What # Rod Bearing failure is most common?
 

Hi everyone,

I recently had a big end rod bearing fail during a track session. This bearing was reduced to the point where the copper was no longer on the bearing surface but floating around in little flakes on my sump screen. There was enough play in the rod end to create a tapping sound from the engine. The valves were not touched by the piston. Yippee!

The bearing in question is # 3 (#3 cylinder).

When I pulled all the others, there was a little scoring on them (probably from the material of the failure) but essentially all the other bearings looked great and they had not at all worn through to the copper.

My question is obviously aimed at finding the cause to this failure. In your book Wayne you say that the big end bearings are one of the last places to receive oil. Is the #3 bearing even worse off than the others? Is it typical the that #3 bearing fails more often or is it just dependant on something else?

BTW, my oil pressure was lowish when I got the car (4 months ago) I'm not sure if the oil pump is the cause or the bearing deteriorating was the cause of my low pressure (pump pitting).

Many thanks.

Tristan

#3 is the first to go when there is oil starvation. #4 is next.

If any other bearing goes first, it is most likely caused by detonation or some other force.

Join the club, I had the exact same failure as you, except my oil pressure was good. The oil starvation in my case was caused by some crap in the oil passage feeding the no3 bearing. It was good job it failed when it did, on teardown found a cracked spring retainer during teardown.
Currently in the process of complete rebuild.

I think it was #6 that failed on mine.
-Chris

The crank oil is fed in through the #8 bearing which is at the rear of the engine. Meaning cylinders 3 and 6 are furthest away from that source...

Have you looked at #6 yet?

Thanks for the good info. I'll look for blocked passeges etc.

Chris,

All others are good. I'll bring them around sometime. Hey, while I'm here, do you have a 12 star flywheel bolt removal tool?

Take care,
Tristan

While your having the crank reconditioned have it crossdrilled. Crossdrilling the crank is a good safeguard for engines that will see track use.

Call Armondo at CCR, he performs magic on cranks

Yup, I'd say that the bearings at the front of the engine (6 & 3) are probably the worst off due to location. This is off the top of my head though, as I don't have a crank in front of me to double-check...

-Wayne

I saw 2 of these this summer, both 2.4S
http://forums.pelicanparts.com/uploa...1059067364.jpg

Bill, that picture shows #5. That is the cylinder most likely to die from detonation. It runs the hottest. That is a rod bearing that failed from detonation.

Bill wins the prize again for the most gruesome engine picture!

Tristan is rebuilding his own motor with much encouragement and moral support from us Chicago area Pelicans! Go Tristan!

Yup, here is an exerpt from Greg Browns article on the subject in "Velocity" the Porsche Owners Club magazine

"how a 911 crankshaft gets oil. The oil for the main bearings comes through a passage that runs from the rear of the case to the front of the case. Each main bearing is fed off of this passage with another passage to each and every bearing. The rod bearing oiling, however, is not quite so simple. The rod bearings are fed oil from the very front (clutch end) of the engine and from the very rear of the engine (pulley end). The pulley end’s main bearing is refered to as the #8 bearing. The flywheel end is also the bearing that controls the thrust of the crankshaft back and forth (end play). The oil then travels through the entire length of the crankshaft and each rod journal is fed oil from this channel. It is important to note that oil is fed from both ends of the crankshaft for the connecting rods. On initial inspection of the oil system,
one would think that the #3 rod bearing (closest to the front and closest to the oil pump) would get the majority of the oil. Not true. To understand this, one must understand how oil (or any fluid) behaves when it is fed through channels to the crankshaft. I think that class on fluid dynamics is where I learned this, but maybe it was when we were trying to get that giant water bong to work. (Since I can’t really remember, it was probably from the water bong thing.) At any rate, fluids tend to back fill. This means that the oil will flow down the main passage and then start to back up and fill the small passages once resistance is met on the far end. So, the oil passes down the channel of the crankcase to the #8 main bearing, where resistance is met from the passages that feed oil out to the heads. Here, the oil begins to back fill and the crankshaft main bearings and the rods bearings get their oil.
From this, it becomes easy to understand which bearings fail first when there is a lack of oil on an intermittent basis. It’s really very simple: #3 rod bearing fails first and is followed by #4 in a 911 engine. In a 944 engine, #2 goes first. In a 928, #6 is the first one out of the block. Engine failures get really easy to figure out from this. If you lose any other rod in a 911 engine, it is not from a lack of oil, but must be sourced from another problem, like detonation. 911 engines frequently lose the #5 rod first
(especially the turbos). This is because this cylinder runs the hottest and starts the detonation process soonest. The small detonations don’t ever show up on the top of the piston, but just hammer on the rod bearing, causing this failure. (There are addditional clues for this failure, by the way) So, if you’ve blown up an engine and #5 was the one to go and your mechanic blamed it on the oil system, you better talk him into taking a few degrees of timing out of the engine before you have another vent in your
new engine. This, of course, is only true if you have oil. If you lose all the oil at once and don’t see your oil pressure gauge or the waving black flag, all bets are off. The rod bearing that sticks first, spews parts first. The final thing to understand is how a 911 engine case is machined. This is pretty simple for this discussion. The main bearing bores are machined perfectly round to a specific dimension with the case bolted together. The aluminum surfaces where the case halves bolt together are bare aluminum. This is true when the case is made at Porsche, and hopefully is true after any subsequent machining processes take place. As you can see from our picture of a GT3RS case going together on the engine stand, the front main bearing (flywheel end) and the #8 main bearing are surrounded by aluminum that has to get sealant in order for the case not to leak. Enter Mr. Flood O’Loctite. (See picture of crankcase with sealant.) Since he can’t squeeze the excess out, the case is held apart in these areas. I’ve actually measured Loctite .005” thick in these areas. What do you suppose this does for the bearing clearances? Well, they just go nuts and you’re lucky if you can get half of the oil to the rod bearings that should be going there. Combine this with the wrong rear thrust bearing (going to get to that, too) and you have a recipe for disaster. So here is the absolute proper way to use Loctite 574. Use a short nap roller that has been rolled full of Loctite 574. Make sure that the Loctite is fresh. This stuff still looks good when it gets old, but it hardens faster. We don’t keep any open bottles longer than 6 months. We also do not keep the roller for any longer than 3 months. Roll on a thin layer on a completely clean, grease free case. Tighten the case halves together as fast as possible once they have contacted each other. Now, here’s a top secret tip that I’m going to let you in on, but don’t tell anyone else! Tighten the perimeter case bolts first, especially on a late engine with “o-rings” that have to be slid over the case bolts with a special tool. This will squeeze out the excess Loctite before it gets a chance to start to harden. If you do the big bolts first, the perimeter may start to “go off” before you get to this hardware, especially if you’re a bit slow. If you do it this way, you’ll never have a problem. Last thing to mention here is the main bearings themselves. Buy the main bearing set from Porsche. It comes
complete with the #8 main bearing in a box for $154.02, retail. The outside suppliers will sell you a set cheaper, but it will not come with the #8 main bearing, which you will need to buy separately. This will cost you more money than if you buy the complete set from Porsche. Not only that, but the main bearings from the outside suppliers have the wrong rear thrust bearing! If you look at the pictures of the thrust bearings, you can see that the aftermarket supplied rear thrust has straight grooves that pass the oil onto the thrust surfaces and allow the oil to escape without any resistance. Now look at the Porsche thrust bearing. Note that the oil grooves are cut notches that do not communicate with the outer edge of the thrust area. This keeps the oil more captive and thus
keeps more oil heading towards the rod bearings. This is the same thrust bearing that is recommended for all of the Porsche race engines, the only difference being that Porsche pays no attention to the size of the rear thrust bearing for street engines. The race engines have three different size rear thrust bearings that can be used, depending upon what size the crankshaft is. You had better get these the correct size, with the engines turning over 8,500 rpm. Of course, Mr. Flood O’Loctite used the wrong thrust bearings in the race engine we just took apart, but at least he only charged $265.00 for the complete set. "

Both cars were tracked, the theory is that the owners may have cranked in too much ignition advance for the gas/conditionns