1st, 2nd, 3rd - POP, PAWOOSHH, WHINNGRRRR! (motor kaput!) pics

[timdhawk]

Pulled the motor, opened her up and found this: a broken ARP stud without the nut:



And this spoke to me in a bad way:


A beautiful collection of WTF leftovers:


Do you think this is bad...?



So, seriously... WTF happened? I found the nut from the broken ARP stud in the bottom with all the chain mess, and its chewed to hell.

 

[lingo]

yep, that is bad

 

[Rich Ard]

what happened is you just bought an engine

 

[VonYinzer]

Stud/nut seems to have been over torqued perhaps?

 

[timdhawk]

Stud/nut seems to have been over torqued perhaps?

I wondered that to, but how would the nut be separate from the broken stud? See 1st pic - the broken stud end is to the left of the stud and the nut was in the case..

 

[neevo]

Maybe the nut backed off the stud, dropped down the bottom, jammed the cam chain and when everything went spastic in the top end it sheared the stud with cam gear fragments?

 

[Brodie]

What is the history of the engine?
Maybe the stud was already like that and the nut was only on one thread.
I only think that because where the hell would the rest of the stud have gone.

 

[timdhawk]

What is the history of the engine?
Maybe the stud was already like that and the nut was only on one thread.
I only think that because where the hell would the rest of the stud have gone.

It was my first 750 complete rebuild 488 miles ago! New pistons, new cam from cyclex, new ARP studs, diligently torqued nuts...I just cant figure it out...o well

 

[Redbird]

I wondered that to, but how would the nut be separate from the broken stud? See 1st pic - the broken stud end is to the left of the stud and the nut was in the case..

Strictly speculation, but my guess is once the stud broke, and there was no more tension on the nut, it vibrated/rotated off the broken end of the stud.

And, btw, that sucks man

 

[Rich Ard]

Re: Re: 1st, 2nd, 3rd - POP, PAWOOSHH, WHINNGRRRR! (motor kaput!) pics

Strictly speculation, but my guess is once the stud broke, and there was no more tension on the nut, it vibrated/rotated off the broken end of the stud.

And, btw, that sucks man

i agree with both of these sentences

 

[Bert Jan]

Damn son.. Thats ugly. Strange problem indeed. When the stud broke, the nut would have fallen down with the end of the stud. There would be no time for the stud to vibrate itself out of the nut before they both went down..

 

[SONICJK]

Cam chain too tight maybe.

 

[Redliner]

Well I can tell you two things.

1. That nut was over-tightened at some point somehow, or just flawed. The sheered end shows that it was a twisting axial break. That's only worth anything in competitive ice skating.

2. The nut was "sawed" by the cam chain and/or sprocket for some time before finally being wedged somewhere. I can't tell from the pictures if it was between the sprocket and head or between the cam chain and sprocket. The cam chain clearly shows a failure by sudden stress, not riding too tight or too loose and not falling off of a broken sprocket. If you look at the short piece of chain, you can see it has changed color do to the incredible temperature difference created in that area when it was suddenly stretched.

Oh, and 3. nice work on the drive chain. My uncle used to go on and on about what bike needed what sprocket ratio to rip that chain up and blow the side of the crankcase to bits. Turns out all you need is a locked up engine?

In summation, you say the bolts were new so you had a flawed bolt. Likely cavitation in the forming process. It just happens.

The exact sound you described actually has a word for it where I'm from. We just say, "buggered".

 

[timdhawk]

Ya know... I had a vague recollection that an HD stud had snapped on me once before when putting this motor back together... I was right:
http://forums.sohc4.net/index.php?topic=82414.msg926653#msg926653
Not the exact same stud, but from the same parts seller...