Engine with thrown rod

logician · 03-11-2005, 06:27 PM

I am looking at a 3.0L engine that is blown. I think it "threw a rod." I'm wondering what the prognosis is for this engine, and whether it might be usable without replacing the crank or case. Here are the specifics:

The previous owner said that the engine started running poorly and making a lot of noise. It then quit altogether. He was not able to restart it. A shop looked at it and noted that the crank is hitting something. I think they must have drained the oil, but I have no information about what they might have found.

The case does not appear punctured. I can see that the #5 piston is at the top of the stroke and does not move when the crank is turned. The crank meets a hard stop on the #5 upstroke, so presumably the rod is somehow wedging up against the piston. The exhaust rocker on #5 is broken. Looking in the plug hole, it looks like the piston is not shattered. The spark plug is blackened, but is not damaged, so it doesn't look like anything was bouncing around inside the cylinder. The cam turns when the crank turns.

I don't really care if the pistons and cylinders are damaged as I would replace them anyway. But I do care about the crank and block. Is it likely that the crank would have been damaged under this kind of circumstance? Because the piston is not shattered, I am hopeful that maybe the forces on the engine were not that great, and so the damage may not be much. Is that hope warranted? Or is this engine likely trash?

Thanks in advance for any advice!

-Juan

trak ratt · 03-11-2005, 07:01 PM

It might be usable, but then again maybe not. At the very least pull #5s head and cylinder. You will then be able to see the inner case. The broken rocker is telling as the exhaust valve usually makes contact first. More than likely the broken rod has beat the crap out of the inside of the case but the fact that it hangs up in the same place all the time might be a good sign. The crank is nearly indestructible thanks to a lot of bearing support. I assume the front pulley and rear flange rotate together?

logician · 11:17 PM

Hey guys, thanks for the info. I'll see if I can get the engine apart to get some more info. I'll post what I find. One other input I got is that when a 911 engine throws a rod, it's likely to be the #5 cylinder because that rod is the last in line for oil.

-Juan

Doug Steinel · Doug Steinel's Garage

Perhaps the exhaust valve broke off and is obstructing the piston's movement?

addictionMS · addictionMS's Garage

Russ of the PRC has two blown motors, both #5 rod, if you are looking to get a cheap motor, I don't think that is the way to go, when they blow, a lot goes wrong.

Jim

logician · 10:14 PM

OK, for your amusement, here are some pictures. It looks like the rod came appart and beat up the case. The #5 spigot has a chunk out of it. I don't know what the other side looks like. There was this very funny thin metal, kind of like very brittle tin foil, in the case.

So what happened? Was the engine starved for oil, and then blew up? Is the "tin foil" an indication of this? What is that tin foil stuff? Is the case usable, assuming what you see is full extent of the damage to the case? Is the case trash, can it be repaired in some way, or can it possibly be used as is? What about the crank? What parts of the engine are salvagable, and what should I consider when trying to use any parts of the engine?

Any input would be greatly appreciated!

-Juan

"Tin foil:"

"Beat up rod:"

"Pictures of case:"

logician · 03-14-2005, 10:57 PM

Jim,

What was the cause of Russ' blown motors? As I understand it, #5 rod is an indication of low oil. If so, why would Russ' engine run low on oil? Was it a problem with the stock oil tank?

-Juan

kolies · kolies's Garage

I have never taken an engine apart yet, am planning to soon though. It looks like one of the rod bolts failed. Would that have been a primary failure, or a secondary after something else?

jpnovak · 03-16-2005, 10:23 AM

Ouch! That hurts my eyes to see. I guess it was a rod bolt failure. That rod is really beat up. The case kicked is a$$.

hcoles · 03-16-2005, 07:00 PM

Juan,
Ouch.....I don't really know what to say. Maybe a rod bolt let go.
If the bearing went out then it might add credance to using the cam oil line restriction upgrade.....puts more oil to the bottom end.
-Henry

cstreit · cstreit's Garage

The "tin foil" like stuff is usually a bearing. Once you lose lubrication (from either a rod bolt loosening and creating a wide tolerance, or just oil starvation) the bearings tend to get beat up and start "extruding from underneath the rod journals.

logician · 03-16-2005, 11:02 PM

Quote:

Originally posted by cstreit
The "tin foil" like stuff is usually a bearing. Once you lose lubrication (from either a rod bolt loosening and creating a wide tolerance, or just oil starvation) the bearings tend to get beat up and start "extruding from underneath the rod journals.

Pulling apart a 911 engine is fun! I've torn the engine down to the case, but have not yet split the halves. Now that I understand how the bearings work, I agree that the "tin foil" stuff is bearing that was extruded. I have not yet found the #5 rod bolts and nuts, although I did see one nut when I first pulled the #5 cylinder off.

I'm curious if the rod bolt broke, or whether one of the nuts backed off. What is the typical seqence of events when a rod bearing is starved for oil?

I'll post more pictures once I have the halves apart.

Henry, this is not an engine out of one of my 911's. It's an engine out of a 911 parts car.

-Juan

hcoles · 03-17-2005, 06:03 AM

OK, glad it's not out of one of your other cars. Good thing you have the M. car while working on all the German machines... ;-)

My understanding of nuts, bolts, torques. Assuming the nut was at least in the ball park of proper torque (bolt streach correct) I don't think the nut will ever back off. I think the failure modes are that the bolt breaks probably starting with a defect or stress concentration of some sort. If the engine over revved and higher than design stress was put on the bolt...there may be a story for that.

-Henry

addictionMS · addictionMS's Garage

I would guess that the bearing went, and that caused the rod to bound back and forth on the crank, than that casue the bolt to stretch, and that caused the nut to loose torque, and that was that. The nut came off and it was finally over.

Juan: in Russ's case I think it was the over rev in the spring that stretched a bolt that was not replaced went fixing the bent valves. Then that bolt loosened. It is interesting that is was #5, because in this scenario, there is no reason for it to have been #5. So the lack of oil must have played a part. just not sure how. Maybe it is just the most stressed so it always fails first. Both Russ and I now have cneter drilled cranks, as will John's motor once we get to it.

camgrinder · 03-17-2005, 07:59 AM

IMHO:
Is the big end of the rod burnt? Black in color? If so the friction between the connecting rod without a bearing insert and the bearing journal heated up the rod bolt causing it to fatigue. The heated bolt will stretch and lose its preload. Eventually breaking, or in some case the nut falling off.
The tin foil was a bearing. When a bearing fails and the clearance opens up, the bearings get hammered. looks to me like the owner of this engine ignored the knock or had his stereo up.

logician · 03-17-2005, 09:03 AM

Quote:

Originally posted by camgrinder
Is the big end of the rod burnt? Black in color?

John, Yes, exactly! I was curious about the cause of the color. The rod end looks like it's been worked too. It seems like the only way for the surfaces to be worked that uniformly is if the rod end was still bolted together, but without any bearing. That would mean that the bearing would have been somehow sqeezed out the side of the rod. I wouldn't have thought that it possible to squeeze the bearing out the side because there is very little clearance there. Maybe it just gets so hot that it melts? Interesting.

The owner said that the engine started to sound loud/bad, but he kept driving. Then the engine died altogether. Don't know for sure how far he drove.

So can that crank surface be restored? How?

Jim, What is a "center drilled" crank?

-Juan

addictionMS · addictionMS's Garage

center drilled crank thread

camgrinder · 03-17-2005, 05:40 PM

The crank needs to be crack tested. Then the journal can be welded up and ground back to standard. Then the crank is nitrided which hardens the crank and stress relieves.
CCR can do the work if your willing to wait. I also believe Ollies can get it done as well.

Russellh911 · Russellh911's Garage

Hi All:

I agree with Camgrinder, that rod bearing went bad and it was ignored. My last motor tossed a rod bearing and ran for about 35 seconds before I shut it down. Still the rod journal scarred and I had to have the crank redone. Figure seven hundred to a thousand bucks for that little goodie. With the damage done to that crank, it may be trash. I have one that looks a lot like it (not the one being repaired).

I think you've got boat anchors on your hands.