Connecting rod bolts coming unscrewed.
Connecting rod bolts coming unscrewed.
(OP)
Ok guys I am going to post this here, because I do not do this stuff for a living.
I am chief of maintenance for a sailing club ( Volunteer position.) and whilst my main areas of expertise are repair and maintenance of wooden and fiberglass boats. Every once in a while I get mixed up with outboard motors.
Now to the question. I have a Propane powered outboard that just died on me at 447 hours. I had two logbook entries on the motor by the members, " motor misfires at high RPM. " followed a couple of days later by " Motor seized one minute after start." I have 4 of these Propane powered outboards on the boats and so far they have been trouble free.
This one when I stripped it down was a bit low on oil but still had oil on the dipstick. The connecting rod had separated at the big end, the bearing cap and both of the bolts were laying in the oil pan. The bearing faces looked perfect and the crankshaft has no dents or nicks on it. Inspection of the connecting rod revealed no signs that the screws had been ripped out, in fact it looks like they had both dropped out at the same time.
Apart from the fact that the connecting rod had smacked the crank case and dented one of the alignment flanges on the bearing half, it looked like I could just bolt it together and the motor would go another 1000 hours.
The bearing shell cap screws have no retainers and it would appear that they just rely on the torque of the screws to keep them in, or maybe some kind of thread retainer like Loctite. My suspicion is that they were never torqued down properly in the first place, however that is beyond my expertise.
I have seen connecting rod failures in the past, but most of them have involved one bolt slacking off and the other bolt bending as the assembly fails.
Any ideas on this kind of small engine failure ?
B.E.
I am chief of maintenance for a sailing club ( Volunteer position.) and whilst my main areas of expertise are repair and maintenance of wooden and fiberglass boats. Every once in a while I get mixed up with outboard motors.
Now to the question. I have a Propane powered outboard that just died on me at 447 hours. I had two logbook entries on the motor by the members, " motor misfires at high RPM. " followed a couple of days later by " Motor seized one minute after start." I have 4 of these Propane powered outboards on the boats and so far they have been trouble free.
This one when I stripped it down was a bit low on oil but still had oil on the dipstick. The connecting rod had separated at the big end, the bearing cap and both of the bolts were laying in the oil pan. The bearing faces looked perfect and the crankshaft has no dents or nicks on it. Inspection of the connecting rod revealed no signs that the screws had been ripped out, in fact it looks like they had both dropped out at the same time.
Apart from the fact that the connecting rod had smacked the crank case and dented one of the alignment flanges on the bearing half, it looked like I could just bolt it together and the motor would go another 1000 hours.
The bearing shell cap screws have no retainers and it would appear that they just rely on the torque of the screws to keep them in, or maybe some kind of thread retainer like Loctite. My suspicion is that they were never torqued down properly in the first place, however that is beyond my expertise.
I have seen connecting rod failures in the past, but most of them have involved one bolt slacking off and the other bolt bending as the assembly fails.
Any ideas on this kind of small engine failure ?
B.E.
You are judged not by what you know, but by what you can do.





RE: Connecting rod bolts coming unscrewed.
I've seen cases where an 'Oh My Gawd' failure was missed on first quick examination. Just wild speculation of course. Good luck.
RE: Connecting rod bolts coming unscrewed.
I will dig it out of the box and take a closer look at it , I ordered a new connecting rod assembly from the factory, but I am curious as to what caused this.
B.E.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
Any chance of a few pictures of the bolts and failed parts?
If we can see the parts it might get better response.
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Connecting rod bolts coming unscrewed.
B.E.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Connecting rod bolts coming unscrewed.
B.E.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
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RE: Connecting rod bolts coming unscrewed.
B.E.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
B.E.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
PS: There's an inline picture insertion feature now. The picture plus button near the 'link' button. It allows pictures to be placed directly in the thread. E.g.
RE: Connecting rod bolts coming unscrewed.
I did not know about that feature, thank you.
So what do you think? Do you think it was run without oil, then oil added after the failure started?
B.E.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
It's still surprising that it didn't so the expected damage when one side let go before the other. But flukes happen, rarely.
RE: Connecting rod bolts coming unscrewed.
Thanks for the pictures!
Well it's hard to see exactly what might of happened but on the 4th picture the one of the bolt, that bolt looks slightly bent to me unless it's an optical illusion.
I was thinking about what you thought the failure mode was and some comments and thoughts on that:-
How long had this engine been in service prior to any log book entries? Whilst it does appear that the bolts loosened over time I'm struggling to understand how maybe two bolts weren't tightened correctly and how both dropped out simultaneously, do you know what torque the bolts were supposedly tightened to originally ?
Anyway I'll think about it some more!
Desertfox
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Connecting rod bolts coming unscrewed.
Is there any wear on the inner edge of the rod or the cap? If it was loose for 'a while' I expect you would see some wear (although I've never looked inside an engine after throwing a rod...); no wear indicates the cap popped off quickly. The bolt doesn't show any wear from the cap sliding up and down, so that again leads me to believe it was a fast failure.
This is a single cylinder engine, right?
Z
RE: Connecting rod bolts coming unscrewed.
Just to check, in the picture that VE1BLL kindly inserted for me I had torqued them to 25 NM to make sure they were not stripped. The one bolt had aluminum plating on the barrel of the bolt , indicating that the bearing cap had been sliding up and down on it. Both bolts were straight ( Checked by rolling on a surface plate.) the other bolt was clean. The bearing cap mating surfaces showed some signs of fretting wear from hammering together, the rotating surfaces showed signs of scuffing indicating that they had run without oil. The threads in the connecting rod , one was clean, the other had about a half thread torn out of the start.
The engines have hour meters fitted , I just went out and read it, it reads 438.0 hours. I would normally expect to see over 1000 hours before any problems show up.
So what this boils down to is : Did the factory under torque the bolts, or did my minions let it get low on oil, and did they add more oil when they heard it knocking without telling me? Then the next question would be , how long after that did it last? It should also be understood that I have more motors of this same make and model with well over 1000 hours on them.
B.E.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
Okay if the failure is due to heating I would expect a scenario something like this:- the aluminium alloy expands more rapidly under heat than the steel bolts which means the bolts would see increased tension and the big end components increased compressive load, now if the original bolt preload was insufficient, then the aluminium alloy components would separate and all the external load would be taken on the bolts themselves and this may have caused the bolts to go beyond their yield stress and taken a permanent set, if this occurred then the bolts would either provide very little preload when the engine cooled or none at all and hence the bolts would be loose but a well designed joint would normally have the bolts fail before damage would occur to the internal threads.
It would be interesting to measure the bolts or look at the threads under magnification to see whether they did in fact yield or not.
One thing you could check is whether the aluminium alloy on the underside of the bearing end cap has imprints of the bolt head.
Of course I have no knowledge of the heat transfer around your engines so it might be that the components got hot enough to reduce their strength during running and failed in a manner due to this.
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Connecting rod bolts coming unscrewed.
One thing you could check is whether the aluminium alloy on the underside of the bearing end cap has imprints of the bolt head.
Well I took the bolts out of the bearing cap and examined the area under the head, because of the integral washer on the bolt head it was hard to determine if there was any plastic deformation there. The one thing I did notice was that the clamping surfaces under the bolt heads on the aluminium part were polished. I am presuming this was from the aluminium striking the underside of the bolt heads while the engine was running with loose cap bolts. I feel that the correct scenario is what you mentioned. The connecting rod overheated from running with insufficient oil. The metal expanded, either compressing the aluminum of the bearing cap , or stretching the grade 10.9 steel bolts, causing the bolts to lose their preload. at which point they started backing out. The engine continued to run with the loose bearing cap, until the bolts had worked their way completely loose. Heat transfer in the engine is that the crank is oil cooled with splash lubrication, the cylinder head and barrel are water cooled.
The logbook said " Engine seized one minute after start.", this would be when the boat would be preparing to leave the dock and the engine would be at idle, and warming up. It is fortunate that the failure occurred then. If it had occurred at full throttle , the rod would most likely have come through the case.
B.E.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
I just got the new parts from china and the instructions for tightening the cap bolts 7mm dia , There are no safety devises of any sort on these bolts. The tighten instructions say " All bolts shall be clean and dry, pre torque to 8 nm then torque to 12nm ."
I think
I am going to throw some thread locker on before tightening in view of what happened last time. Good or Bad idea?
B.E.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
Thread locker will alter the tightening torque.
I will post more shortly.
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Connecting rod bolts coming unscrewed.
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Connecting rod bolts coming unscrewed.
Are you sure about the 7mm dia? that's not a preferred size.
I would be worried about anything I received from china, are the cap screws 10.9 does it have marking on the head of the cap screws?
If they are grade 10.9 then the bolts are only stressed to a quarter of there yield stress, however the internal aluminium threads which the bolts mate with may have a yield stress a lot lower than the bolt stress yield, talk to a manufacturer of any thread locker you might want to use and get there advice.
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Connecting rod bolts coming unscrewed.
The bolts are embossed 10.9 on the head. The maker says assemble dry. I have 4 other motors still running without coming apart. So I will trust the maker .
I guess this pre torque final torque is similar to an angle of turn after pre torque without the precision.
B.E.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
Thanks for the response!
I just looked in bs3643pt2 at screw thread major diameters and 6.786 would be marginally outside any of the three classes of thread for M7 although it looks like its M7 you have as nothing else is close.
For interest have a look at this link
http://www.boltscience.com/pages/casestdy.htm
The torqueing of bolts isn't a very accurate method even worse with dry bolts, however I think the two stage torque figures is an attempt to achieve a more even distribution of clamping force.
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Connecting rod bolts coming unscrewed.
http://www.scribd.com/doc/190688884/Bolt-Mike-III
Keith Cress
kcress - http://www.flaminsystems.com
RE: Connecting rod bolts coming unscrewed.
We will see how long it lasts.
We might as well close this thread.
Thanks Desertfox , and itsmoked That is a really neat gadget .
B.E.
You are judged not by what you know, but by what you can do.
RE: Connecting rod bolts coming unscrewed.
I'd use the old conrod as a test article, as follows:
Coat the bolt threads with Loctite PST, with maybe a little Never-Seez mixed in.
Measure the torque to break the bolt.
Torque the next new one, similarly lubed, to ~50...80 pct of that number.
Mike Halloran
Pembroke Pines, FL, USA