Bearing clearances for high rpm sbc?

[Walter R. Malik]

In my opinion ... Large clearances are usually a band-aid for suspect parts or shoddy machine work, or both.

What would be large clearances to you?

More than .001" per inch of diameter; unless there is enough torque made to distort parts more than that.

[Walter R. Malik]

Darin Morgan stated awhile back......that he hadn't run less than .003 or .0035 on any clearance, rod or main, in any engine in many many years. I believe that is verbatim. And I have a good feeling he uses NO shoddy parts. And I also have a feeling that he has probably built, or been involved with building more horsepower than everyone on this thread combined. Just something to consider.

Just something to consider ...
Jon Kaase is a member here.

[dwilliams]

Callies has some graphs of bearing clearance vs. load capacity and oil flow. They show a "sweet spot" around .002."

Their graphs are in line with the SAE and IMechE papers I have.

[statsystems]

Callies has some graphs of bearing clearance vs. load capacity and oil flow. They show a "sweet spot" around .002."

Their graphs are in line with the SAE and IMechE papers I have.

Have a link to the graphs? I tried to find them and didn't see them.

Thanks

[DaveMcLain]

Darin Morgan stated awhile back......that he hadn't run less than .003 or .0035 on any clearance, rod or main, in any engine in many many years. I believe that is verbatim. And I have a good feeling he uses NO shoddy parts. And I also have a feeling that he has probably built, or been involved with building more horsepower than everyone on this thread combined. Just something to consider.

Those recommendations are ludicrous for a build like this guy is wanting to do. I'd recommend shooting for about .002 on the rod and .0025 on the mains, I think that's plenty with any reasonable oil.

[BlitzA64]

"A little too loose and you know, a little too tight and everyone knows" not sure if this was Confucius or not???!!!

[nickpohlaandp]

.0028/.0022 With a very strait crank.

How do you know the crank is straight when running?

Well you probably can't tell for 100% sure what's going on with the crank when it's running unless you have some badass telemetry sensors hooked up to pick up vibrations and interpret those vibrations into usable data, but generally when you're looking at straightness of the crank while it's running you check crank runout prior to installation (too much and get a different crank) and then go for good balancing. If your runout is good and your rotating assembly is properly balanced, your crank, theoretically, should be running straight.

Edit: The way I check runout (others may have different ways) is to install the #1 and #5 main bearing in the block, put the crank in, and put a dial indicator on the center main journal. Spin the crank over and see how much your needle moves. In a perfect world it wouldn't. realistically you'll see a little movement, maybe 0.001"

[engineguyBill]

How do you know the crank is straight when running?
[/quote]

If the crank checks-out straight prior to, or during, engine assembly it will remain relatively straight while running in a NA engine combination. Excessive crankshaft flex usually shows up in blown and/or turbocharged applications - especially when exotic fuels such as nitromethane are used. Some stroked crankshafts "may" exhibit some flex at high RPM is the main pin to rod pin overlap is significantly reduced - or nonexistent.

[engineguyBill]

[/quote]

Edit: The way I check runout (others may have different ways) is to install the #1 and #5 main bearing in the block, put the crank in, and put a dial indicator on the center main journal. Spin the crank over and see how much your needle moves. In a perfect world it wouldn't. realistically you'll see a little movement, maybe 0.001"
[/quote]

That's the way I check for runout, too. - works for me. have also chucked the crank in the lathe and turned the crank by hand with dial indicator on the mains. The result is exactly the same as with the in-block method described in the previous post.

[DaveMcLain]

Edit: The way I check runout (others may have different ways) is to install the #1 and #5 main bearing in the block, put the crank in, and put a dial indicator on the center main journal. Spin the crank over and see how much your needle moves. In a perfect world it wouldn't. realistically you'll see a little movement, maybe 0.001"
[/quote]

That's the way I check for runout, too. - works for me. have also chucked the crank in the lathe and turned the crank by hand with dial indicator on the mains. The result is exactly the same as with the in-block method described in the previous post.
[/quote]

Then what does the crank do when you press on it with your thumb?

[Fireonthemountain]

Things touch boom. Not good.

And what's the actual harm of bigger clearances?? Oil pressure at idle really matters little, as long as it jumps up with the rpms to good pressure numbers.

There are cranks that turn into "wet noodles" at higher rpms. Then you get into block flex and all the rest.

[needforspeed66gt]

In my opinion ... Large clearances are usually a band-aid for suspect parts or shoddy machine work, or both.

This.

I rarely ever run over .001" per inch assuming good quality parts, and of course, precise machine work.

The cup engines run just under .001" per inch and they're spinning almost 10k rpm...if you use cheap parts that move too much under load, skip line honing the block, or you just take the rods out of the box and torque them up, that's when you'll spin a bearing and then blame the oil clearance rather than the machine work or assembler's attention to detail.

[cjperformance]

Edit: The way I check runout (others may have different ways) is to install the #1 and #5 main bearing in the block, put the crank in, and put a dial indicator on the center main journal. Spin the crank over and see how much your needle moves. In a perfect world it wouldn't. realistically you'll see a little movement, maybe 0.001"

That's the way I check for runout, too. - works for me. have also chucked the crank in the lathe and turned the crank by hand with dial indicator on the mains. The result is exactly the same as with the in-block method described in the previous post.
[/quote]

Then what does the crank do when you press on it with your thumb?
[/quote]

Exactly ...

[MadBill]

...And what's the actual harm of bigger clearances?? Oil pressure at idle really matters little, as long as it jumps up with the rpms to good pressure numbers...

The load capacity of the "hydrodynamic wedge' that supports the crank and rods drops rapidly as the clearance increases, so it's important to not run much more than enough to ensure distortion doesn't cause parts to 'ground out'. i.e., the ideal rod/main clearance in a 500 HP @ 7,000 RPM SBC might be 0.002"/0.0025", but the identical parts in a 1500 HP blown 8500 RPM application might need a thous or more greater.

On the other hand, the flow to maintain sufficient pressure will also increase with more clearance, thus increasing the oil's capacity for carrying off heat..

[cjperformance]

...And what's the actual harm of bigger clearances?? Oil pressure at idle really matters little, as long as it jumps up with the rpms to good pressure numbers...

The load capacity of the "hydrodynamic wedge' that supports the crank and rods drops rapidly as the clearance increases, so it's important to not run much more than enough to ensure distortion doesn't cause parts to 'ground out'. i.e., the ideal rod/main clearance in a 500 HP @ 7,000 RPM SBC might be 0.002"/0.0025", but the identical parts in a 1500 HP blown 8500 RPM application might need a thous or more greater.

On the other hand, the flow to maintain sufficient pressure will also increase with more clearance, thus increasing the oil's capacity for carrying off heat..

Dont start talking pressure Bill !, that will start a whole new debate! haha. But yes more clearance means more potential flow (if the pump is able)BUT typically with more clearance an according increase in oil viscosity choice would take place which theoretically will then increase the observed pressure , SO , for instance have we achieved more 'flow volume' going from say .002 with x oil and y pressure when compared to say .003 on the same bearing with x'thicker' oil to achieve the same y pressure ? Or have we simply achieved a resistance to flow that shows the same pressure reading BUT has a different flow volume? i think ive just confused myself! So 2 ways to tackle opening up the clearance, ,,more flow volume same vis lube,, OR same flow volume increase vis of lube.