Very interesting read from CP-Carillo

[gectek]

Running a lower viscosity due to tighter clearances is not the proper way though, nor is running a higher weight the proper way to mask a poorly built engine. When I worked for a company that ran alot of engines through their shop, we noticed more people had a problem with running .002 than with .0025 even on a small journal sbc rod, which is the 2.000 size. The motors were built with the intent to be abused obviously, but when the machine shop did not check clearances good enough or they figured it would work, it wound up biting us in the ass. One engine lasted through 4 races and he won his event, but the engine was done after that. Rods were blue, so no reusing them either.

[TGP37]

On topic of different oil viscosity, I've read how lubricating an oscillating system requires different viscosity versus a rotating system such as the turbine shaft for a turbo spinning 120K+ rpms.

Something with jet engine oil used in a turbo closed oil system separate from the motor oil.

So what becomes neglected? The rotary lubrication (crank, cam, turbo) or oscillating lubrication (rods, rings, rockers)? Because the motor uses 1 oil type (lol obviously) and can only be optimal for one or the other.


Edit: Just learned how too much vacuum in the crankcase can lead to wrist pin failure. In part by a lack of oil vapor, I instantly remembered this thread.

If you wanted to put a moderate vacuum in the crankcase, using rods with wrist-pin oiling should alleviate that issue. Allowing for safe, strong crankcase vacuum.

[NateD4]

gectek I don't quite buy it. If the shop wasn't mic'ing the journals and bearings prior to assembly and then verifying with plasti-gauge then they could have had sub 0.002" clearances.

The oil viscosity, brand and type matter. 0.0025" is safe, but I certainly can't think of a single reason the proper oil with .002" bearings won't work. I've heard that some modern OEM engines run as tight as 0.0015" with 5W oil. It really gets down to pressure loading and heat break down capacity of the oil.

I really don't think clearances mask a bad engine build. Not checking them, and improper oil selection show a builder who doesn't care or was rushed.

There really are two schools of thought in the engine building world on bearing clearances. 1) Tight clearances and high oil pressure and 2) loose clearances and high volume for cooling.

You want tight bearings to control the crankshaft and reduce the impulse loads to the bearing surface. While you want loose bearings to prevent laminar flow drag from requiring high pump pressure and power. That way the oil can flow the friction heat away and out the sides of the rod/journal cheek faces.

I certainly expect #2 in a top fuel car where bearing loads are close to an order of magnitude more than a street car would be.

However I'd be willing to bet that a number of people on this forum are running turbos with 0.002" clearance on the rod bearings.

I'll also bet the ones that spun bearings spun them for one of a few reasons. 1) lower then required oil pressure, 2) lack of cross drilled oiling on their cranks, 3) not enough of a blend on the oil holes, 4) bearings that don't have a long enough oil hole.

Also you have to keep in mind the purpose of a tri-metal bearing is to absorb dirt. Race bearings don't have the soft later of metal on them that absorbs dirt. SO, having a clean oil system and top notch filter with the right oil are important.

I'd be curious the brand oil those with turbos and spun bearings are running.

Just my thoughts for whatever they are worth.

[NateD4]

TGP37, When you size a bearing you need to know the loads it is exposed to. You then select your width and diameter and run a friction analysis. This helps determine oil viscosity and to a degree a second round of sizing.

The rod journal size though is really a function of bearing loads, packaging and bending loads in the cranks (why the new 3.9L have large diameter journal).

The bearing designer is concerned about the bending of the rod journal and flexure of the rod cap. As the clearances are so tight a slight bend in the journal or rod cap can remove the clearance and break the oil film causing metal to metal contact.

In that regard the rigidity of the parts and load they see probably has as more to do with bearing clearances and viscosity then anything else.

When you get cyclic loadings (oscillating loadings) you see two things happen. An oscillating pressure load on the bearing and a deflection of the rod journal and main cap.

The pressure loading is a matter of figuring out the peak pressure at worst loading and using that for the sizing calculation.

The deflection of the rod journal is handled by eccentricity of bearings where the area near the bolt joint is larger in diameter then the primary loaded area. This adds extra clearance for the rod cap to flex in the rod axial direction, while the bolted sides actually deform inward tightening up the bearing.

Clevite/MAHLE discusses this in their literature on bearings.

If you read this it might help answer your question better.

http://www.mahle-aftermarket.com/C125790900540AC0/vwContentByKey/W28P2MXF358STULEN

http://www.aera.org/ep/downloads/ep8/EPQ409_26-27.pdf

As far as you question goes on jet engine oil. It is a complex question as the mutli-viscosity indexers found in common engine oil break down at high temperatures (as they are a type of plastic). While the other additives are added to clean and resist combustion products. Additionally the materials used in automotive seals can be quite different then aerospace seals in engines.

Jet turbine oils don't have the same additives because their requirements are different. Primarily the seal materials and thermal break down requirements.

Additionally jet engine oils are designed to operate at lower temperatures and pressures.


You might want to try to call one of the engine oil companies like Mobil to ask them. They make aircraft oil and auto oil.



Vaporizing the oil can be an issue, though I'm not super familiar with it, my recent quest for pistons has uncovered the use of DLC coatings on wrist pins to help with lubrication retention. Additionally in my case I'm running the 3900 block with oil squirters so oiling isn't as big a concern.


Again just my thoughts on the subject.

[TGP37]

Again just my thoughts on the subject.

By all means, I enjoy reading the high end stuff. You are definitely a knowledgeable person.

[NateD4]

A really good reference on bearings: http://www.mahle-aftermarket.com/C125790900540AC0/vwContentByUNID/97C716A640FD0B1AC12579AB0062923E/$FILE/EB-40-07.pdf

Some may have already read this, for everyone else I consider it a must read if you want some insight into bearing clearances.

[gectek]

You do not have to buy it. I saw it. You do not believe what I sold, saw and found, then you do not have to. It is a fact though. And if you have a bore gauge as sophisticated as this machine shop, then plastigauge is for shade trees.

[NateD4]

It's not a question of buying it or not. It's engineering.

I'm not sure you read all that I wrote following your comment. There is no 'correct' clearance. It is a matter of application. You cant run .0025 clearances in every engine and expect them to work. There is a bit of math and experimentation involved. Read the entire Clevite catalog I posted they talk about much of what I mentioned and discuss some additional things regarding clearances.

Running a lower viscosity due to tighter clearances is not the proper way though, nor is running a higher weight the proper way to mask a poorly built engine. When I worked for a company that ran alot of engines through their shop, we noticed more people had a problem with running .002 than with .0025 even on a small journal sbc rod, which is the 2.000 size. The motors were built with the intent to be abused obviously, but when the machine shop did not check clearances good enough or they figured it would work, it wound up biting us in the ass. One engine lasted through 4 races and he won his event, but the engine was done after that. Rods were blue, so no reusing them either.

As your quote above states: "but when the machine shop did not check clearances good enough or they figured it would work, it wound up biting us in the ass"

A few comments on that 1) If they didn't check the clearances at all whether by bore and micrometer or plasti gauge I'm not at all surprised things bit them in the ass. 2) If they did use some super fancy bore gauge to size them that isn't the whole picture.

You have to realize that adding an extra 0.0005" clearance is really a safety margin due to improper engineering. The people who design OEM car engines spend millions of dollars to determine the exact bearing clearances for a given engine. Any race shop with perhaps a few top level race engine shops don't have the budget or time to determine an exact clearance, so they use rules of thumb.

There is another point you didn't mention in the above statements. Which is what eccentricity bearing they were running and what RPM they ran. It is very possible to run 0.0025 or 0.003" clearance and still fail a rod bearing due to high flexure in the rod cap causing an out of round condition of the bearing bore. In such cases it is possible to break the oil film by straining the cap. High end racing bearings have extra eccentricity to allow for this.

As I said previously there is a lot that goes into picking .002" or 0.0025" clearances and the oil to run with them. Additionally crank flex and rod flex play a huge role in picking the clearance. In all reality a properly built engine has the bare minimum clearance to allow the oil to flow properly for cooling to help eliminate component flex. Additionally components in the bottom end should be designed with sufficient rigidity so that they don't break oil film due to flexure. This doesn't happen all the time.

A good example is a Pontiac or Olds engine. Which have rod journal diameters almost 2.5", while nascar is running a narrow bearing with a 1.88" honda journal (per Celvite's data).

One bearing will last longer than another, one will have more drag as well. All is a trade off. So you really have to know your engine in order to get the right clearances.

If the rods failed on that engine it could have been for a number of reasons. Too loose main clearances, improper radius of the oil holes, poor oil hole diameter selection or a slew of other interacting factors.

When I asked my local machinist how he picks bearings he says we put something in with a bit extra clearance and then run it a season and see what the wear pattern is like, then adjust to get what we want. Not an exact science but I guess it works considering the effort required to engineer an exact solution.

[gectek]

It's not a question of buying it or not. It's engineering.

I'm not sure you read all that I wrote following your comment. There is no 'correct' clearance. It is a matter of application. You cant run .0025 clearances in every engine and expect them to work. There is a bit of math and experimentation involved. Read the entire Clevite catalog I posted they talk about much of what I mentioned and discuss some additional things regarding clearances.



As your quote above states: "but when the machine shop did not check clearances good enough or they figured it would work, it wound up biting us in the ass"

A few comments on that 1) If they didn't check the clearances at all whether by bore and micrometer or plasti gauge I'm not at all surprised things bit them in the ass. 2) If they did use some super fancy bore gauge to size them that isn't the whole picture.

You have to realize that adding an extra 0.0005" clearance is really a safety margin due to improper engineering. The people who design OEM car engines spend millions of dollars to determine the exact bearing clearances for a given engine. Any race shop with perhaps a few top level race engine shops don't have the budget or time to determine an exact clearance, so they use rules of thumb.

There is another point you didn't mention in the above statements. Which is what eccentricity bearing they were running and what RPM they ran. It is very possible to run 0.0025 or 0.003" clearance and still fail a rod bearing due to high flexure in the rod cap causing an out of round condition of the bearing bore. In such cases it is possible to break the oil film by straining the cap. High end racing bearings have extra eccentricity to allow for this.

As I said previously there is a lot that goes into picking .002" or 0.0025" clearances and the oil to run with them. Additionally crank flex and rod flex play a huge role in picking the clearance. In all reality a properly built engine has the bare minimum clearance to allow the oil to flow properly for cooling to help eliminate component flex. Additionally components in the bottom end should be designed with sufficient rigidity so that they don't break oil film due to flexure. This doesn't happen all the time.

A good example is a Pontiac or Olds engine. Which have rod journal diameters almost 2.5", while nascar is running a narrow bearing with a 1.88" honda journal (per Celvite's data).

One bearing will last longer than another, one will have more drag as well. All is a trade off. So you really have to know your engine in order to get the right clearances.

If the rods failed on that engine it could have been for a number of reasons. Too loose main clearances, improper radius of the oil holes, poor oil hole diameter selection or a slew of other interacting factors.

When I asked my local machinist how he picks bearings he says we put something in with a bit extra clearance and then run it a season and see what the wear pattern is like, then adjust to get what we want. Not an exact science but I guess it works considering the effort required to engineer an exact solution.

I am sure you just agreed with me when you said that there is experimentation involved. I am telling you this. In a real life machine shop that does hundreds of motors for Pro Mod dragsters, hundreds of engines for nascar style races in south america, and hundreds of motors for the LS community, when they ran less than .002-.0025 the mains burned up.

These engines are built with new parts every time, but also as you can imagine, they leave nothing to the manuf and check it all themselves. Results dont lie.

And as far as what I said about them no checking like they should, did I ever say they did not check them at all? No. They had some people that were new hires that were supposed to check them, and they did in their way, not the companies way. The other problem is people deliberately asking them to build them with a certain clearance. When the company did that, the customers got what they asked for, but def not what they wanted.

I mean honestly, what else do you drive besides a search engine? Get in the real world. I would trust a company that works with it every day rather than a company that manufactures the parts that can be used in any way the end user wants, but their tolerances and batches have shown to be crap quite often.

[NateD4]

No, I don't think I quite agreed with you. You made it sound like running 0.0025" clearance is the only thing that will work, which isn't true.

Additionally if the shop didn't check clearances there's something wrong. I can't imagine any Pro Mod or Nascar style engine builders not checking clearances. That's just unfathomable.

Maybe I'm a bit odd, but everything should get checked 3 times.

There are soo many factors that can attribute to bearings burning up that you can't simply point the finger at the clearances and say 'yep, thats the problem'. What oil pressure were they running? What oil temps? What type and brand of oil? What Viscosity? What bearing types were they running? What was the surface finish on the crank? What was the surface deviation of the journal? What heat treat were the journals put at? How much rod deflection did they have? How clean were the parts on assembly? What kind of oil hole lead ins are there? What size oil holes? What oil options did the bearings come with? What are the clearances of the main bearing caps?

Unless you can answer all these you really can't accurately say the clearances were wrong. Perhaps they were correct for the builds but usually extra clearance is just the easy way to fix a slew of other hidden problems.


As an example of hidden problems: I found the mains on my LZG block are off by .0003 out of plane. This was after 3 measurement sessions on a CMM machine. I also found out without the caps on they were out of round by almost a thousandth (not surprising considering the load the mains puts on the cast iron). Also the block shifted (or perhaps because the caps are off) and the centers of the mains are off by a max of 0.0003". This means the main bearing bores are not all on the same center. An extra .0005" gives me an extra 0.0002" margin simply on the non concentric main bores.

I doubt many shops go to that level of detail to make sure everything fits. But I can say blocks do some weird things which is one of the reasons you add extra clearances especially on high performance engines. The amount of clearance will depend on some of the above factors. But that doesn't mean there is a one size fits all clearance that 'has' to be run on a specific engine. I'll probably be running 0.0020" clearance on the LZG with turbo, we'll see if I damage a rod bearing or not.

[gectek]

Checking with plastigauge is not the norm. Most clearances the first time are checked in at least 3 places. They are also checked more than once. The machine shop does not assemble those other engines NORMALLY since they do all the extra stuff, but they set the clearances up to what is specified.

I never meant to imply that .002 was never acceptable, but that from my own experience and the machine shops exp, .0015 is too tight. You started spouting rule of thumb, but then changed to exact exact measurements. Its all over the place. I can tell you what happened with our engines. I can tell you how they were set up. You are trying to delve too far into something honestly to defend your point.

We had 2 engines being ran in mud bog race trucks in canada. One was set up at .0025 and one at .002. Both had the same parts go into them. Identical save for one had a diff style carb intake on it and camshaft. They ran close to the same oil pressure as i was in contact with the guy who ran them, but the one with the tighter clearances had the engine run a tad hotter. You can tell what that was from.

[NateD4]

Checking with plastigauge is not the norm. Most clearances the first time are checked in at least 3 places. They are also checked more than once. The machine shop does not assemble those other engines NORMALLY since they do all the extra stuff, but they set the clearances up to what is specified.

I never meant to imply that .002 was never acceptable, but that from my own experience and the machine shops exp, .0015 is too tight. You started spouting rule of thumb, but then changed to exact exact measurements. Its all over the place. I can tell you what happened with our engines. I can tell you how they were set up. You are trying to delve too far into something honestly to defend your point.

We had 2 engines being ran in mud bog race trucks in canada. One was set up at .0025 and one at .002. Both had the same parts go into them. Identical save for one had a diff style carb intake on it and camshaft. They ran close to the same oil pressure as i was in contact with the guy who ran them, but the one with the tighter clearances had the engine run a tad hotter. You can tell what that was from.

That is the risk of inspecting at 3 points. Today the best way to inspect is electronically with a laser scan or similar tool. But I don't think the OEMs even use this method. Though I bet they have something similar.

If the shop checks them every time more than once and the parts are measured at the same temperature.... they should fit properly.

Without other data this leads me to believe there is an issue with the grind of the crank journal. Clevite's literature gives a surface ripple requirement for surface deviations (this is different than the Ra roughness).

If all that checks then who knows. Plastigauge should be the last line to checking clearances. Though I think it is more then adequate on its own if the crank is manufactured properly.

I'm not trying to delve to far into this, but rather have a discussion and perhaps learn something in the process (or allow others to learn).

In that type of extreme racing I can understand the temp differences. However oil type matters as does the rigidity of the crank and most importantly the viscosity.

Any oil that isn't right for the clearances is going to get hot simply due to shear stresses in the bearings and oil system.

Any chance they ran it on a different viscosity oil?


Interesting discussion.

[gectek]

They only run a few weights of oil due to the intense cold up there and the tried and true oil they have up there. Their oil requirements are alot diff though, but since the oil companies use the same base stock for our oil, or theirs, then it does not matter really.