Supercharged '91 GT Refresh
01-08-2014, 10:40 PM
#121
Former Sponsor
Does anyone know anybody that ever rebuilt a 928 engine and didn't end up using the standard 928 101 901 07 main bearings? Anyone seen a crank with a main bearing journal worn below 69.971 mm? I'm sure it has happened, but even the 100K mile, regularly tracked GTS crank that I nearly killed by running the 2/6 rods dry had main journals that measured as new.
The crankshafts do not wear, unless something catastrophic occurs.
And this complete "lack of wear" has caused problems, over the years.
Not sure how many times I've told people how this works....could someone please highlight this...so I don't have to keep typing it?
Each and every crankshaft came with a paint dot on each main bearing and each rod bearing.....either blue, yellow, or red. The difference between these journals and their corresponding bearings is .01mm (.0004"), per color, per bearing shell. If you do the math...the difference between a set of "reds" and a set of "blues" is .04mm (.00158").
When the engines were assembled, the builder simply reached into his "bin-o-bearings" and plucked out the corresponding color bearings.....both mains and rods.....and assembled the engine.
If he had a "blue" journal, he used a set of "blue" bearings on that journal. If he had a "red" journal, he used a set of "red" bearings on that journal.
Perfect clearances, every time.
And for a few years, one could order the size bearing needed.
No longer.
Now, if you order a set of rod bearings or main bearings, you always get two "yellows" or one "blue" and one "red" in one box of shells. They will never send a double set of "blue" bearings or a double set of "red" bearings, in the same box. Never. The idea, from Porsche, is that the "average" crank will accept the "average" size bearings.
Not always true, unfortunately, and problems can and do result.
If you have a "red" journal and you somehow stuff a set of "blue" bearings on it....you are going to have a catastrophic failure. Guaranteed.
And most of the crankshafts were, indeed, "red"....which means they measured to the high side of the specification and needed the bearings with the most clearance.
This can be a huge "problem", when buying a set of Glyco rod bearings.....they don't put a set of double "yellows" or a "blue" with a "red", in one box.
If you order a set of Glyco rod bearings....they still allow the same tolerances when they make the bearings....but they don't get sorted. So it is possible to get a set of 16 rod bearings that are all "blue" (the tightest) and if you have an original "red" crankshaft (needed the bearings with the most clearance) you can/will be in deep trouble. It is also possible to get a set of all "red" bearings....and if you have an original "blue" crankshaft....you are going to need a really, really good oil pump!
And because the original bearings are very "soft", many people are now sending their bearings out to be coated. Not hard to imagine what happens if you add a little bit of coating onto an already tight set of bearings....no room for any oil!
Many 928 engines have failed from improper bearing clearance. You don't have to look very hard or very far to find a story about someone having a rod bearing failure shortly after installing a new set of Glyco rod bearings.
Measure carefully. The "colors" are long gone....they designed the "paint" to quickly disappear. If you have a crankshaft that measures to the high part (or low part) of the specification.....figure out a way to get a set of appropriate bearings. Buy multiple sets of bearings from Porsche and be prepared to mix and match to get the clearance you need. Buy several sets of Glycos from several different suppliers and pick through them to find the size you need. Scotch Brite the bearing shells, to get the clearance you need. Scotch Brite them more, and send them out to be coated. Send the crankshaft out and have it micro-polished to a smaller size, in the specification.
There's multiple ways to skin this cat, but you need to end up with the proper bearing clearance.
Oh yeah, the smart money puts about .0125mm more clearance into #2 and #6.....but that is a completely different story.
Last edited by GregBBRD; 01-09-2014 at 02:38 PM.
01-08-2014, 11:40 PM
#123
Supercharged
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Correct.
The crankshafts do not wear, unless something catastrophic occurs.
And this complete "lack of wear" has caused problems, over the years.
Not sure how many times I've told people how this works....could someone please highlight this...so I don't have to keep typing it?
Each and every crankshaft came with a paint dot on each main bearing and each rod bearing.....either blue, yellow, or red. The difference between these journals and their corresponding bearings is .01mm (.0004"), per color, per bearing shell. If you do the math...the difference between a set of "reds" and a set of "blues" is .04mm (.00158").
When the engines were assembled, the builder simply reached into his "bin-o-bearings" and plucked out the corresponding color bearings.....both mains and rods.....and assembled the engine.
If he had a "blue" journal, he used a set of "blue" bearings on that journal. If he had a "red" journal, he used a set of "red" bearings on that journal.
Perfect clearances, every time.
And for a few years, one could order the size bearing needed.
No longer.
Now, if you order a set of rod bearings or main bearings, you always get two "yellows" or one "blue" and one "red" in one box of shells. They will never send a double set of "blue" bearings or a double set of "red" bearings, in the same box. Never. The idea, from Porsche, is that the "average" crank will accept the "average" size bearings.
Not always true, unfortunately, and problems can and do result.
If you have a "red" journal and you somehow stuff a set of "blue" bearings on it....you are going to have a catastrophic failure. Guaranteed.
And most of the crankshafts were, indeed, "red"....which means they measured to the high side of the specification and needed the bearings with the most clearance.
This can be a huge "problem", when buying a set of Glyco rod bearings.....they don't put a set of double "yellows" or a "blue" with a "red", in one box.
If you order a set of Glyco rod bearings....they still allow the same tolerances when they make the bearings....but they don't get sorted. So it is possible to get a set of 16 rod bearings that are all "blue" (the tightest) and if you have an original "red" crankshaft (needed the bearings with the most clearance) you can/will be in deep trouble. It is also possible to get a set of all "red" bearings....and if you have an original "blue" crankshaft....you are going to need a really, really good oil pump!
And because the original bearings are very "soft", many people are now sending their bearings out to be coated. Not hard to imagine what happens if you add a little bit of coating onto an already tight set of bearings....no room for any oil!
Many 928 engines have failed from improper bearing clearance. You don't have to look very hard or very far to find a story about someone having a rod bearing failure shortly after installing a new set of Glyco rod bearings.
Measure carefully. The "colors" are long gone....they designed the "paint" to quickly disappear. If you have a crankshaft that measures to the high part (or low part) of the specification.....figure out a way to get a set of appropriate bearings. Buy multiple sets of bearings from Porsche and be prepared to mix and match to get the clearance you need. Buy several sets of Glycos from several different suppliers and pick through them to find the size you need. Scotch Brite the bearing shells, to get the clearance you need. Scotch Brite them more, and send them out to be coated.
There's multiple ways to skin this cat, but you need to end up with the proper bearing clearance.
Oh yeah, the smart money puts about .0125mm more clearance into #2 and #6.....but that is a completely different story.
The crankshafts do not wear, unless something catastrophic occurs.
And this complete "lack of wear" has caused problems, over the years.
Not sure how many times I've told people how this works....could someone please highlight this...so I don't have to keep typing it?
Each and every crankshaft came with a paint dot on each main bearing and each rod bearing.....either blue, yellow, or red. The difference between these journals and their corresponding bearings is .01mm (.0004"), per color, per bearing shell. If you do the math...the difference between a set of "reds" and a set of "blues" is .04mm (.00158").
When the engines were assembled, the builder simply reached into his "bin-o-bearings" and plucked out the corresponding color bearings.....both mains and rods.....and assembled the engine.
If he had a "blue" journal, he used a set of "blue" bearings on that journal. If he had a "red" journal, he used a set of "red" bearings on that journal.
Perfect clearances, every time.
And for a few years, one could order the size bearing needed.
No longer.
Now, if you order a set of rod bearings or main bearings, you always get two "yellows" or one "blue" and one "red" in one box of shells. They will never send a double set of "blue" bearings or a double set of "red" bearings, in the same box. Never. The idea, from Porsche, is that the "average" crank will accept the "average" size bearings.
Not always true, unfortunately, and problems can and do result.
If you have a "red" journal and you somehow stuff a set of "blue" bearings on it....you are going to have a catastrophic failure. Guaranteed.
And most of the crankshafts were, indeed, "red"....which means they measured to the high side of the specification and needed the bearings with the most clearance.
This can be a huge "problem", when buying a set of Glyco rod bearings.....they don't put a set of double "yellows" or a "blue" with a "red", in one box.
If you order a set of Glyco rod bearings....they still allow the same tolerances when they make the bearings....but they don't get sorted. So it is possible to get a set of 16 rod bearings that are all "blue" (the tightest) and if you have an original "red" crankshaft (needed the bearings with the most clearance) you can/will be in deep trouble. It is also possible to get a set of all "red" bearings....and if you have an original "blue" crankshaft....you are going to need a really, really good oil pump!
And because the original bearings are very "soft", many people are now sending their bearings out to be coated. Not hard to imagine what happens if you add a little bit of coating onto an already tight set of bearings....no room for any oil!
Many 928 engines have failed from improper bearing clearance. You don't have to look very hard or very far to find a story about someone having a rod bearing failure shortly after installing a new set of Glyco rod bearings.
Measure carefully. The "colors" are long gone....they designed the "paint" to quickly disappear. If you have a crankshaft that measures to the high part (or low part) of the specification.....figure out a way to get a set of appropriate bearings. Buy multiple sets of bearings from Porsche and be prepared to mix and match to get the clearance you need. Buy several sets of Glycos from several different suppliers and pick through them to find the size you need. Scotch Brite the bearing shells, to get the clearance you need. Scotch Brite them more, and send them out to be coated.
There's multiple ways to skin this cat, but you need to end up with the proper bearing clearance.
Oh yeah, the smart money puts about .0125mm more clearance into #2 and #6.....but that is a completely different story.
Thanks for your input. I've read you posts about the red, blue, yellow dotted bearing before but never really paid attention... Well guess what. I'm paying attention now. Makes sense to me now.
I replaced my Rod bearings several years ago with some glyco bearings and maybe I just got lucky. I will measure them, since I have everything apart. I will also look at the mains and clearances there too. Would appreciate any insight you have there when I get there.
If you've been able to get another batch of those super nifty intermediate shafts made, I'm in the market for one of those.
01-09-2014, 10:04 AM
#125
Rennlist Member
Correct.
The crankshafts do not wear, unless something catastrophic occurs.
And this complete "lack of wear" has caused problems, over the years.
Not sure how many times I've told people how this works....could someone please highlight this...so I don't have to keep typing it?
Each and every crankshaft came with a paint dot on each main bearing and each rod bearing.....either blue, yellow, or red. The difference between these journals and their corresponding bearings is .01mm (.0004"), per color, per bearing shell. If you do the math...the difference between a set of "reds" and a set of "blues" is .04mm (.00158").
When the engines were assembled, the builder simply reached into his "bin-o-bearings" and plucked out the corresponding color bearings.....both mains and rods.....and assembled the engine.
If he had a "blue" journal, he used a set of "blue" bearings on that journal. If he had a "red" journal, he used a set of "red" bearings on that journal.
Perfect clearances, every time.
And for a few years, one could order the size bearing needed.
No longer.
Now, if you order a set of rod bearings or main bearings, you always get two "yellows" or one "blue" and one "red" in one box of shells. They will never send a double set of "blue" bearings or a double set of "red" bearings, in the same box. Never. The idea, from Porsche, is that the "average" crank will accept the "average" size bearings.
Not always true, unfortunately, and problems can and do result.
If you have a "red" journal and you somehow stuff a set of "blue" bearings on it....you are going to have a catastrophic failure. Guaranteed.
And most of the crankshafts were, indeed, "red"....which means they measured to the high side of the specification and needed the bearings with the most clearance.
This can be a huge "problem", when buying a set of Glyco rod bearings.....they don't put a set of double "yellows" or a "blue" with a "red", in one box.
If you order a set of Glyco rod bearings....they still allow the same tolerances when they make the bearings....but they don't get sorted. So it is possible to get a set of 16 rod bearings that are all "blue" (the tightest) and if you have an original "red" crankshaft (needed the bearings with the most clearance) you can/will be in deep trouble. It is also possible to get a set of all "red" bearings....and if you have an original "blue" crankshaft....you are going to need a really, really good oil pump!
And because the original bearings are very "soft", many people are now sending their bearings out to be coated. Not hard to imagine what happens if you add a little bit of coating onto an already tight set of bearings....no room for any oil!
Many 928 engines have failed from improper bearing clearance. You don't have to look very hard or very far to find a story about someone having a rod bearing failure shortly after installing a new set of Glyco rod bearings.
Measure carefully. The "colors" are long gone....they designed the "paint" to quickly disappear. If you have a crankshaft that measures to the high part (or low part) of the specification.....figure out a way to get a set of appropriate bearings. Buy multiple sets of bearings from Porsche and be prepared to mix and match to get the clearance you need. Buy several sets of Glycos from several different suppliers and pick through them to find the size you need. Scotch Brite the bearing shells, to get the clearance you need. Scotch Brite them more, and send them out to be coated.
There's multiple ways to skin this cat, but you need to end up with the proper bearing clearance.
Oh yeah, the smart money puts about .0125mm more clearance into #2 and #6.....but that is a completely different story.
The crankshafts do not wear, unless something catastrophic occurs.
And this complete "lack of wear" has caused problems, over the years.
Not sure how many times I've told people how this works....could someone please highlight this...so I don't have to keep typing it?
Each and every crankshaft came with a paint dot on each main bearing and each rod bearing.....either blue, yellow, or red. The difference between these journals and their corresponding bearings is .01mm (.0004"), per color, per bearing shell. If you do the math...the difference between a set of "reds" and a set of "blues" is .04mm (.00158").
When the engines were assembled, the builder simply reached into his "bin-o-bearings" and plucked out the corresponding color bearings.....both mains and rods.....and assembled the engine.
If he had a "blue" journal, he used a set of "blue" bearings on that journal. If he had a "red" journal, he used a set of "red" bearings on that journal.
Perfect clearances, every time.
And for a few years, one could order the size bearing needed.
No longer.
Now, if you order a set of rod bearings or main bearings, you always get two "yellows" or one "blue" and one "red" in one box of shells. They will never send a double set of "blue" bearings or a double set of "red" bearings, in the same box. Never. The idea, from Porsche, is that the "average" crank will accept the "average" size bearings.
Not always true, unfortunately, and problems can and do result.
If you have a "red" journal and you somehow stuff a set of "blue" bearings on it....you are going to have a catastrophic failure. Guaranteed.
And most of the crankshafts were, indeed, "red"....which means they measured to the high side of the specification and needed the bearings with the most clearance.
This can be a huge "problem", when buying a set of Glyco rod bearings.....they don't put a set of double "yellows" or a "blue" with a "red", in one box.
If you order a set of Glyco rod bearings....they still allow the same tolerances when they make the bearings....but they don't get sorted. So it is possible to get a set of 16 rod bearings that are all "blue" (the tightest) and if you have an original "red" crankshaft (needed the bearings with the most clearance) you can/will be in deep trouble. It is also possible to get a set of all "red" bearings....and if you have an original "blue" crankshaft....you are going to need a really, really good oil pump!
And because the original bearings are very "soft", many people are now sending their bearings out to be coated. Not hard to imagine what happens if you add a little bit of coating onto an already tight set of bearings....no room for any oil!
Many 928 engines have failed from improper bearing clearance. You don't have to look very hard or very far to find a story about someone having a rod bearing failure shortly after installing a new set of Glyco rod bearings.
Measure carefully. The "colors" are long gone....they designed the "paint" to quickly disappear. If you have a crankshaft that measures to the high part (or low part) of the specification.....figure out a way to get a set of appropriate bearings. Buy multiple sets of bearings from Porsche and be prepared to mix and match to get the clearance you need. Buy several sets of Glycos from several different suppliers and pick through them to find the size you need. Scotch Brite the bearing shells, to get the clearance you need. Scotch Brite them more, and send them out to be coated.
There's multiple ways to skin this cat, but you need to end up with the proper bearing clearance.
Oh yeah, the smart money puts about .0125mm more clearance into #2 and #6.....but that is a completely different story.
01-09-2014, 10:06 AM
#126
Rennlist Member
I have said many times if I could, I would quit everything and go work for him for free.
01-09-2014, 11:22 AM
#127
Burning Brakes
I did stop by his shop on the way to a concert in Los Angeles. He was very kind, told me stories of his 1983 restoration with his son, and that gave me the idea of a Greg Brown Fantasy Camp. It didn't materialize because soon after bringing up the idea I was on the road for months at a time, but someday...
01-09-2014, 02:47 PM
#128
Former Sponsor
Imagine if all you had done, for forty years, was conduct the same piece of music, every single day. You'd know every little detail about that piece of music. Every note. every single little tiny nuance. You'd know which musicians could play the best particular instrument, in order to get the very best results.
You'd have studied other conductors and taken what good things they could do and added/modified your own technique using parts of their improvements. You'd have watched terrible conductors mess the entire piece of music up....rejecting everything they do, from them on out.
It's virtually the same thing....
01-09-2014, 03:07 PM
#129
Former Sponsor
Greg...
Thanks for your input. I've read you posts about the red, blue, yellow dotted bearing before but never really paid attention... Well guess what. I'm paying attention now. Makes sense to me now.
I replaced my Rod bearings several years ago with some glyco bearings and maybe I just got lucky. I will measure them, since I have everything apart. I will also look at the mains and clearances there too. Would appreciate any insight you have there when I get there.
If you've been able to get another batch of those super nifty intermediate shafts made, I'm in the market for one of those.
Thanks for your input. I've read you posts about the red, blue, yellow dotted bearing before but never really paid attention... Well guess what. I'm paying attention now. Makes sense to me now.
I replaced my Rod bearings several years ago with some glyco bearings and maybe I just got lucky. I will measure them, since I have everything apart. I will also look at the mains and clearances there too. Would appreciate any insight you have there when I get there.
If you've been able to get another batch of those super nifty intermediate shafts made, I'm in the market for one of those.
Porsche prefers to have their crankshaft be at the very "top" of the specification. That's the goal, when the crankshaft makers are making the cranks. Most end up there.....red journals, needing red bearings.
If the crankshafts wore a tiny bit, there would not be a problem...but because they are very resistant to any wear, they remain at these "high tolerances".
I recently got to go look at a couple of pallets of brand new 944 3.0 liter crankshafts that came into the country. Every single crankshaft was a "red" journal crankshaft. Given that the "average replacement bearing" was going to be a set of double yellows (if purchased from Porsche), anyone that assembles an engine from one of these crankshafts is going to have .02mm (.0008") tighter bearing clearance than if the engine was assembled, by Porsche.
Might work, if driven carefully, when new and when cold.
However, if someone gets a set of "blue clearance" bearings From Glyco) bad things are very likely going to happen, because there is now .04mm (.0016) of oil clearance missing. That's probably going to make bearing clearances that requires pre-warming the engine and the engine oil (very common in high output aluminum racing engine applications.)
Don't forget, a aluminum small block Chevrolet (with steel caps) grows over .002" in main bearing clearance (according to the Chevy engineers) from cold to warm. There's absolutely no reason our 928 engines don't do the same thing....
01-09-2014, 03:39 PM
#130
Nordschleife Master
How reasonable or realistic would it be to have a competent machine shop take a little bit off of the crank journals?
I.e. take it from "red" to "blue" tolerances?
Is there a hardened surface layer that would be affected?
Just curious, I hope I never have to go this far into my motor.
I.e. take it from "red" to "blue" tolerances?
Is there a hardened surface layer that would be affected?
Just curious, I hope I never have to go this far into my motor.
01-09-2014, 04:03 PM
#131
Former Sponsor
How reasonable or realistic would it be to have a competent machine shop take a little bit off of the crank journals?
I.e. take it from "red" to "blue" tolerances?
Is there a hardened surface layer that would be affected?
Just curious, I hope I never have to go this far into my motor.
I.e. take it from "red" to "blue" tolerances?
Is there a hardened surface layer that would be affected?
Just curious, I hope I never have to go this far into my motor.
I certainly have my own solutions and my own approaches to the unique problems found inside the 928 engine....since I build these engines on a continuous basis.
This is only one "stumbling block" in a long line of seemingly tiny little nuances with the 928 engines....the majority of which are completely overlooked.
These engines are far more complex and have a far larger amount of little details than people are aware of.
We've frequently marveled at how many of the engines that are disassembled, by unknowing people and "rebuilt", which either never run again or run for a very short period of time.
And in these totals, you can certainly add in the number of stroker engines that have either never ran or ran for very short periods of time....
It's really sad, for someone, like me, that loves these vehicles and loves what they were.
01-09-2014, 06:29 PM
#132
01-09-2014, 07:09 PM
#133
Greg - still .002 to .003 depending on many other variables for clearance?
01-09-2014, 09:30 PM
#134
Former Sponsor
The universal .001", per inch of journal diameter, never gets you too far away from what you should be doing.
Certainly, there are exceptions and reasons to "cheat" on this formula....but you better have a damn good reason!
Certainly, there are exceptions and reasons to "cheat" on this formula....but you better have a damn good reason!
01-09-2014, 09:34 PM
#135
Rennlist Member
So Greg, not to hijack Andrew's thread but if I just put glyco "standard" rod bearing in my GTS engine, I could be asking for trouble?