Broken bolts.

[pandos]

Today I noticed a strange vibration in the engine at low revs...there was also a sort of a knocking sound.

I checked the engine bay at the side only to find that the doors were not properly secured and I surmised that this was the knocking sound.

At this stage I was hard on the wind flat seas, sun shining doing about 5 knots with the tide going in the right direction beautiful days sailing was anticipated...but the noice was bugging me...

So I looked under the front of the engine only to find that all 3 bolts from one engine mount bracket had broken and fallen onto the base of the engine compartment...

I cleaned this part of the boat last week so all of these bolts fell out today.

All 3 bolts have sheared, they all look as if I had overtightened them and sheared them as I was fitting them...that did not happen...(plainly had they been broken when being fitted they would have fallen out already...)

The cure is obviously new bolts if I can get the stumps out...

any exotic theories on why these bolts would go go all at the same time...

The bolts are m10 with 13mm hex heads 8.8. afaik they are original to the engine. 1980... I had them out and refitted in June..and about 5 years ago.

No signs of rust ... But all three bolts are dull dark grey where they broke..?

Bolts are 8.8 grade steel.

 

[gregcope]

Are they Stainless bolts? Do you know what they are?

 

[pandos]

Are they Stainless bolts? Do you know what they are?

They are steel 8.8 grade..

 

[Daverw]

I had same issue 3 years ago, one had sheared and was only fLund when I checked a loose other foot, from the wear on the face it looked like it had done so some time ago and had a lovely polished surface. I found a cracked bracket on the engine. Not sure which had come first. Event found slight bent shaft which I think was the root cause

 

[vyv_cox]

Photo not the clearest but I would be very surprised if this was not fatigue. Most likely cause is undertightening. In the case reported by daverw this even more probable, polishing of the fracture face being characteristic of fatigue. There is a lot on the subject at Fatigue

 

[Boater Sam]

Crystalline appearance at the fracture looks like fatigue. One gets a crack, overloads the others till they all shear under load.

 

[pandos]

Thanks for those useful replies. Plain as day from vyv-cox link that fatique was the cause.

I think this may be caused by paint on the faces of the joint so that they were not pulled together firmly enough thus the bolts were not actually tight enough...

Next trick will be getting out the remaining sections of bolts...

Probably need to pull engine out into the saloon again...??

 

[LadyInBed]

Probably need to pull engine out into the saloon again..

Is it not possible to just jack / leaver up the 'corner'?

 

[pandos]

Is it not possible to just jack / leaver up the 'corner'?

The engine is tight against the beds and cabinet on the port side...

I will jack it and remove the bracket and engine mount, and maybe I will be able to get a few fingers in to try and wind out the stumps but if any kind of drill or tool is needed it will require a lot of space..

The bolts run at a 45degree slope up into the block almost at the joint with the oil sump.

 

[vyv_cox]

Crystalline appearance at the fracture looks like fatigue. One gets a crack, overloads the others till they all shear under load.

Sorry, not true. Fatigue is characterised by beach marks or polishing, developed as the crack propagates through the metal under cyclic load. All explained on my website. The crystalline appearance is common in higher strength steels in which beach marks do not develop so readily.

 

[pandos]

I have taken some HD photos with my phone and enlarged to get a better view. (They are too big to upload)

They look exactly the same as the first photo below the heading "Fatigue fracture in bolts" on the link.

I wonder should I opt for higher tensile replacements?

I'll change all the bolts in the mount on the other side also...(and remove all paint so I have flat clean mating surfaces)

 

[pandos]

 

[vyv_cox]

View attachment 123140

That's better! I can just about see beach marks moving 4:00 - 11:00 and ratchets between 2:00 and 5:00. These are formed by multiple inititation sites, either due to high stress concentration factor (e.g. thread roots) or relatively high tension. Final fracture appears to be at 12:00.

 

[vyv_cox]

I have taken some HD photos with my phone and enlarged to get a better view. (They are too big to upload)

They look exactly the same as the first photo below the heading "Fatigue fracture in bolts" on the link.

I wonder should I opt for higher tensile replacements?

I'll change all the bolts in the mount on the other side also...(and remove all paint so I have flat clean mating surfaces)

No need to go stronger than 8.8. You run the risk of increased brittleness and 8.8 is plenty strong enough for the job.

 

[pandos]

Thanks for th

No need to go stronger than 8.8. You run the risk of increased brittleness and 8.8 is plenty strong enough for the job.

Thanks for the excellent advice analysis.

 

[pandos]

Thought I'd provide an update...

Jacked up engine today managed to get bracket and engine mount out.

With the aid of a sail needle and plenty of wd40 I managed to wind out the stumps of two of the bolts...

The 3rd I had to wind in and run a tap into the hole and then wind that one out.

So all good... Extended the threaded section on the new bolts and put things back together.

Sods law dictated that I would leave my small torque wrench at home but next trip will swap out the bolts on the other mount and torque all 6 bolts...( They are only m8 so pretty thin...I thought they were m10.)