Main Bearing Clearances - Team Camaro Tech
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post #1 of 35 (permalink) Old Mar 25th, 15, 09:35 PM Thread Starter
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Main Bearing Clearances

Hi all. I've started on the 383, and had a question in regard to main bearing clearances. For the first 3 main bearings, I've gotten measurements of .0015, .0018, and .002. Are the first 2 too tight? I contacted SCAT, and they said .0015 was at the bottom end, but still acceptable. I've seen conflicting info to that on multiple forums, so thought I'd consult you guys.

If they are too tight, two questions.

1. I see main bearings that are +.001 undersized, which would put me around .0025/.0028 for the first two. Is that a viable alternative, or is there something else I can do t save the $100 on new bearings.

2. If new bearings are in order, can I mix sizes, where I keep the third bearing in place at .002.

Again, the engine is for street, not strip.

First time with an engine build, so learning the ropes.

Thanks.

67 Black RS Coupe, 383, M21 & 10 Bolt Posi. Edelbrock E-Tech 170 Heads, 2204 Roller Cam, Air Gap Intake.
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post #2 of 35 (permalink) Old Mar 25th, 15, 10:17 PM
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Re: Main Bearing Clearances

You are fine at .0015.
I do wonder why the difference between the .0015 and the .0020.

Just FYI, on the production 302 GM specs were .0008 to .0030 on all five.
On the 350 the GM specs are .0008 - .0020 on 1 through 4, and .0018-.0034 on #5.

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post #3 of 35 (permalink) Old Mar 25th, 15, 10:25 PM Thread Starter
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Re: Main Bearing Clearances

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Originally Posted by bilodeaulynn View Post
You are fine at .0015.
I do wonder why the difference between the .0015 and the .0020.

Just FYI, on the production 302 GM specs were .0008 to .0030 on all five.
On the 350 the GM specs are .0008 - .0020 on 1 through 4, and .0018-.0034 on #5.
Thanks Lynn! That's great info. I was hating the thought of delaying my build a few more days. Interesting enough, I was looking at the "Rebuilding the Small Block Chevy" SA Series book that I have, and it cites acceptable clearance ranges of .002-.0035 for stock street, and .0025-.0037 for Hi-Pro Street. But as I mentioned, SCAT said it was bottom end, but acceptable.

Why are there all of these clearance discrepancies?

67 Black RS Coupe, 383, M21 & 10 Bolt Posi. Edelbrock E-Tech 170 Heads, 2204 Roller Cam, Air Gap Intake.
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post #4 of 35 (permalink) Old Mar 26th, 15, 04:45 AM
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Re: Main Bearing Clearances

These are my thoughts,
Oil cools and lubes a bearing and its journal as the oil goes through the bearing into the crankcase.
The 302 was designed for higher RPM's than the standard 350.
Higher RPM's has more oil shear - read friction/heat generated as oil is pulled apart between the journal and bearing, the more heat generated.
Higher RPM's, bigger bearing clearances allows more volume of oil for cooling and lubing.

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post #5 of 35 (permalink) Old Mar 26th, 15, 05:26 AM Thread Starter
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Re: Main Bearing Clearances

This engine won't see the high side of 6K. Again, not sure why there are so many discrepancies between current reading and the GM specs, which support what you are saying Everett, based on Lynn's comments.

By the way, I had some assembly grease on the bushing the first time around. I took that off and measured again 'dry'. Almost the same readings, but more like .0016/.0017/.0020 for 1, 2, and 3.

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post #6 of 35 (permalink) Old Mar 26th, 15, 06:28 AM
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Re: Main Bearing Clearances

Just out of curiosity what are you using to measure clearances? I have never been able to measure in tenths and would like to be able to do it. Any info would be appreciated.
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post #7 of 35 (permalink) Old Mar 26th, 15, 08:39 AM
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Re: Main Bearing Clearances

Plastigage gives you that capability, although it's purely an 'estimate' based on reading the width of wax against the standard on the paper. Note to OP: always assemble dry when using plastigage, but you can use a single drop of oil to hold the plastigage in place while you assemble and torque. I've always used 1.5 to 2.5 when assemblying, with the 1.5 on the tight end being for stock street engines, and towards the higher end for a performance build. I expect all the measures to be within a couple of ten-thousandths of one another.

PS. Don't forget to clean and lube before final assembly (after doing the plastigage measurements!

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post #8 of 35 (permalink) Old Mar 26th, 15, 09:45 AM
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Re: Main Bearing Clearances

Just my opinion but I don't think it is rational to talk about 1/10s when using plastigauge. You might be able to say it's closer to 0.0015 than 0.002 but can't say it's 0.0016. That's from my background in manual machining and precision instruments.

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post #9 of 35 (permalink) Old Mar 26th, 15, 10:05 AM
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Re: Main Bearing Clearances

I can't speak to the 'accuracy' of the plastigage, but it is easy to read the clearance to a precision of +/- 0.0001.. (ie.. generally it reads between two of the values very clearly). 'Precision' is how repeatable the measure is.. Accuracy means absolute value!
Plastigage is great for measuring precision of machining and assembly. If I get all my numbers between 1.7 and 1.8 or 1.9, I'm happy that they are precise! Whether the *real values* are 1.7-1.9 or 2.0 to 2.1, I care less about that (unless we are close to one end of the allowable range).

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post #10 of 35 (permalink) Old Mar 26th, 15, 10:58 AM
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Re: Main Bearing Clearances

That's too tight, I would absolutely use an X bearing. Even though it's not a high rpm build there is no way I would run less than .0025 to 003 on the mains.

Even the bearing manufacturers typically tell you to go .0010 X diameter of journal (more if lots of rpm's). If you are running 400 mains (2.65") that's .00265".

I assure you too tight is a wayyyy bigger concern than too loose. I shoot for .003" mains and .0025"-.003" on the rods.
Also depending on how you measure will matter, but with even plastigauge it's pretty close. Call your bearing manufacturer, they will be the best resource. But "on the tight side" is not were you want your bearing clearance, it's not a stock 350 in a taxi cab, even if you won't lean on it at all.

Like Everett pointed out, a bit more oil is a good thing, within reason of course. Too little oil cushion is a very bad thing.

Sean

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post #11 of 35 (permalink) Old Mar 26th, 15, 12:16 PM
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I agree with Sean

And some of the best advise may be to "call the manufacturer" to get their input
They should be able to pinpoint a range that will give you the best performance of their bearing for your application.
Be sure to have the information of the oil you intend to run for them also, if you're planning on a modern multi-vis-syn with a 'low' starting index they can advise on best set-up.

I've seen some pretty large spreads on some of the 383 'kits' from some of the on-line suppliers - nothing to be worried about (in most cases) as long as you adjust your clearances as required for correct clearance specs.
Years ago they used to call this 'Blueprinting' ...

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post #12 of 35 (permalink) Old Mar 26th, 15, 12:30 PM
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Re: Main Bearing Clearances

"Balanced and blue printed, 3/4 race cam....."

Sean

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post #13 of 35 (permalink) Old Mar 26th, 15, 12:51 PM
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Talking Re: Main Bearing Clearances

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... 3/4 race cam....."
You got a Cam ... Click image for larger version

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post #14 of 35 (permalink) Old Mar 26th, 15, 01:37 PM
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Re: Main Bearing Clearances

Well, as you can see, there are lots of opinions.

I respect all of the other opinions, but stick to my statements based on GM recommendations.
I AGREE that too loose is better than too tight. However, according to GM, .0015 isn't too tight.
If you are running a stock type bearing, no reason that it MUST be as loose as has been recommended (i.e. .0010 plus .00265 for a total of .00365). Now, would it HURT for you to run that clearance? Absolutely not. Might you even gain a bit of power if you are running thick oil? Maybe.

However, as you have stated, this isn't a blue print build, and it is a street motor.
If in doubt, by all means call your bearing manufacturer.

You asked: Why are there all of these clearance discrepancies?

I assume you are talking about the clearance discrepancies from one recommendation to another. Simple answer is: differences of opinion. That is why I cited the GM specs. If someone was taught to always use a clearance of x to xx, and they built every engine for the last 20 years at that spec, they are going to swear by it. Nothing wrong with them touting that spec as "correct". It is correct in his or her experience.

I highly doubt you are going to find someone who had a clearance of .0015 on a main bearing who experienced a failure as a result.

And as for measuring in "1/10s", I am certain that poster meant to state they wanted to measure in ten thousanths of an inch instead of thousanths.

As for the discrepancies on the clearances on YOUR crank, that is either a result of poor quality control via the crank manufacturer, the bearing manufacturer, or both.

If you haven't already tried, you might swap bearings from one main journal to another and see if clearances are more uniform. Because of stacked tolerances, you may be able to get them all closer to what you want.

Lynn
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post #15 of 35 (permalink) Old Mar 26th, 15, 02:02 PM
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Re: Main Bearing Clearances

General rule of thumb we have always gone by
Is daily driver type use smaller the clearance as performance / hi rpms go up and/ or dynamic compression ratio (as in a daily lpg hi octane engine) 2 thou up.
But when u the bigger tolerance go the bigger hi vol oil pump.
A daily low rpm can have bigger clearance around the 2 thiu but the hi performance should not have close clearance
Same goes for piston clearances
As to plasti gauge...for many yrs used as indicative .. but it used correctly.. by the book, it is quite reliable and accurate....personally I think over thew yrs the bad rap plasti gauge has got is from miss use or not stored well... always used fresh.

My Spelling is not incorrect...it is creative

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