I'll find an excuse to stop at Greg's, likely tomorrow or Thursday, and grab the a bore gauge.

 

Oh, wow. I didn't see that. What was the owner noise like? 991 GT3 owners will be a "select" bunch to be sure.

I'll have to check it out. Need to filter out the eventual whining.

 

Good luck with asking Porsche to do that on a 20 year old engine....

I've never seen a broken R1 rod, unless the bearings spun, first.

 

I've never known what to call the area where the beam attaches to the big end. " sounds great!Saddle"

I've become a sort of a detective, having seen so many engines apart and so many engines fail, over the years. Racing gave me plenty of engines to look at.....

If you can't figure out what happened to the engine that is blown up in front of you, how would you be able to ensure the next one doesn't do the same thing? I had a good friend who used to say, "You have to be 1% smarter than what you are working on!"

There are clues to every failure, if you are logical. "Advanced reading of the tea leaves."

Anytime a fastener to a rod breaks or becomes compromised, an engine fail dramatically, very quickly. The rod simply will not stay on the crankshaft for more than a few seconds, before the forces tear the rod into pieces. This leaves its own set of clues. There will be pieces of rod bearing that will still be intact...all of the layer will still be attached and it will look normal. This is because the rod separates at the parting line and part of the bearing will remain untouched.

Clearly, Rob's failure was bearing first....there was barely any of the steel backing left and the crank was worn considerably.

 

GTS crank 2/6 journal, 5-17-09




Repaired 2/6 journal, 10-22-11:

 

I rather suspect a miracle would be more likely. I have a set of R1 rods and pistons that appear to be in good nick. My GTS crank is probably good for little given the wear on the rear thrust face that ground/polished out leaving what would be a crank TB float of 0.5mm but most of it on one side. Whether it could be re-nitrided and a custom bearing made no idea but would probably cost more than a used GTS crank.

It would be nice to convert my 90 S4 motor to GTS spec but the more I read the more I reckon it is probably better to leave it as an S4. Even the 8 bob crank seems to get indifferent reviews from you experts.

Regards

Fred

 

I am not necessarily saying that the conrod nuts backing off is the reason for the engine failure. Since it's 2/6 journal, it's probably more likely the usual 928 problem of sending anything but oil to the 2/6 journal. If it were some other journal, then I'd entertain more novel theories more seriously as the underlying cause. I am merely commenting on how it's possible that the nuts came off from the bolts in one piece, regardless of what the original failure cause was, which I understood to be the question asked.

When the rod fastener fails at high rpm, I would expect the engine to stop rotating in a short number of cycles. However, if the rod fastener fails at low rpms, it may not be as instantaneous. If the redline rpm is 6000, and the rod fastener starts to fail at 1500 rpm, then it's only being subjected to about 6% of the forces that it would be subject to at 6000 rpm. This makes me believe that things can happen at cruise rpms that would never happen at redline rpm. If Rob's engine failed at cruise rpm, rod bolt or nut backing off relatively slowly is a lot more plausible theory than if it failed at the redline rpm.

It appears from a casual search that airplane and marine engines have conrod nuts back off relatively frequently compared to other kinds of failures. Those engines are often operated at very low rpms (relative to redline) for long periods of time. Especially airplane engine failures get researched thoroughly, so there are a lot interesting investigative techniques for doing post mortems on piston aircraft engines. For example, if the rod nut backs off while the engine is still running for a little while, one can see coked oil on the rod cap in the area that is usually covered by the rod nut. The same holds for the rod bolt backing off from the saddle, if there's coked oil on the saddle the bolt backed off before the engine stopped.

If the engine rattled (at cruise rpm) for a while before stopping, that could be consistent with the rod running with the rod cap loose. (It would also be consistent with garden variety rod bearing issue.) The rod running loose, even if properly oiled, will generate a very large amount of heat even at cruise rpm. The resulting bearing shell and journal will probably look just like a garden variety rod bearing failure due to oil starvation, even though the bearing was never starved of oil.

So I guess my questions to Rob would be the following:
- Did the engine fail at cruise rpm or near redline rpm? If it was closer to cruise rpm, then rod bolt or nut backing off slowly is more plausible theory. If it was closer to redline rpm, then oil starvation is the more plausible theory.
- Did the rod big end "saddle" have oil coked under where the bolt should have been covering the surface? How about the cap under the nut? If there's oil coking there, then the fastener backing up is a more plausible theory.

 

I recall Rob saying he was pulling up the big hill on the 5 , the grapevine, taching about 4,000 rpm....big long hill with about 2,600 ft of elevation change.

 

I was in the far left lane, 4th gear, ~4000 rpm for 5-7 miles uphill, when the temp gauge rose, and the oil pressure gauge dropped to ~2.5 bar. Dropped over a lane or two, put it in 5th, turned on the heat, and lugged it for a while while I was thinking about what to do. so probably 2700 rpm or so in 5th gear. Put up the windows so I could actually hear the motor, I heard an audible rod knock for a couple of seconds just as I was passing the Tejon pass sign, and it let go in a big way- audible explosion, and felt it through the wheel, my feet, my butt... Much more explosive than when the interim motor in the Zombie let go, and there's a lot more insulation between motor and me in the GTS. The jolt was probably the #2 rod being bent 90 degrees between the crank and the block:



The crank was worn .040" on backside of the 2/6 throw, and the crank ws bent .008"

So it was turning less than 3K when it went.

RE: the coking under the rod bolt end, dunno. I think I still have the pieces, somewhere.

 

I wasn't saying you were wrong. Anything is possible inside an internal combustion engine.

My background is racing, not airplanes, so hardware failures are instantly a disaster.

 

I am glad I don't have a GTS.

 

Yeah, poor guys huh

 

A GTS owner down under added some mild boost to a 93 GTS and the rods bent IIRC.

 

Yes, seen a few bent ones, myself.

There was a local Porsche shop that was doing 968 engines and using Carrillos.

To increase their profits, they then were selling the 968 rods to the 951 guys.

Fixed a couple of those with broken/bent rods.

 

What does a set of Carrillos retail for these days?