More on engine coupler

[Constans5555]

On a 1984 Mark Twain 25' cabin cruiser with a 350 and a 260 mercruiser outdrive, it had been recomended that I use a 15.25x15 and it states on the engine that I need to be running between 4200 and 4600 RPMs and with a 15.25x15 I am only getting 4100 RPMs, which Michigan Wheel recommended.The reason I am writing you is to see if you have any ideas on the problem I am having with my engine coupling failing.This will be the third coupling I have had replaced this year. The mechanic I am dealing with, states that with him aligning the shaft perfectly the only two reasons a coupling fails is if there is an impact to throw the shaft out of alignment or a faulty coupling and there has been no impact and it is hard to believe that I have been through three faulty couplings, anyway after having the second coupling replaced I took the boat out about a mile or so and reached back in to feel the shaft and outer part of the coupling and discovered that it was generating a huge quantity of heat, enough that you could barely touch either one, I had contacted the mechanic about my findings in which he contacted mercruiser about their coupling generating heat in which they had stated to him that the heat should not cause any problems just to make sure that the alignment is perfect in which he checked again and it was perfectly aligned. My mechanic suggests that since I am not reaching the RPMs that is recomended with a 15.25X15 that I am putting additional strain on the engine which in turn could be the cause of the coupling failing. Just wondering if you could give many any suggestions on this matter. My mechanic has talked with his prop man and suggests that I switch to a 16x13 to reach the RPMs that are recomended.Iam definitely going to have my mechanic double check for any soft spots where the motor mounts, but I don't see that being the problem since the engine has been pulled 3 times and I'm sure that that possibility has been looked into. Just wondering if there could be any other possibilties of what would be causing the coupler to fail?Thanks for all your help.

 

[Constans5555]

Re: More on engine coupler

Oh yea I had forgot to add that the gimal bearing is also new and everything seems to be in sound condition on the outdrive.

 

[Don S]

Re: More on engine coupler

The 4100 rpms indicate you are overpropped. This puts excess strain on your engine. You need to go with something like a 15x14 or a 15.25x14 to gain some more rpm.<br />Any manufacture can put you "In the ballpark" with diameter and size, but it's up to the individual to make sure he gets the proper rpm. It would be impossible for any prop manufacturer to state an exact size. It's always a trial and error thing.

 

[Constans5555]

Re: More on engine coupler

Thanks Don for the info on the prop but can you tell me if you have ever heard of it putting enough strain on the engine that it would throw out the engine coupler almost instantly after installing new ones 2 times in a row, also I would not think that I would be putting to much strain on the engine when I'm running right at the minimum RPMs which is 4200 at max throttle, but that is why I'm writing you guys cause I don't have a clue. Also what do you think about the coupler and shaft generating heat enough to where you cannot touch either one? Any ideas are much appreciated

 

[Don S]

Re: More on engine coupler

That little bit of extra strain shouldn't cause the coupler to go out. I would sure have them put in new rear mounts on the next coupler change, no mater what they looked like. Did they ever change the splined shaft that goes into the coupler? If it is too worn out it can cause a lot of extra wear on the coupling.

 

[tryan]

Re: More on engine coupler

look at the mode of failure on your coupler. if the rubber is failing, you have a high torsional load somewhere. if the spline insert is getting munched, something is moving around or worn. is the right shaft being used? there are two sizes (edit.. lengths) of black anchor yokes.<br /><br />you might use spline grease on the shaft also.<br /><br />is the transom solid?

 

[Constans5555]

Re: More on engine coupler

Thanks Don I have also heard about if the spline is worn it could cause problems and I will be sure to have them check it out,the answer is no they did not replace the spline, and I will also look into them changing out mounts, does this involve a lot of time and money?

 

[Constans5555]

Re: More on engine coupler

Thanks Tom for your input, and the failure to the coupler the second time was at the rubber on the inside of it where the spline goes in, the teeth in the coupler look in good condition, but when he pulls the outdrive I will make sure he checks the size and condition of the shaft and from what I've been told by both mechanics that have worked on the boat have stated that the transom seems to be pretty solid, but is there a special way to make sure that this is the case?

 

[Don S]

Re: More on engine coupler

does this involve a lot of time and money?

Nope<br />The mounts can be changed in about a half hour or so while the bell housing is off to change a coupler.<br />The splined shaft is only held on by the ujoint and doesn't take more than about 15 minutes to change once the drive is off.<br />So on top of the labor of changing the coupler, add about an hour for the other two items.<br />The mounts are about $50 for the pair and the shaft is around $100.

 

[Constans5555]

Re: More on engine coupler

Thanks Don I appreciate your time and knowledge.But while I am on here what do you think about the heat that was being generated back at the coupler and shaft, mercruiser supposedly said that it is ok but I think it it is odd that it gets so hot that you cannot touch either one of them. Thanks again

 

[Constans5555]

Re: More on engine coupler

Hey Tom Ryan just wondering about what you said on the high torsional load, can you explain to me what you mean and where this can be coming from.Thanks for your time and Knowledge.

 

[Constans5555]

Re: More on engine coupler

Hey Don just thought of something my dad had mentioned about a possibility that I may not have the proper stern drive for this engine and how can I tell that it is the proper stern drive for this engine.My father had experienced a problem with jasper sending them the wrong replacement stern drive due to wrong gearing in the top end, and could this be a possiblity in the problem I'm having with the coupler. Thanks again for your time.

 

[Constans5555]

Re: More on engine coupler

Hey Don just wondering about the diagram you sent my dad(meemaw777) in which it shows the rear motor mounts and was wondering what those mont to.

 

[Don S]

Re: More on engine coupler

If the mounts are bad, or the splined shaft is bad, it will create too much heat and it will destroy couplings. Are they putting plenty of spline grease on the splines when they put the drive on????<br />The mounts sit on the transom shield. Just go look at the back of the engine and you will see. And seriously if you are going to be working on these boats, get yourself the Mercruiser service manuals for the engine and drive.

 

[Capt Ken]

Re: More on engine coupler

I can't believe that no one has hit upon the reason for this problem. Bet if you check, you will find your transom getting soft and is flexing under load. This throws the alignment off and causes couplier failure. The reason the rubbber is getting hot is because its compressing on one side and stretching on the other. Common on a boat that is 20 years old. Raise the outdrive and have someone watch the transom as you put your weight on the drive. Should not be any flex in the transom.

 

[Don S]

Re: More on engine coupler

Yea Ken, it was brought up in his dads post. It's hard to keep track of problems over several different posts by different people on the same boat.

 

[Constans5555]

Re: More on engine coupler

Thanks Capt Ken I just got back from checking out the transom and it does flex when weight is applied so I guess now is my question, what are my options on fixing the weak transom. Thanks again

 

[tryan]

Re: More on engine coupler

hey transam, you might have to change your screen name after this. the rotdoctor.com has some secret stuff to fix the flex in the transom, but the only true repair is to cut it out FROM THE INSIDE. <br /><br />OR...........a nice stainless steel plate across the back would keep people from tailgating with their high beams on.<br /><br />when you have the coupler off this time, stick the existing shaft in. it should be tight. <br /><br />merc changed the shaft length about 1/2 inch which may or may not be the root cause of your problem. you would have to look at the wear pattern.<br /><br />as far as the torsional load statement, you need to find out what the numerical value of reduction your drive is and figure out what prop you need from there. as don stated about your being over propped, old boats, i have a few, tend to take on water and thus weight. i had a 16' i/o that gained 300# or so every summer.

 

[Constans5555]

Re: More on engine coupler

Thanks Tom !

 

[Constans5555]

Re: More on engine coupler

Thanks