Sheared Coupling Bolts

[sailsmanship]

I have a 1997 33' Searay Express with twin mercruiser 7.4 mpi motors straight inboards - I have twice sheared the coupling bolts from the transmission on the port motor- Does ANYBODY know what may be causing this ?

 

[xtraham]

Re: Sheared Coupling Bolts

the engine needs to be line up with the shaft, remove all the coupling bolts and check the clearance around the coupling, it should not be more than .010 gap @ any point adjust the engine mounts to correct it........

 

[Haut Medoc]

Re: Sheared Coupling Bolts

Poor alignment.......JK
Beat me to it, Ham....

 

[Don S]

Re: Sheared Coupling Bolts

I try to keep the alignment to within .005 and shoot for .003
If I had .010, I went searching for tools.

 

[Bt Doctur]

Re: Sheared Coupling Bolts

I`m pretty sure it`s .001 per inch of coupler dia.

 

[sailsmanship]

Re: Sheared Coupling Bolts

Have 4 replacement bolts and I am 8 hours away from destination -- Any temporary fix to help us get home ?

 

[Don S]

Re: Sheared Coupling Bolts

Have 4 replacement bolts and I am 8 hours away from destination -- Any temporary fix to help us get home ?

Yea, don't run that engine.
I have to assume from your posts that this happened all in the same trip, and not over the course of the summer.
Maybe you should give some other information. Did you hit something? Any vibration? When was the last time the alignment was done? Are you using grade 8 fine thread bolts? Were these bolts changed before this trip to SS bolts perhaps?
If the shaft is bent, there is no repair you can do on the water. Same with a damaged prop causing a serious vibration.
Sorry, no easy way out, either fix it right, or it will break them again.
Personally, I would bolt it up with the 4 remaining bolts and not use the engine unless absolutly necessary, as in docking. Keep it at low rpm till you find the problem.

 

[xtraham]

Re: Sheared Coupling Bolts

how long does it take to schear them ? two bolts at a time will buy you some time if you take it easy on the throttle, just snug them up, not extreamly tight, if you still don't make rob two from the other shaft, you may have to limp on one engine

 

[sailsmanship]

Re: Sheared Coupling Bolts

Thanks for all the help. The reason I couldn't give more info was because I was in a remote area trying to relay this post via bad cell phone connections to my son who was posting this for me. This was my buddies charter boat that I was helping take back from Lake Michigan where he runs salmon charters in late summer till fall to Lake Erie where we live and he runs walleye in spring and early summer. He had said that this happened last year about 150 hours ago and he did replace bolts. That is why he kept some more or board. The ones we sheared this time where either Grade 2 or Grade 5 and they were coarse thread. We were in pretty rough seas at the time they sheared and I think that contributed to it.
Make a long story short, we put it the other four, ran in some moderate seas, than checked at fuel stop. Broke one. We had pretty much flat seas the rest of the way and ran at a 3000 rpm and made it back alot later but without breaking another one. (lucked out ). He's pulling boat tomorrow for season and will get it aligned. Thanks again for your help and I didn't mean to bore you guys, but I notice alot of people get advice and never tell how it worked out..

 

[Don S]

Re: Sheared Coupling Bolts

It needs to be in the water for final alignment, not sitting on blocks. The boat hull will sag a little and throw off the adjustment. But I would sure make sure the shaft is straight, the cutlass bearings are good and the prop isn't damaged before doing the final alignment.

 

[rodbolt]

Re: Sheared Coupling Bolts

after breaking a few I would also carefully measure the bolt holes on both flanges for distortion.
use the correct shank and hardness coupler bolts as well. hardware and auto parts stores wont have the correct type usually.
it does not take long to check the shaft with a dial indictor. anythig more than .005" TIR is cause to pull the shaft and find out whats causeing the excessive TIR.
I am with DonS and AGAINST pulling any bolts from the good side.
run on one unless you have to have the other for docking.
I wont settle for anything less than .005" misalignment on the coupler after the hull has been back in the water with a normal load for 8 hours.

 

[Boomyal]

Re: Sheared Coupling Bolts

What is with the 'transmission'? I'm not sure how that fits in to the Merc Picture?

 

[rodbolt]

Re: Sheared Coupling Bolts

boomyal
mercruiser makes a series called MIE, they had a clutch or reverse gear. inboard stuff with only a propshaft exiting the hull.
some were straight drive like the 71 and 72 seriers borg warners,some were down angle like the hurth and some used a V drive of various makes.
the 71 72 series are simple, the down angle a pain the V drive is almost more trouble than its worth to line the coupler flange faces. merc says .003" is max, Ill buy .005" on used equipment.
any more misalignment than .005" will need to have the shaft and coupler disasembled for inspection. I find usually either a bent shaft or a distorted flange,especially at the area the bolts go.
local machineshop in wanchese will chuck them up and indicate off the bore and true up the flange so its pernendicular to the bore centerline.
occasionally we find the strut got whacked out of alignment.
I use a laser pointer in a fixture that fits in the strut and a target that fits in the shaft log and also look to see if the laser dot hits very close to the tranmision output shafts center drill mark.
thats the mean thing about inboard shafting, the centerline of the shaft strut at the prop end must be centered with the shaft log where the shaft passes into the hull which both must be centered to the transmisions output shaft. on new it must be within .003" on used Ill buy as much but no more than :005".
when reassembling the last step is to attach the shaft packing box to the log so the packing box is centered on the shaft and not dragging on any one side. a loose cutlass bearing in the shaft strut can also create problems. the cutlass bearing is simply a rubber bushing with a brass shell pressed into the strut. running them dry at haulouts will destroy them rapidly as will running in very sandy water.

 

[Boomyal]

Re: Sheared Coupling Bolts

Thanks rodbolt. Wasn't aware of these variations.