Result of stretched rodbolts?
 

Was cleaning up in the garage and the broken nr3 rod bearing from my engine caught my eye, i have never seen a bearing crack this way before. Is this the result of stretched rod bolts? I can add that the engine failed at high rpm.http://forums.pelicanparts.com/uploa...1380383951.jpg

The quintessential story is always determining the sequence of events when an engine fails and that's part of the forensic process. :)

One needs to carefully examine EVERYTHING in close detail to determine what happened and in what order to learn anything constructive.

So i need to give CSI a call then?......Slowmotion video of the sun reflecting in the paint from a cool angle as the car races along the forest road. Cutscene showing down shift in to 3rd and the tach climbing past 6000 as the Porsche flies by a Volvo stationwagon. Cutscene to the engine bay, internal view of the engine, oil getting hotter as the preasure increases ......knock, knock, knock, game over!:)

Yessir!! :)

All those parts tell a story, one simply needs to learn the "language" to read it. :)

You have done that once or twice. Only delete to knock, knock part. ;)

One of the advantages of using the stretch gauge method of installing rod bolts is that you can check them for stretch (exceeding the elastic limit) on the next teardown.

I've not seen or heard of a bearing breaking like this, but compared with the professional engine builders I have a rather small (although, alas, still too large) personal data base. But I don't recall seeing mention of this here or on other likely suspect communications media.
What other damage did this motor sustain?

"Normal" bad rod bolt stretch causes a substantial loss of oil pressure at the bearing, and it doesn't take much stretch to do that. Then the bearing overheats. At some point the stretched and somewhat weakened rod bolt or bolts simply break. You can usually see how the shaft of the bolt has necked down, and broke where at its narrowest.

Hard to imagine that the cap could get far enough away from the rod before disaster strikes that the bearing could rotate to where the broken off ends were unsupported in the gap between cap and rod body and snapped off fairly cleanly.

Bolt Stretch seems a very unlikely cause for a failure of this type and the surface of the shells doesn't seem to show any damage of the type that would occur if the rod bolts had lost preload.

Rod bolts should be preloaded so that the alternating loads developed during running are less than the preload.

This means that even if a bolt were installed at a load which exceeded its elastic limit it is very unlikely that any further deformation would occur due to piston acceleration.

I also don't think that a 'stretched' bolt can be said to be weakened.

The type of steel used for the manufacture of Rod Bolts will exhibit work hardening once it has been loaded to beyond its yield point.

This means it will increase in strength - possibly by as much as 25%- and continue to support the applied load although its area is reducing (Poisson's Ratio = 0.5 beyond yield).

At the onset of necking the increase in strength is insufficient to compensate for the reduction in area and the load that the bolt can support would fall.

This means that if you tightened a bolt until it necked it would not be able to support the applied torque and it would break.

If you tightened it beyond yield but below the onset of necking it is unlikley to fail - hence the general use of 'stretch bolts' in modern engine.

I don't believe modern 911 rod bolts are 'stretch' bolts and the comments regarding their replacement are only found in Volume I on the Workshop Manual.

If, however, they did deform plastically when they were originally installed I would certainly replace them.

[QUOTE=Walt Fricke;7680270]One of the advantages of using the stretch gauge method of installing rod bolts is that you can check them for stretch (exceeding the elastic limit) on the next teardown.

I've not seen or heard of a bearing breaking like this, but compared with the professional engine builders I have a rather small (although, alas, still too large) personal data base. But I don't recall seeing mention of this here or on other likely suspect communications media.
What other damage did this motor sustain?

Thats the strange thing, the broken bearing was the only dmg i could find? The rods are in the shop now but there was no visual dmg to the rod. Perhas a material failure, ive seen plenty of failed bearing in my days but never one like this? I can add that the nuts seemed to have the correct tourqe when i removed the rod.

Forgot to ask if the arp 10mm The Official ARP Web Site | Kits is the correct bolts for the 2,7 rods?

I guess no one reading this has seen a failure like this one. And none of us can even imagine just how a bending force could be applied to the same relative place on each of the bearing halves with enough room to cause an actual break. Especially with no other apparent damage.