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Old 02-27-2007, 10:16 AM   #1
dug-e-fresh
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Default Thoughts on main bearing clearances...

I wanted to gather some thoughts on what people consider to be the best way to clearance the main bearings on our cranks.

The factory calls for a tight 0.0004-0.0012" clearance on the main and 0.0012-0.0045" clearance for the main thrust.

The recommendations I recieved for a "race" motor called out for 0.0020" on the mains with no specific thrust clearance (I assume factory specs would be fine).

That said, what are the your thoughts on 0.003 for both? I was told that this would significantly reduce system oil pressure, by as much as 15 psi.

Obviously thats not too good for high rpm operation... any other thoughts? I was told that the motor would rev extra quick, it just wouldn't be able to safely rev that high... I am talking 8500-9000 rpms.

Assuming 0.003/0.003 is bad for high rpm, what are your thoughts on 0.0025/0.003?

Would this be ok for near 9000 rpm operation? I know most things in life are not linear, but that may only be a 7-8 psi pressure drop. Assuming an EJ22T pump shimmed for extra pressure... thoughts??

Yea I know, when it comes to oil pressure, "only" should never be used...

def
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Old 02-27-2007, 10:32 AM   #2
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Loose is fast?



Well, I suppose it really depends on how long you want the motor to last. You know with extra clearances you'll have extra wear, it just depends on how much you're really willing to accept.
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Old 02-27-2007, 12:38 PM   #3
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I just did this on my crank. I had two sets of specs, Subaru and those of a well-known Subaru engine builder. The builder recommended 0.0018-0.0022 on the mains and 0.0012-0.0016.

Unless you are a machinist, you are not going to get an exact number, and there will be variation.

I ended up with 1.6, 2.0,1.6, 2.0,1.6 thousands on the mains plus or minus a tenth or two.

All the rods were at 1.4-1.5 thousands.

Your main issue with loose is oil pressure. Too loose and you'll not have sufficient oil pressure. If you inspect the Cosworth bearing very closely, you'll see that they are ACL HX bearings, which means they have an extra thousandth of clearance. Now Cosworth may have have their bearings specially made, but I'd guess they did not. So, you are looking at 0.001 over Subaru spec if you use those bearings. So, Cosworth and the most experienced and trustworthy engine builder I know agree that's about the limit before "loose is too loose."
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Old 02-28-2007, 12:50 AM   #4
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man, this brings me back to my high perfromance piston engines course...I think it was chapter 7...all kinds of fun calcs for this one...
you can see what you would actually gain and loose (theoretically) with one vs. the other and make your own decision based on what we know to be the faulty point in high rpm operation.

I'm building my first factory tight...the second shortblock is getting all kinds of fun experiments.

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Old 02-28-2007, 12:53 AM   #5
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I would think 2.5 thousansds
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Old 02-28-2007, 03:21 AM   #6
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Rule of thumb; .001 of clearance for every inch of shaft dia. i.e. 2 inch shaft would get .002 clearance.

A little loose is better than a little tight.

You can have those ACL bearing, just had some start to come apart, same clearance as a Subaru bearing .0015 the Subaru bearings look fine.
Saw some particles in the oil filter, removed the engine and tore down, crank was fine, replaced with Subaru bearings.
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Old 02-28-2007, 08:05 AM   #7
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I recommend to stick with subaru factory upper limits. These engines are designed to run with tight clearances. We use coated bearings in order
to run almost stock clearances in high performance applications.

I agree, ACL HX bearing (cosworth) usually gives you too much
main clearance in stock block, causing oil pressure to drop.

Thanks, Tomi
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Old 02-28-2007, 10:07 AM   #8
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The factory mains have a 2.36" nominal diameter. So "rule of thumb" would yield 0.00236" for clearance.

My clearances are 0.0025"for the mains and 0.0030" for the thrust.

Talking to some Subaru engine builders, they seem to think that 0.0025 would be fine for a smooth, quick revving, reasonably high rpm motor. But 0.0030" would be too much of a trade off (loss of oil pressure).

def
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Old 02-28-2007, 11:25 AM   #9
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I'm going subaru bearings with wpc treatment...that way I can should be just a tad loose from the factory but still retain that oil layer because of the treatment...I'll see if I eat my first billet crank

regardless, the real problem with our engines is still the oil pressure/volume
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Old 02-28-2007, 01:08 PM   #10
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I'm setting my new 2.5L closed deck block (thrust #3) up with 0.0015 main and 0.002 rod. I'm using ACL RACE series bearings, un-coated. These are not the duraglides.

Cobb Tuner Crank, Pauter rods so I hope the bearings are fine as that's a lot of cash down the drain is the bearings fail.

Quote:
You can have those ACL bearing, just had some start to come apart, same clearance as a Subaru bearing .0015 the Subaru bearings look fine.
Were these the ACL Duraglides or RACE series?
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Old 02-28-2007, 11:35 PM   #11
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Quote:
Originally Posted by mick_the_ginge View Post
I'm setting my new 2.5L closed deck block (thrust #3) up with 0.0015 main and 0.002 rod. I'm using ACL RACE series bearings, un-coated. These are not the duraglides.

Cobb Tuner Crank, Pauter rods so I hope the bearings are fine as that's a lot of cash down the drain is the bearings fail.



Were these the ACL Duraglides or RACE series?
I bought these bearing about 2 years ago, from a shop that is on this form. I check the clearance back then, check good, installed on crank and did not like the feel, so I removed them and reinstalled the Subaru bearings. Two years of hard abuse, pulled the motor, bearing still looked good, had crank checked and re-polished. Checked clearance on ACL bearings, installed and still did not like the way they felt, but decided to run them (1/2 Subaru 1/2 ACL). 2 dyno sessions and one trip, started to see some debris in oil. Pulled engine and inspected. ACL bearing were coming apart. Subaru’s look fine.

ACL coated bearings

When I bought them, I talked on the phone about these bearings; they said they were better than Subaru, NO WAY!!! At this time I did not know if ACL made a race series?

Now looking at the bearing and ACL web site the part number does not look like the race series part number.

060C03
8296
STD

The race series number is 4B8296H

If I was not the type of person that checks everything all the time, I would have lost a bearing. Every oil change I inspect the oil filter, Canton.
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Old 03-01-2007, 01:13 AM   #12
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Given a sufficient oil pump flow, greater clearances mean better cooling of the bearings. I'm sure the bearings wear more quickly with more clearance, but how long do you think this short block will last before being rebuilt. It ends up being judgement.
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Old 03-02-2007, 04:21 AM   #13
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Quote:
Originally Posted by mick_the_ginge View Post
I'm setting my new 2.5L closed deck block (thrust #3) up with 0.0015 main and 0.002 rod. I'm using ACL RACE series bearings, un-coated. These are not the duraglides.

Cobb Tuner Crank, Pauter rods so I hope the bearings are fine as that's a lot of cash down the drain is the bearings fail.
Are there any users here who Plastigauge their main bearings? That will tell you if your clearances are where you want them.

I am familiar with other import motors, where good builders do this for all built engines. However, I haven't seen any mention of it here in the Subie forums.
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Old 03-02-2007, 07:14 AM   #14
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Quote:
Originally Posted by Silverpike View Post
Are there any users here who Plastigauge their main bearings? That will tell you if your clearances are where you want them.

I am familiar with other import motors, where good builders do this for all built engines. However, I haven't seen any mention of it here in the Subie forums.
there are mixed reviews on plastigauge... but its on here.

def
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Old 03-02-2007, 11:32 PM   #15
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Can someone recommend a nice rod bearing clearance to aim for? I'm using Clevitte and the builder is getting close to that step.
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Old 03-02-2007, 11:49 PM   #16
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.0015" is a nice number to shoot for.
Tomi
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Old 03-03-2007, 07:49 PM   #17
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I went with 0.0025 on the rods.

def
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Old 03-05-2007, 07:09 PM   #18
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doug, what are you using to measure your clearances, a dial bore gauge or plastigauge?
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Old 03-06-2007, 04:17 PM   #19
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I've only gotten plastiagauge to work. Maybe a better bore gauge could have worked.
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Old 03-10-2007, 05:54 AM   #20
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How would you adjust the clearances if they were out? take material off the bearings?
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Old 03-10-2007, 08:21 AM   #21
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I was told emery cloth works - but that WAS'NT from a real motor guy.. just a friend that seems to know a little more than I do (which isn't much).
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Old 03-12-2007, 02:15 PM   #22
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Doesn't sound like a very accurate way of doing but hey.....does anyone know what a pro engine builder would do?
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Old 03-12-2007, 05:45 PM   #23
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Grind the rod journal to the next size bearing would be my guess.
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Old 03-12-2007, 09:14 PM   #24
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With the engines I work on, non Subaru, main and rod bearings are available in different thicknesses in .003mm increments. This way we can set the clearances exactly the way we want them. I would assume Subaru does something similar.

DaveW
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Old 03-12-2007, 11:09 PM   #25
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Subaru does have some options for bearing thicknessess, I would not use emery cloth to remove material, find a shop with a line hone thats the best way to do it! I used subaru specs on my first biuld and spinning 8,500rpm's i lost a rod bearing, I did my second useing cosworth bearings and shimmed the oil pump to make up for lost pressure and its still running strong!
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