2/6 Rod bearing fix?
02-27-2014, 09:21 PM
#16
Shameful Thread Killer
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I suffered a 2/6 bearing failure after about 15 minutes of track use on a mild temp day. Using good oil, at the proper fill level did not help in a bone stock 90GT.
My 'fixes' that I incorporated were;
1. Check the clearance on the rod bearings carefully for correct film or squish size. I used plastigauge on the journal several times to get it perfect.
2. Install the GTS baffle in the oil pan.
3. Grind smooth and clean up the low pressure side of the oil pickup right up to the oil pump.
4. Radius, and grind smooth all the delivery passages from the oil return line to the crank gallery feeds.
5. Deburr the oil return channels in the edge of the heads, and also at the base of the block where oil returns to the pan. Remove all flashing from the casting where oil would be flowing.
6. Install the cam cover vents on both sides of the engine and plumb them together.
7. Replace the oil pump gears, and insure they were very tight in the oil pump housing.
8. Had machine shop resize and hone the rod big ends to take out some oval shaping.
I haven't run the engine much yet, so I have no empirical evidence that this will improve resistance to rod bearing seizures, but I think all these things will help or I wouldn't have done them.
My 'fixes' that I incorporated were;
1. Check the clearance on the rod bearings carefully for correct film or squish size. I used plastigauge on the journal several times to get it perfect.
2. Install the GTS baffle in the oil pan.
3. Grind smooth and clean up the low pressure side of the oil pickup right up to the oil pump.
4. Radius, and grind smooth all the delivery passages from the oil return line to the crank gallery feeds.
5. Deburr the oil return channels in the edge of the heads, and also at the base of the block where oil returns to the pan. Remove all flashing from the casting where oil would be flowing.
6. Install the cam cover vents on both sides of the engine and plumb them together.
7. Replace the oil pump gears, and insure they were very tight in the oil pump housing.
8. Had machine shop resize and hone the rod big ends to take out some oval shaping.
I haven't run the engine much yet, so I have no empirical evidence that this will improve resistance to rod bearing seizures, but I think all these things will help or I wouldn't have done them.
02-27-2014, 11:59 PM
#17
Nordschleife Master
There's been enough written on this subject that if the OP did a search, he'd spend an entire day weeding through the various threads.
Regarding the Clevite CB981P bearing....it is very narrow. The 944 rod is several millimeters wider than the 928 rod, so it looks even narrower in that application than in the 928 application.
However, the 944 piston pushes squarely down on the 944 piston and rod....there is no rod offset, because it is an inline 4 cylinder and it is a simple thing to make everything push straight down. This is not the case with the 928 engine. There is considerable offset built into the rods, since each rod shares a rod journal on the crankshaft with another rod. This means that the piston never pushes squarely on the crankshaft rod journal.
The result is that the rod has a "twisting moment" and essentially tries to "rock" the rod and the bearing on the rod journal.
The Clevite bearing was narrow enough that the bearing load math was very far off and I completely dismissed this as a possible solution.
That being said, I have never tested them....
Regarding the Clevite CB981P bearing....it is very narrow. The 944 rod is several millimeters wider than the 928 rod, so it looks even narrower in that application than in the 928 application.
However, the 944 piston pushes squarely down on the 944 piston and rod....there is no rod offset, because it is an inline 4 cylinder and it is a simple thing to make everything push straight down. This is not the case with the 928 engine. There is considerable offset built into the rods, since each rod shares a rod journal on the crankshaft with another rod. This means that the piston never pushes squarely on the crankshaft rod journal.
The result is that the rod has a "twisting moment" and essentially tries to "rock" the rod and the bearing on the rod journal.
The Clevite bearing was narrow enough that the bearing load math was very far off and I completely dismissed this as a possible solution.
That being said, I have never tested them....
02-28-2014, 12:16 AM
#18
Nordschleife Master
It's my understanding that the bearing area center is exactly in the middle of the rod beam for both the 928 and the 944. No difference. The bearing location relative to the beam is exactly the same in the both rods, the only difference being the 2mm that is shaved off of the 944 rods to make them 928 rods, with no impact on the bearing area center relative to the rod beam. The bearing area center of the stock 928 rods is exactly under the rod beam. The cheeks of the rods are there just to locate the rod relative to the crankshaft (in crankshaft guided rod engines like these), and they have no bearing (pun intended) on the bending moment of the rods.
02-28-2014, 01:01 AM
#19
Instructor
Thread Starter
Sorry Greg, I'm a computer guy not an engine builder so it's only intellectual curiosity that led me to ask the question. I really didn't know that pistons or rods would be offset in a V8 as compared to a I4. I have enough mechanical aptitude to have built a Cobra replica but that was only assembling completed subsystems. Just getting the Jaguar suspension put together right with all the shims, etc. was enough of a challenge for me. There's no way I could paint it or build the engine or transmission. Like Clint Eastwood said - "A man's got to know his limitations".
I'm truly in awe of someone like you that can build an engine that doesn't explode or seize the first time you start it up. Don't ask me about the Lotus 907 engine I tried to rebuild to "save money". My poor Jensen Healey didn't know what hit it. Literally.
Tom
89 S4 Auto
I'm truly in awe of someone like you that can build an engine that doesn't explode or seize the first time you start it up. Don't ask me about the Lotus 907 engine I tried to rebuild to "save money". My poor Jensen Healey didn't know what hit it. Literally.
Tom
89 S4 Auto
There's been enough written on this subject that if the OP did a search, he'd spend an entire day weeding through the various threads.
Regarding the Clevite CB981P bearing....it is very narrow. The 944 rod is several millimeters wider than the 928 rod, so it looks even narrower in that application than in the 928 application.
However, the 944 piston pushes squarely down on the 944 piston and rod....there is no rod offset, because it is an inline 4 cylinder and it is a simple thing to make everything push straight down. This is not the case with the 928 engine. There is considerable offset built into the rods, since each rod shares a rod journal on the crankshaft with another rod. This means that the piston never pushes squarely on the crankshaft rod journal.
The result is that the rod has a "twisting moment" and essentially tries to "rock" the rod and the bearing on the rod journal.
The Clevite bearing was narrow enough that the bearing load math was very far off and I completely dismissed this as a possible solution.
That being said, I have never tested them....
Regarding the Clevite CB981P bearing....it is very narrow. The 944 rod is several millimeters wider than the 928 rod, so it looks even narrower in that application than in the 928 application.
However, the 944 piston pushes squarely down on the 944 piston and rod....there is no rod offset, because it is an inline 4 cylinder and it is a simple thing to make everything push straight down. This is not the case with the 928 engine. There is considerable offset built into the rods, since each rod shares a rod journal on the crankshaft with another rod. This means that the piston never pushes squarely on the crankshaft rod journal.
The result is that the rod has a "twisting moment" and essentially tries to "rock" the rod and the bearing on the rod journal.
The Clevite bearing was narrow enough that the bearing load math was very far off and I completely dismissed this as a possible solution.
That being said, I have never tested them....
02-28-2014, 01:23 AM
#20
Nordschleife Master
Am I not right?
02-28-2014, 01:42 AM
#21
Nordschleife Master
I would have to actually go out and measure myself to confirm Tuomo. I haven't delved that deep into the piston offsets etc. But likely will be later this year.....
A very interesting thread indeed. I would be very curious to see these changes under higher pressure, with oil, and also with gravity compensation. IE tilting the girdle at X deg both left and right to see the effect under g-force. As the failures happen during cornering, it would be very interesting to see what effect gravity would have.
A very interesting thread indeed. I would be very curious to see these changes under higher pressure, with oil, and also with gravity compensation. IE tilting the girdle at X deg both left and right to see the effect under g-force. As the failures happen during cornering, it would be very interesting to see what effect gravity would have.
02-28-2014, 05:42 AM
#22
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Åke
02-28-2014, 05:47 AM
#23
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Åke
02-28-2014, 06:01 AM
#24
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There's been enough written on this subject that if the OP did a search, he'd spend an entire day weeding through the various threads.
Regarding the Clevite CB981P bearing....it is very narrow. The 944 rod is several millimeters wider than the 928 rod, so it looks even narrower in that application than in the 928 application.
However, the 944 piston pushes squarely down on the 944 piston and rod....there is no rod offset, because it is an inline 4 cylinder and it is a simple thing to make everything push straight down. This is not the case with the 928 engine. There is considerable offset built into the rods, since each rod shares a rod journal on the crankshaft with another rod. This means that the piston never pushes squarely on the crankshaft rod journal.
The result is that the rod has a "twisting moment" and essentially tries to "rock" the rod and the bearing on the rod journal.
The Clevite bearing was narrow enough that the bearing load math was very far off and I completely dismissed this as a possible solution.
That being said, I have never tested them....
Regarding the Clevite CB981P bearing....it is very narrow. The 944 rod is several millimeters wider than the 928 rod, so it looks even narrower in that application than in the 928 application.
However, the 944 piston pushes squarely down on the 944 piston and rod....there is no rod offset, because it is an inline 4 cylinder and it is a simple thing to make everything push straight down. This is not the case with the 928 engine. There is considerable offset built into the rods, since each rod shares a rod journal on the crankshaft with another rod. This means that the piston never pushes squarely on the crankshaft rod journal.
The result is that the rod has a "twisting moment" and essentially tries to "rock" the rod and the bearing on the rod journal.
The Clevite bearing was narrow enough that the bearing load math was very far off and I completely dismissed this as a possible solution.
That being said, I have never tested them....
Installing the Clevite CB-1628HX high performance rod bearing using the existing cut in the con rod (the width of the bearing tab has to be filed down at one side) the bearing will sit 0,35mm offset the middle of the rod beam. Do not think this is an issue. The alternative is to machine new cutouts in the rod which of course will add to the costs if you do not have your own mill.
Using the CB-1628HX bearing for the OEM con rod and a new GTS crank I have measured an oil clearance of 0,07mm which is perfect.
Åke
02-28-2014, 06:13 AM
#25
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About oil clearance.
http://www.mahle-aftermarket.com/C125790900540AC0/vwContentByUNID/8B0AD29525AD100AC1257A79006F8720/$FILE/CL77-1-205R.pdf
Åke
http://www.mahle-aftermarket.com/C125790900540AC0/vwContentByUNID/8B0AD29525AD100AC1257A79006F8720/$FILE/CL77-1-205R.pdf
Åke
02-28-2014, 07:55 AM
#26
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CB-981P are difficult to find and it seems we have bought just about all what was still available. Oversizes are still fairly easily available and some other manufacturers make std size but they might not be same level as Clevite. One good thing is that there are much thicker oversizes available than what Porsche has ever offered. Cranks what have been unrepairable in the past are fixable without having to change rods other than make second cuts to keep non stock bearings in place.
02-28-2014, 02:14 PM
#27
Nordschleife Master
02-28-2014, 03:14 PM
#28
What is wrong with width.
Under 8000rpm, the additional friction from a wider bearing that can take more stress seems like a good trade off.
Under 8000rpm, the additional friction from a wider bearing that can take more stress seems like a good trade off.
02-28-2014, 04:11 PM
#29
Nordschleife Master
It's a philosophical question. One could start calling a hunting rifle an accordion and vice versa, but it may lead to inefficient communication as not everyone will immediately get it if you ask them "Are you sure that your accordion is loaded?"
The thing about tradeoffs is that they are quantitative. I can think of only two ways to decide what is a good trade off and what is bad in this context. First, punch the data in a bearing calculator. Second, get data on later engines and compute their forces using the reciprocating and rotating masses at their redline rpms, select those that have forces similar to the 928, and then see what kinds of bearings are used in those engines.
As a point of reference, Subaru EJ25 has lighter components and 7k redline so the forces are somewhat lower than with the 928, and the bearings are 16.5mm long. That's more than 30% shorter than the 928 rod bearing.
As a point of reference, Subaru EJ25 has lighter components and 7k redline so the forces are somewhat lower than with the 928, and the bearings are 16.5mm long. That's more than 30% shorter than the 928 rod bearing.
02-28-2014, 04:59 PM
#30
It's a philosophical question. One could start calling a hunting rifle an accordion and vice versa, but it may lead to inefficient communication as not everyone will immediately get it if you ask them "Are you sure that your accordion is loaded?"
The thing about tradeoffs is that they are quantitative. I can think of only two ways to decide what is a good trade off and what is bad in this context. First, punch the data in a bearing calculator. Second, get data on later engines and compute their forces using the reciprocating and rotating masses at their redline rpms, select those that have forces similar to the 928, and then see what kinds of bearings are used in those engines.
As a point of reference, Subaru EJ25 has lighter components and 7k redline so the forces are somewhat lower than with the 928, and the bearings are 16.5mm long. That's more than 30% shorter than the 928 rod bearing.
The thing about tradeoffs is that they are quantitative. I can think of only two ways to decide what is a good trade off and what is bad in this context. First, punch the data in a bearing calculator. Second, get data on later engines and compute their forces using the reciprocating and rotating masses at their redline rpms, select those that have forces similar to the 928, and then see what kinds of bearings are used in those engines.
As a point of reference, Subaru EJ25 has lighter components and 7k redline so the forces are somewhat lower than with the 928, and the bearings are 16.5mm long. That's more than 30% shorter than the 928 rod bearing.