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Loose prop shaft coupler bolt causes


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My 2015 23LSV has 2 of the 4 coupler bolts coming loose after a few running hours. I’ve been told it may be due to broken engine mounts. The boat has the Corvette 450 engine and I bought it used last year. It doesn’t vibrate noticeably and runs great. 
Does anyone have thoughts on this?

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@BrianTD - Is this a persistent issue based on your post, happening to you multiple times as you accumulate hours?  If yes, as noted above a good look at the fasteners (need to be good nyloc or nuts with lock washers), a check of the running gear and an engine alignment are steps to solve your problem.  If just a one time, good chance the fasteners are not properly locking.  That joint takes a lot of vibration and pulsing loads so good locking fasteners are important.  You probably should also check to make sure the output flange is also properly torqued.

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On 9/15/2021 at 6:19 PM, dalt1 said:

Common issue, tighten them up and keep an eye on them.

 

On 9/16/2021 at 8:32 AM, Woodski said:

@BrianTD - Is this a persistent issue based on your post, happening to you multiple times as you accumulate hours?  If yes, as noted above a good look at the fasteners (need to be good nyloc or nuts with lock washers), a check of the running gear and an engine alignment are steps to solve your problem.  If just a one time, good chance the fasteners are not properly locking.  That joint takes a lot of vibration and pulsing loads so good locking fasteners are important.  You probably should also check to make sure the output flange is also properly torqued.

Only 2 of the 4 on right side as facing engine are coming loose. I just tightened once in August and within 10-15 hours of use the same 2 were loose again. I asked the Marina to diagnose. Just picked the boat up yesterday and they diagnosed it as extremely worn engine mount bushings which need to be replaced and $1100 to fix. Mounts not broken just worn bushings. I didn’t have them do the work just winterize. Does this makes sense for a 2015 23LSV with 600 hours and well maintained?

if the engine bolts were loose for a lot of hours could the vibration have destroyed the bushings?

 

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You are noting 2 different sets of fasteners, first post notes coupler bolts, latest infers 2 on the right side but not specific to which ones.

The tendency is that a loose bolt or fastener will be a cause of bushing wear.

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12 minutes ago, oldjeep said:

2 of the 4 on the right side?

Are you talking about bellhousing or transmission bolts? 

My loose bolts are the prop shaft coupler bolts. When I asked marina to diagnose why 2 of 4 are coming loose after 10-15 hours of use they told me I had worn bushings on the engine mounts which need replacement or my prop shaft coupler bolts will keep coming loose. Does this diagnoses make sense to the Malibu Crew?

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Agree with 85 Barefoot.  I have a 2015 23LSV which had a slight vibration which progressively got worse over the previous season.  I assumed it was prop related, as my prop wasn’t perfect.  Fast forward to the later part of the season, where we were out boating and the vibration got significantly worse quickly because 2 of the 4 shaft coupling bolts had sheared.  After this drawn out mishap, which mind you the local Malibu marina could not diagnose, I ended up replacing the prop shaft, dripless seal, coupling, prop, and strut bushings.  Major job that requires special tools and a lot of patience.  Come to find out, the cause of this whole thing was a severely misaligned engine.  So, I definitely agree, get to the bottom of why the hardware is coming loose. 

Edited by BDFD1206
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Thanks! Makes a lot of sense. Spend $1100 for bushings and a proper realignment versus thousands plus downtime if the bolts shear during the season. 

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3 hours ago, BrianTD said:

Thanks! Makes a lot of sense. Spend $1100 for bushings and a proper realignment versus thousands plus downtime if the bolts shear during the season. 

Actually, this makes no sense (to me).   I would not think (nor have I ever observed) that the design would ever want any play whatsoever which could come through bushings.  I could be wrong, and I'll look, but that just seems weird.  

If you have tightened the 2 coupler bolts that have come loose, then just undo all 4 and check the alignment yourself.  Plenty of threads on that, but if you can't find any, I'll explain.  Need a feeler gauge.  .003".  

Edited by 85 Barefoot
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