are probably just chucked in with a shaft order. BICBW.  I have not been told that the original ones that I have reinstalled have experienced any vibrations or other adverse wear. IIWM, I would use what I had. 

I was told the manufacturers strongly recommend replacing the two together, and it’s difficult even for vendors to get them to sell a shaft without the coupler. Just trying to understand why?

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New prop shafts usually come with new couplers.  You will likely need to remove the coupler from the new prop shaft to install it, unless you intend to remove the rear seat, v-drive, deck floor, and fuel tank to slide the shaft with the coupler already installed into place from the inside of the boat.

If you have an attachment to your original coupler and you really feel like throwing away the new coupler that comes with the shaft so you can use your old one, I doubt anyone will try to stop you.

Edited by csleaver
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5 hours ago, csleaver said:

New prop shafts usually come with new couplers.  You will likely need to remove the coupler from the new prop shaft to install it, unless you intend to remove the rear seat, v-drive, deck floor, and fuel tank to slide the shaft with the coupler already installed into place from the inside of the boat.

If you have an attachment to your original coupler and you really feel like throwing away the new coupler that comes with the shaft so you can use your old one, I doubt anyone will try to stop you.

Why the sarcasm?? I’m asking WHY the new shafts come with a new coupler? The coupler itself costs about $200 and it doesn’t SEEM necessary, but maybe it is. 
 

If you don’t know the answer, just say so, or don’t reply at all. 

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3 hours ago, jmack said:

Why the sarcasm?? I’m asking WHY the new shafts come with a new coupler? The coupler itself costs about $200 and it doesn’t SEEM necessary, but maybe it is. 
 

If you don’t know the answer, just say so, or don’t reply at all. 

I think that @oldjeep is on to something with the balancing.  It is also possible that they lap the tapers so that you get maximum power transfer with minimum issues.  With today's machining accuracy, things aren't generally lapped any more, but it sure can't hurt. 

I would recommend lapping the prop to the new shaft also, but what do I know?  Slapping your prop on it will probably work, but getting a good mating surface helps ensure that you don't have vibration or a high spot that puts all the torque on the key.

As for the sarcasm, you have asked us to turn on our hypothecators and conjure up explanations for you.  If you really want to know the true answer, contact the company that made the shaft and coupler and sell them as a set (and then report back here with your findings).  I was shocked in 2005 that Ford only sold their power window regulators as a set when it was their crappy plastic coated metal that failed (due to poor manufacturing control).  They refused to sell me the part that I needed to fix the regulators, so I made my own.  Sometimes a company has a real reason to sell everything as a unit, and sometimes they just want to for their convenience.  In the case of a shaft and coupler, I expect that they have a real reason. 

If they told you that you had to lap the new shaft to your old coupler to make it work correctly, would you do it, or just slap it on and expect success?

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34 minutes ago, justgary said:

As for the sarcasm, you have asked us to turn on our hypothecators and conjure up explanations for you.  If you really want to know the true answer, contact the company that made the shaft and coupler and sell them as a set (and then report back here with your findings).  

@justgary I appreciate the response and you make some valid points about the possible reasons why the two parts should be replaced together. 
 

However, I’m not asking anyone to hypothesize about why the coupler should be replaced. I’m asking if anyone knows. I did pose the question to two vendors I spoke with and they couldn’t give me an answer. Manufacturers normally aren’t keen to take calls from consumers with these types of inquiries, so I was asking here, in hopes someone had more knowledge and experience than I do. 
 

I’ve done some reading on Marine Hardware’s site and they claim the two parts are “fitted” as a pair and should be replaced together. Maybe that’s the case in large yachts and industrial applications, but I doubt it applies to our boats. The couplers are anodized after machining, so they go through more processes and likely shipped to another facility after machining is done. If both the shafts and the couplers aren’t stamped with serial numbers, how would they ever match them back up?

Obviously I’m hypothesizing now, which is why I asked the question- hoping someone had some inside knowledge. 

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15 minutes ago, jmack said:

Maybe that’s the case in large yachts and industrial applications, but I doubt it applies to our boats.

I suppose that if a fellow is asking about a shaft and coupler, it may be because his broke or bent.  I understand the bending part, but if it broke, perhaps correct fitting *does* apply to our boats.

By the way, the coupler in my boat is steel, not aluminum, and thus is not anodized.  Any lapping or balancing could easily be done after anodizing, so the probably are processed in bulk until they are mated with a shaft (and a key).  Heck, for that matter, the key itself may be a big part of the "matching."  Any work they do to keep you from having trouble later due to a mis-fit drive train is warranty work they don't have to do (even though mis-fitment shouldn't even be covered, but people will skip steps and then blame trouble on "bad parts").

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If you are looking for a replacement propeller shaft this one will fit the 2015 Malibu 23 LSV, Malibu has at least 20 of them, and it should be available from your Malibu dealer:

3431915, Prop Shaft, 1-1/8 x 44, w/ZF Ski Vee Coupler

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5 hours ago, jmack said:

Why the sarcasm?? I’m asking WHY the new shafts come with a new coupler? The coupler itself costs about $200 and it doesn’t SEEM necessary, but maybe it is. 
 

If you don’t know the answer, just say so, or don’t reply at all. 

You are way over thinking this and may want to have a professional do the repair instead.  That is my honest recommendation, no sarcasm.

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1 hour ago, csleaver said:

If you are looking for a replacement propeller shaft this one will fit the 2015 Malibu 23 LSV, Malibu has at least 20 of them, and it should be available from your Malibu dealer:

3431915, Prop Shaft, 1-1/8 x 44, w/ZF Ski Vee Coupler

Thanks @csleaver , I’ve got one on the way. 

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1 hour ago, csleaver said:

You are way over thinking this and may want to have a professional do the repair instead.  That is my honest recommendation, no sarcasm.

Maybe so- I just like to understand things, and certainly not spend an extra $200 if I don’t need to. 
 

I’ve pulled shafts before to replace cutlass bearings. I’m capable of performing the work. 

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  • 4 weeks later...

Just a quick update - I replaced the coupler, prop shaft, and strut a couple of weeks ago and did the engine alignment. The job wasn't as bad as I thought it might be - I was able to reach the nuts on the strut without removing any hoses or accessories from the engine. The biggest pain was tightening the coupler nut, which required cutting down a socket and welding a piece of flat bar to it as a handle - no socket and ratchet would come close to fitting between the coupler an vdrive flange.

As for the coupler and prop shaft fitment - I'm convinced they're not a fitted pair. The coupler is anodized with no machining / lapping marks in the tapered bore. It's not touched after being anodized and not serialized to keep it matched to a certain shaft. Both ends of the shaft have the same machining pattern on the tapers - no special fitting is evident on the coupler end. 

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