Bearing clearances for high rpm sbc?

General engine tech -- Drag Racing to Circle Track

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travis
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Bearing clearances for high rpm sbc?

Post by travis » Mon Oct 23, 2017 5:56 pm

For a 355" sbc with a forged crank, rods, and light weight pistons, what kind of rod and main clearances do you like to see? Shift point on this will probably be somewhere around 7500 I'm guessing. Would you run a touch more ring gap than a "normal" street engine? I'm stepping outside of my comfort zone here

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Re: Bearing clearances for high rpm sbc?

Post by engineguyBill » Mon Oct 23, 2017 6:20 pm

.0020" clearance on the rod bearings, .0025" on the mains. Have your crank checked to verify that it is absolutely straight. Use 5W20. 5W30 or 10W30 oil. Minimum .0045" per inch cylinder diameter for the top ring gap and minimum .0050" per inch cylinder bore for the second ring gap.
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Re: Bearing clearances for high rpm sbc?

Post by S10LS2 » Mon Oct 23, 2017 8:33 pm

^^^ Eazy peezy,^^^ just like that. The haynes or chilton's got them same specs. Or any car mag of the last 25 years. 5w20 is a good all around advice #-o Especially when no idea what you a buildin. ?? Enlighten us? Up here in the northeast junk circle track classes run close to your rpm, not much effort. I am positive you are guessing rpm along with the guy who is guessing said 5w20 is close to ballpark. When he has no clue on what you are doing. Imo only so do not get upset.

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Re: Bearing clearances for high rpm sbc?

Post by KnightEngines » Mon Oct 23, 2017 10:39 pm

If it's an aftermarket block, good crank & good rods then I'd go with what Bill suggested - though to me that's a bit on the tight side.

I'd likely go for .00275-.003" on the mains & .0025" on the rods.
With a factory block .003-.00325" on the mains - coz they move around more.

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Re: Bearing clearances for high rpm sbc?

Post by Walter R. Malik » Mon Oct 23, 2017 11:06 pm

In my opinion ... Large clearances are usually a band-aid for suspect parts or shoddy machine work, or both.
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Re: Bearing clearances for high rpm sbc?

Post by rewguy » Mon Oct 23, 2017 11:24 pm

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.

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Re: Bearing clearances for high rpm sbc?

Post by kimosabi » Mon Oct 23, 2017 11:42 pm

Walter R. Malik wrote:
Mon Oct 23, 2017 11:06 pm
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?

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Re: Bearing clearances for high rpm sbc?

Post by vortecpro » Tue Oct 24, 2017 12:18 am

.0028/.0022 With a very strait crank.
Last edited by vortecpro on Tue Oct 24, 2017 12:20 am, edited 1 time in total.
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Re: Bearing clearances for high rpm sbc?

Post by vortecpro » Tue Oct 24, 2017 12:19 am

vortecpro wrote:
Tue Oct 24, 2017 12:18 am
.0028/.0022 With a very strait crank.
Racing a NA NHRA stocker should be mandatory before any posting.

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Re: Bearing clearances for high rpm sbc?

Post by travis » Tue Oct 24, 2017 1:44 am

This is a GM 4 bolt block out of a '70's era truck, GM steel crank, and a new set of Scat 6" rods and Wiseco 2vr lightweight flat tops (replacing a set of cheap CAT rods and L2256 pistons). The block needed a lot of work...things was clearly moving around in there and the guy said he couldn't keep bearings in it. The crank was chewed up pretty bad and had to be ground .030" under. This guys budget was blown at the machine shop with all the work done plus the new rods, pistons, and balancing. He had me come do some measuring on it...I come up with 0.0032 on the mains and 0.0028 on the rods. Seemed loose to me but I dunno...this isn't what I normally build.

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Re: Bearing clearances for high rpm sbc?

Post by cgarb » Tue Oct 24, 2017 2:14 am

If this is a drag race build...I would use them as is. That's right in the ballpark of what I like to see on clearances for my drag race builds, but I use heavier oil. Depending on the season I use SAE30 and SAE40. Spring and fall 30W summer 40W. I'm probably giving up a few HP I know, but I'm not having any bearing issues and my stuff generally stays together for years. My $.02

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Re: Bearing clearances for high rpm sbc?

Post by travis » Tue Oct 24, 2017 3:15 am

I forgot to mention...it is a low budget drag car, with a little street driving once in a while.

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Re: Bearing clearances for high rpm sbc?

Post by RCJ » Tue Oct 24, 2017 6:25 am

.001 X Journal diameter.+.0002,-.0003

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Re: Bearing clearances for high rpm sbc?

Post by kimosabi » Tue Oct 24, 2017 6:33 am

vortecpro wrote:
Tue Oct 24, 2017 12:18 am
.0028/.0022 With a very strait crank.
How do you know the crank is straight when running?

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Re: Bearing clearances for high rpm sbc?

Post by Racerrick » Tue Oct 24, 2017 8:27 am

RCJ wrote:
Tue Oct 24, 2017 6:25 am
.001 X Journal diameter.+.0002,-.0003
X2 that formula works so for a sbc 0025 -.0027 mains and .002-0022 is plenty unless its blown or you have weak parts and/or shoddy machining.

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