Reliability of OA as gauge of bearing wear?

Messages
12
Location
Austin, TX
Hello, This is my first post here. I tried doing a search, but didn't have any luck finding an answer to my specific question. I have a 1992 Porsche 968 (thus my user ID) with about 107K very well-cared for miles. The precedessor to the 968 (the 944) was known to have issues with #2 and #3 rod bearing wear, though this seems to be less of an issue in the 968 due to improvements in oil pan baffling. Anyway, I did an oil analysis with Blackstone a year ago at my last oil change, and it showed no sign of elevated copper or tin, though lead was a little high (but Blackstone said this could be due to a particle that could be temporary). I'm about to do another OA, and am wondering how reliable oil analysis is in determining the health of the bearings. In other words, if there's still no sign of elevated bearing metal, should I rest easy and not worry about doing a pre-emptive bearing replacement? My concern is that on the one hand, bearing failure on these engines can lead to catastrophic results, but on the other hand, replacing the bearings on this engine is a HUGE and very expensive job, so I don't want to do it if I don't have to. I use Redline 10W40 synthetic in the engine, and since the car isn't a daily driver, I change the oil once a year, which works out to every 5000 miles or so. From the records, the previous owners kept up a very regular schedule of oil changes. Thanks in advance.
 
Messages
4,563
Location
NW Ohio
If you search in the oil analysis section, you will find a fairly recent debate on just this topic. It reached no totally firm conclusion. It would seem that UOA is more a reliable weathervane for the condition of the oil as opposed to the engine. From what I gathered, to make a determination on an engine, you have to have a long UOA history, watch for big changes and know what you are looking at with repect to the particular engine. A few PPM change in one element is insignificant (mostly). The general consensus is that if you do UOA at oil changes, you may catch the problem too late. If you were to pull samle every 500 or 1000 miles, perhaps you might catch something. I hope you aren't THAT pedantic (: < )! It's also vital to have someone very qualified to interpret the results of the tests (the name Terry Dyson comes up a lot in that regard) but again, it comes down to charting trends over a long period. My opinion, which is free and worth every penny of what you paid, is that you are worrying too much. If this is indeed going to happen, it's going to happen. A preemptive overhaul would be a waste of money in my opinion because you should be able to catch a bearing failure early, long before a spectacular failure, and if you catch it early, it should be a normal overhaul. My advice is to continue caring for your "Porsh" well, watch the oil pressure and listen for noises which would indicate a bearing problem. Perhaps do UOA at each change and start tracking the trends. After several UOAs you will better be able to track big changes.
 

Porsche968

Thread starter
Messages
12
Location
Austin, TX
Jim, Thanks for the detailed and well-written response. You're probably right, I'm probably worrying too much. The 968 community is a small but very active one, and every horror story tends to get blown out of proportion. A big part of the reason for my concern, which I failed to mention in my first post, is that I track the car a fair bit, which means at least one trip to the redline each lap. But these engines are designed to be driven hard, and it seems that as long as the key components are getting oil (i.e. there's no starvation happening, which should have been taken care of with the baffles Porsche added to the 968's pan), and that said oil is clean and of top quality, the key engine components should have a very long life. I'll ask around if anyone who has experienced bearing failure has had any early warnings. All I've read about is the catastrophic situations, such as "When the coatings wear off the bearings they like to connect themselves to the crank. Usually the rod snaps and bandsaws the crankcase." Yikes - $10K+ repair. And I'll of course compare the results of the analysis I'm about to have done with the one I did at the last oil change.
 
Last edited:
Messages
320
Location
ohio
I would do the rod bearings no matter what the UOA says at that mileage...We have been racing 944's for many years and doing UOA's along the way...believe me, you will never know what hit you when it does go through the bearing...and none of my UOA's were out of normal range. Granted, we are using them under severe service but it really is not that expensive to do the rod bearings on these cars. i can do it myself in around 7 hours and the bearing is about 60 bucks. if you throw one you'll be lucky to get away with $2000 worth of parts and a whole lot more in labor to clean up the mess it makes in the engine.
 

Porsche968

Thread starter
Messages
12
Location
Austin, TX
 Originally Posted By: oliver88
I would do the rod bearings no matter what the UOA says at that mileage...We have been racing 944's for many years and doing UOA's along the way...believe me, you will never know what hit you when it does go through the bearing...and none of my UOA's were out of normal range. Granted, we are using them under severe service but it really is not that expensive to do the rod bearings on these cars. i can do it myself in around 7 hours and the bearing is about 60 bucks. if you throw one you'll be lucky to get away with $2000 worth of parts and a whole lot more in labor to clean up the mess it makes in the engine.
Hmmm... OK, a vote to replaced the bearings, and you definitely speak from experience. I have heard, though, the the 968's 3.0 liter is not as prone to rod bearing failure as the older engines, largely because of better oil pan baffling, but there may be other factors as well. My concern isn't so much with the amount of work involved, but with all the other "while you're in there" items (might as well install a windage tray, replace the steering rack bushings, put in a Lindsey racing 3-piece cross-member to make future bottom end work much easier, and then there's the necessary wheel alignment needed as a result of removing the castor blocks) that bring the total for the job to well over $1000, even if I do it myself. Not exactly "cheap insurance." Are there no early warning signs of any kind? Like a drop-off in oil pressure (mine is rock solid)? Or some tell-tale noise? Thanks for the input - plenty of food for thought...
 
Messages
320
Location
ohio
I have the Lindsey baffles and windage tray installed on my race motors...helps, but does not cure the issues...cross drilling the crank may help...also, keep looking for ways to eliminate aeration in the race motors...street cars can go a looong time on a set of rod bearings but i would definitely do them at 100,000 plus miles...no question about that. The early sign will be a slight tick followed by a loss of oil pressure....the problem we have is that it is usually wide open throttle when it happens!
 
Last edited:
Messages
320
Location
ohio
BTW, we just unbolt the strut towers from the frame and drop the whole unit, power steering attached...plug the brake lines too. saves the alignment...
 
Messages
6,388
Location
Washington St.
Are the bearing failures due to gradual, excessive wear? Analysis will show the bearing material from the accelerated wear. Are the failures sudden due to material strength failure? There is nothing in the oil for analysis to find in this case.
 
Messages
320
Location
ohio
In racing environments the 944 engine can cavitate the oil and there are many guesses as to what the best solution is. In reality the only real way to solve it is to go to a dry sump system like the Porsche 911's use. The failure can be minimized by changing out the rod bearings yearly in a race car. we run them 15-20 races and change them. They are always worn,some a little bit, some quite thoroughly! The worst is usaully the #2 bearing although i had a #4 let go on me last August at Mid-Ohio and it was nasty. The block was OK but the crank and pistons were wasted.
 

Porsche968

Thread starter
Messages
12
Location
Austin, TX
 Originally Posted By: oliver88
BTW, we just unbolt the strut towers from the frame and drop the whole unit, power steering attached...plug the brake lines too. saves the alignment...
Interesting, and very clever... So as not to bore everybody with the gory details of how to work on our specific cars, I'm sending you a PM for more details about this method. Thanks in advance.
 
Top