250 Pro XS Lower Unit

[ChuckTR21]

Does anyone recall seeing posts here, or hearing of, bolt failures on Pro XS lower units such that the lower unit shears off while running? I seem to recall seeing posts about such issues in the fairly recent past. I'm almost certian i had a failure along these lines today as I lost my complete lower unit while running around 35mps in water that was between 20-30 feet deep. Was running along just file and motor gave a shudder and immediately came off plane. No impact, no kick-up by motor, just the shudder and then stopped. Trimmed up and lower unit gone. The rear-most bolts are gone and the forward bolt on one side is sheared off clean and the other still has the lock-nut with a small piece of the LU still attached.
Really hard to imagine how this could happen without impact but after thinking about this I seemed to remember some discussion of similar issues so thought I'd ask and see if anyone might remember.

[Savage]

My money is on the LU fasteners were not properly torqued with a torque wrench, probably after a water pump replacement, or some other maintenance. It is virtually impossible for every fastener to fall off at the exact same time.

Depth of water is not a relevant factor, because big logs can float under the surface of the water at any depth.

Check for signs of an impact.

[tomloans]

Wow...! Following.

[bass.man65]

What year is your engine? This will happen if the rear bolt is not there. I have seen 1 that came from Mercury that did not have rear bolt installed from factory. Had 70hrs when lower unit come off.

[ChuckTR21]

What year is your engine? This will happen if the rear bolt is not there. I have seen 1 that came from Mercury that did not have rear bolt installed from factory. Had 70hrs when lower unit come off.

2016; S/N: 2B122173
I'm thinking this has to be something like that as there was simply no impact when this occurred. With all due respect to the floating/suspended log possibility, that would have generated an impact and the typical inertia of the boat being somewhat halted which typically pushes your body forward. There was none of that and no damage to the trim/tilt or hydraulic jackplate. No appearance of damage and they all function just as expected. Hoping to get it to the dealer tomorrow or Tuesday and see what we can find out.

[clayshooter100]

Had the lower unit been removed for service at some point or was it un-touched since the motor came from the factory?

[Savage]

2016; S/N: 2B122173
I'm thinking this has to be something like that as there was simply no impact when this occurred. With all due respect to the floating/suspended log possibility, that would have generated an impact and the typical inertia of the boat being somewhat halted which typically pushes your body forward. There was none of that and no damage to the trim/tilt or hydraulic jackplate. No appearance of damage and they all function just as expected. Hoping to get it to the dealer tomorrow or Tuesday and see what we can find out.

You would be surprised what a impact feels like...….I lost part of a skeg at +70 mph and all we felt was a small bump early in the morning. I didn't see or feel any results of this impact, till the end of the tournament, when we put the boat on the trailer...….part of the skeg was GONE!

[Savage]

What year is your engine? This will happen if the rear bolt is not there. I have seen 1 that came from Mercury that did not have rear bolt installed from factory. Had 70hrs when lower unit come off.

I'm not buying that ONE missing bolt on the rear CAUSED 4 properly torqued nuts (on studs) to lose their preload torque, and ALL fall off on the SAME DAY.

[clayshooter100]

I'm not buying that ONE missing bolt on the rear CAUSED 4 properly torqued nuts (on studs) to lose their preload torque, and ALL fall off on the SAME DAY.

How do you come to the conclusion that the 4 nuts lost their preload torque? The OP clearly states that one of the front studs was sheared off cleanly, and the other front stud and nut was still attached holding on a small chunk of the gearcase. Sure it is possible that the nuts had not been torqued properly after re-installing the lower unit at some point, but I don't think we have enough info yet to jump to conclusions. I have also heard of a missing rear bolt causing a chain reaction of overloading the main studs and nuts causing them to fail together.

[bass.man65]

I'm not buying that ONE missing bolt on the rear CAUSED 4 properly torqued nuts (on studs) to lose their preload torque, and ALL fall off on the SAME DAY.

I guess fatigue never crossed your mind? If your missing the rear bolt running down river crossing waves or in rough water you will get a loaded and unloaded effect on rear of lower unit. where bolt should be were lower unit and exhaust housing meet you will have a gap no gap eventually the aluminum will give and on this one the lower unit went forward into the bottom of the boat. Mercury did agree and replaced all broken parts.

[EuropeanAM]

I'm not buying that ONE missing bolt on the rear CAUSED 4 properly torqued nuts (on studs) to lose their preload torque, and ALL fall off on the SAME DAY.

This one has been proven conclusively a number of times- Loose OR missing hardware can result in inadequate clamp load of the gearcase to the driveshaft housing, eventually resulting in movement of the case, wear of the surface, and then fatigue and failure of castings and/or other fasteners.

Remember- loss of any one fastener is a minimum of 20% of the clamp load lost (and that aft bolt receives some of the heaviest lateral forces in turns).

2016; S/N: 2B122173
I'm thinking this has to be something like that as there was simply no impact when this occurred. With all due respect to the floating/suspended log possibility, that would have generated an impact and the typical inertia of the boat being somewhat halted which typically pushes your body forward. There was none of that and no damage to the trim/tilt or hydraulic jackplate. No appearance of damage and they all function just as expected. Hoping to get it to the dealer tomorrow or Tuesday and see what we can find out.

Hopefully it had been off several times by now... hopefully it was torqued properly.

Do you have pictures (side view often most useful)?

[Savage]

How do you come to the conclusion that the 4 nuts lost their preload torque? The OP clearly states that one of the front studs was sheared off cleanly, and the other front stud and nut was still attached holding on a small chunk of the gearcase. Sure it is possible that the nuts had not been torqued properly after re-installing the lower unit at some point, but I don't think we have enough info yet to jump to conclusions. I have also heard of a missing rear bolt causing a chain reaction of overloading the main studs and nuts causing them to fail together.

Because the lower unit would not have fallen completely off WITH NO IMPACT, if all the fasteners had the correct preload torque...… In my opinion.

A more likely scenario is there probably WAS an impact, or loose fasteners.....The no impact statement however, can not be proven without examining the lower unit...….which is at the bottom of the lake.

OP Your best bet is to call your insurance and see where that leads, because you will get no relief with Mercury!

[lpugh]

I strongly agree with Savage and Don on their points on this matter. A close inspection of the mating surfaces and the holes where the rear studs were will tell the story I think. I believe you will find signs of surface to surface movement on the mating surfaces and the holes for the missing studs will be enlarged a little (wallowed out), probably visible to the eye and can be felt when installing a new stud into the hole. It lost most of its clamp force and hammered around until the rears studs fell out and sheared the front ones off.
Checking those fasteners is part of a normal maintenance procedure

Very Remote possibility of poor machining of stud holes, in which the threads would probably be totally pulled out of housing, Possible but vey unlikely I think. If this were the case there would be a rash of this problem of similar ser numbers and a recall

[ChuckTR21]

This one has been proven conclusively a number of times- Loose OR missing hardware can result in inadequate clamp load of the gearcase to the driveshaft housing, eventually resulting in movement of the case, wear of the surface, and then fatigue and failure of castings and/or other fasteners.

Remember- loss of any one fastener is a minimum of 20% of the clamp load lost (and that aft bolt receives some of the heaviest lateral forces in turns).



Hopefully it had been off several times by now... hopefully it was torqued properly.

Do you have pictures (side view often most useful)?

Don,

Thanks for your reply and input. Photos are attached and I really believe this was a failure of the bolts. Interested to hear your thoughts as I plan to take to dealer tomorrow.



IMG]http://i68.tinypic.com/r1zodv.jpg[/IMG]

[lpugh]

May be the picture, but looks to me the rear bolt backed out and ran that way long enough to cause a chain reaction. Close up of that hole would be helpful and of the starboard frontal looks like it may have been cracked before total failure ( the dark area is of interest )

Service history review might be in order as well as contacting insurance Co. Might be in your best interest to not tell them what you think, let them decide. You don't want to start a blame game.

[Savage]

After seeing the large portion of the front midsection broken off, with the rear section of the mid still intact, my best guess would be that a impact did occur. IMHO.

[Jtreed99]

After seeing the large portion of the front midsection broken off, with the rear section of the mid still intact, my best guess would be that a impact did occur. IMHO.

I had this same thought when it was obvious from the pictures that he needed more than just a lower and some studs. At least it makes for an easy inspection by the insurance adjuster.
Just doing some thinking, I imagine loose hardware with lack of clamping force combined with a stick style motor toter and road vibration could set up some opportunities for metal fatigue.

[tomloans]

I hope this was from an impact... Good grief. The only other explanation would be what Don said quoted below or a combination of both...

"This one has been proven conclusively a number of times- Loose OR missing hardware can result in inadequate clamp load of the gearcase to the driveshaft housing, eventually resulting in movement of the case, wear of the surface, and then fatigue and failure of castings and/or other fasteners.

Remember- loss of any one fastener is a minimum of 20% of the clamp load lost (and that aft bolt receives some of the heaviest lateral forces in turns)."

[JOE54]

respectfully guys I have seen 3 mtrs with this exact break 2 of which I know the rear most bolt was not there (recent gearcase removed ) , I would advise those that have had cases removed to check torque on all fasteners regularly , 45ftlbs on 4 nuts ,55.3 on rear bolt, 39.8 on anode plate ,hope this helps ya JOE

[clayshooter100]

I agree Joe. That failure looks like it started from lack of clamp load at the rear of the gearcase, which then caused the nut and stud next in line to fail (and the gearcase broke on the side where a piece is left). After that only the front two nuts and studs were attached. The extreme over loading and forward pressure on those two studs caused the midsection to break in my opinion.

I don’t believe an impact was likely after seeing those photos.