Oil Clearance Theories for stout motors

[Mad_Trbo]

Just a quick question for the experts.

I have been reading about oil clearances for the rods and mains. I am in the midst of a rebuild and would like to gain a better understanding of the importance of tighter vs loser clearances depending on the use of the motor you are building.

My focus is primarily why the experts prefer to have a loser set of oil clearances when building a performance motor vs your daily grocery getter. I know oil clearance on the mains dictates oil pressure when looking @ mains vs rods. Why would you want loser, seems like that means low pressure.


And then what dictates performance, if I am building a street car but I want it to have power levels ranging from 750 - 800 hp. Does that mean I need to follow the school of thought for race or hi po engines? I obviously want to have proper oil pressure for daily driver.

Finally with rods, I have read on the board that half a thousandths is way to close even though it is listed as ok by GM. I understand that is on the tight side and you probably never want to be at the end of either side of the spec. But say I have 1 thousandths clearance on the rods with a set of coated bearings. Am I really in trouble although not in the middle of GM specs I am not on the absolute tight end either.

Any help and guidance much appreciated as I continue to learn the nuances of building a stout motor. Any reading suggestions appreciated.

 

[Lee Thompson]

Here we go again

750 HP with .001 on the rods and .0012-.0014 on the mains. Great oil pressure with a blueprinted stock front cover, smaller cam bearing oil feed holes and oil passages radiused. Hot idle is 25 psi with 65 psi @ cruise. I do have a LARGE external oilcooler behind front mount.

 

[BadAssGN]

750 HP with .001 on the rods and .0012-.0014 on the mains. Great oil pressure with a blueprinted stock front cover, smaller cam bearing oil feed holes and oil passages radiused. Hot idle is 25 psi with 65 psi @ cruise. I do have a LARGE external oilcooler behind front mount.

What size are you drilling your cam bearings ?

 

[turbocody]

it's my understanding that the extra clearances on the rod/mains is for an extra cushion for oil to keep from eating the bearings up. and with all the FLEX that goes on with the crank/block,etc.....it would makes sense. my 2cents

 

[Mad_Trbo]

750 HP with .001 on the rods and .0012-.0014 on the mains. Great oil pressure with a blueprinted stock front cover, smaller cam bearing oil feed holes and oil passages radiused. Hot idle is 25 psi with 65 psi @ cruise. I do have a LARGE external oilcooler behind front mount.

Maybe a poll is in order, but curious to know how long you been running these sort of clearances. Have you had an opportunity to pull a motor a part that had these clearances and if so. What did the bearings look like?

 

[jdpolzin]

I have always heard that .0020 is pretty much a good goal on both rods and mains.

 

[Mad_Trbo]

I have always heard that .0020 is pretty much a good goal on both rods and mains.

Understood, but I am trying to get a little deeper than hearsay. I was hoping someone could dig into the advantages and disadvantages of going tighter or loser.

Or at least provide a link to some information.

 

[Alky V6]

In a high performance engine (HP levels above OEM levels), heat buildup and oil film breakdown in the bearings becomes a problem. The extra clearance (more than OEM specs) is to allow more oil flow to help cool the bearings. Also, at higher HP levels than OEM, more flexing of the bottom end components including some deforming of the rod big end and pinching of the bearing shells take place. As was mentioned earlier, the extra clearance helps to avoid metal to metal contact when all this flexing and deforming starts to take place.

Higher bearing loads because of higher HP levels than OEM will usually require a heavier oil to help maintain a stable oil protection film. Heavier oils need extra clearance so that a certain level of free flow can take place in the bearing clearances for cooling purposes.

Having clearances that are too tight for the HP levels the bearings are seeing can lead to overheating at the bearings due to inadequate cooling oil flow.

Keep in mind that oil not only prevents metal to metal contact, but must also carry away heat buildup in the bearing clearance.

 

[Lee Thompson]

What size are you drilling your cam bearings ?

#1 --no change
#2-3-4--.080 hole

 

[turbofabricator]

I have always shot for .0017" but no more than .002" Any more and you'll need 20w-50 oil just to fill the "gap". Tighter seems to be the trend these days in everything I have been involved in. Everything from Sprint cars, drag, and endurance circle track. But this is a topic that has just as many theories as to who killed Kennedy. If .0024" has worked for you, then keep using it. I will continue to set my engine clearances between .0017-.002" I have not had a bearing problem in the last 15 years or so ('cept thrust bearings, which is another topic). Good oil pump mods and good oil passage mods will go a long way in keeping everything together. Never leave anything overlooked. I even set the timing chain roller clearance to .00175" :biggrin: I have set StageII engine clearances to .0025" and they have worked great, but it also had a dry sump pump hangin' on it, too. The stock oil pump is questionable at best. It works inspite of itself. Now back to more theories in oil clearance................

 

[Alky V6]

I have always shot for .0017" but no more than .002" Any more and you'll need 20w-50 oil just to fill the "gap". Tighter seems to be the trend these days in everything I have been involved in. Everything from Sprint cars, drag, and endurance circle track. But this is a topic that has just as many theories as to who killed Kennedy. If .0024" has worked for you, then keep using it. I will continue to set my engine clearances between .0017-.002" I have not had a bearing problem in the last 15 years or so ('cept thrust bearings, which is another topic). Good oil pump mods and good oil passage mods will go a long way in keeping everything together. Never leave anything overlooked. I even set the timing chain roller clearance to .00175" :biggrin: I have set StageII engine clearances to .0025" and they have worked great, but it also had a dry sump pump hangin' on it, too. The stock oil pump is questionable at best. It works inspite of itself. Now back to more theories in oil clearance................

Crank thrust problems. Let me guess. A turbo 400 trans? If it is, give me a call. I may be able to help you with that.

 

[Mad_Trbo]

Good stuff

In a high performance engine (HP levels above OEM levels), heat buildup and oil film breakdown in the bearings becomes a problem. The extra clearance (more than OEM specs) is to allow more oil flow to help cool the bearings. Also, at higher HP levels than OEM, more flexing of the bottom end components including some deforming of the rod big end and pinching of the bearing shells take place. As was mentioned earlier, the extra clearance helps to avoid metal to metal contact when all this flexing and deforming starts to take place.

Higher bearing loads because of higher HP levels than OEM will usually require a heavier oil to help maintain a stable oil protection film. Heavier oils need extra clearance so that a certain level of free flow can take place in the bearing clearances for cooling purposes.

Having clearances that are too tight for the HP levels the bearings are seeing can lead to overheating at the bearings due to inadequate cooling oil flow.

Keep in mind that oil not only prevents metal to metal contact, but must also carry away heat buildup in the bearing clearance.

I would venture to guess there is probably no formula for determining how you should open oil clearances as you increase horsepower levels.

I have heard that .0015 - .0017 is good place to be for the rods. Thanks for the discussion.

I had my rod journals turned .010 and wnet with a coated bearing from calico that lead to oil clearances of .0009.

I am slightly annoyed, I explained to the machinist that I was building a motor that would be in the 700 hp range and he said go 10 under with 10 undersize bearings and that would put me where I needed to be.

No for getting that extra 5 - 7 ten thousandths extra material off the journals. I should be able to go for a polishing and this should take care of it, agreed?

 

[Alky V6]

If the rod big end bore size is on the tight side, have it sized out a little to cause a little less bearing crush. Don't go larger than the recommended high side limit of the big end bore size. I would do this before polishing the crank that much. If after the rod resizing still leaves you on the tight side, hopefully, you won't need to polish off so much of the crank to get your clearance. The heat treatment of the crank journals are very thin and you could polish through it.

 

[Mad_Trbo]

Not following, I already had the crank grinded .010 under. Did this go through the heat treatment?

 

[Alky V6]

Not following, I already had the crank grinded .010 under. Did this go through the heat treatment?

If it hasn't been treated, you should have it done. Tuftriding or nitriding are two available processes.

 

[Lee Thompson]

Maybe a poll is in order, but curious to know how long you been running these sort of clearances. Have you had an opportunity to pull a motor a part that had these clearances and if so. What did the bearings look like?

Have been running these clearances for many years. Bearing look great. Everything must be RIGHT to run tight clearances. The Buick stock oiling system has many 90 degree turns to make before it gets to the job site as compared to say a sbc. If it is a race only eng. or you have a drysump system then I can go along with .002 + clearances. The Buick Power Source book show the larger clearances but keep in mind these are dry sump oiling systems. Hardly a week goes by without a post on low oil pressure. But it is your eng. do as you please.

 

[BlackBandit]

delete

 

[Alky V6]

Me too.

 

[BlackBandit]

???

You are correct sir. After actually using a calculator I realize that I'm an idiot. I'll remove the above post so as not to confuse anyone. Thanks. james

 

[Alky V6]

You are correct sir. After actually using a calculator I realize that I'm an idiot. I'll remove the above post so as not to confuse anyone. Thanks. james

They're called brain farts. Some say they never do it (like my wife) or they can hold it forever (sure), but we all know everyone does it.