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Main Bearing Clearances

5K views 34 replies 17 participants last post by  hhott71 
#1 ·
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.
 
#3 ·
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?
 
#4 ·
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.
 
#5 ·
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.
 
#7 ·
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! :)
 
#9 ·
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).
 
#19 ·
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!.
You are absolutely correct. I've developed/ validated enough protein characterization assays to know better. My terminology was sloppy but the clearances aren't sloppy enough.

Glad you got the under bearings. There's no down side.
 
#10 ·
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.
 
#11 ·
I agree with Sean :yes:

And some of the best advise may be to "call the manufacturer" to get their input :thumbsup:
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' ... :p
 
#14 ·
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.
 
#17 ·
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. ...

If in doubt, by all means call your bearing manufacturer.

You asked: Why are there all of these clearance discrepancies? ...
... As for the discrepancies on the clearances on YOUR crank, that is either a result of poor quality control via the crank manufacturer ...
Boy Lynn I sure hope you don't think anyone was blowing off your information posted or 're-inventing' GM spec.s - that was certainly not my intent :eek:
I was agreeing with Sean's point of it being the 'big' journal 400 crank clearance specs vs. the 302~350 Large journal specs.
Everything you posted is the GM recommendation for clearances for those crank journals. :yes:

The best take-away here is the advise to follow the bearing suppliers specifications for clearancing 'their' bearings!
GM gives specs for only 'their' bearings ... no matter whom the original supplier was ...
Anything else IS just an opinion/expertise/observation as a result of building engines over the years, as mine was.

And ... in addressing the OP's original question about "discrepancies" ... you've echoed my point about my own 'observations' of the machining of some of the 383 items supplied by some sources - they vary ... to put it mildly ... depending on the company.
 
#15 ·
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.
 
#16 ·
As always, you all are great and appreciate the responses. What I have learned is that the tighter the clearance, the more precise everything else has to be, which is why many lean towards the .002+ side of things.

Ironically, I spoke with the same guy at SCAT twice, and he gave me two different answers, then contacted both Eagle and C&C, and they both gave me ideal clearances between that of .002 and .003.

I went ahead and ordered a set of Clevite X bearings that will give me the .001 breathing room. Could have spent the extra $90 to a new bellhousing, but that's just the way it goes. I'll have installed by tomorrow, and let you guys know how it goes..
 
#18 ·
Good call Chad. :thumbsup:

And Lynn,
After I finished posting I did a quick search for "Clevite bearing clearance spec" since I was working from memory, and they recommend .00075 to .0010 on a normal build, and say to add .0005 to that for performance builds, as a general rule. Pretty much what I have always done, and also the opinion of the bearing manufacturer. So even on the manufacturer's "tight side" spec, a 2.65" journal crank should be just shy of .002".
What does GM specs say about factory clearances on a 400?

It always sounds good to go with what the motor had when it was built, and it would probably be fine also. But it's not a factory GM build so you should always use what the mfg. of your new parts recommends.
I don't know if Chad has Clevite bearings, but what they say falls right in line with what I have always heard/ done, so there must be something to it....

I personally wouldn't use an old spec on a new build (you are not using 100% factory parts right?), and besides that, even a lightly used 383 that will still see past 6k once in awhile is already not a factory build.

As said there are a lot of opinions, but many are based on fact from manufacturers and lots of past experience. :beers:
 
#20 ·
IMO, it's too tight. If you're that tight checking with plastigage I would definitely have it rechecked with mics and a bore gauge. I would shoot for .002-.003. If you decide to run it tight, make sure you check for runout on the crank, as any runout would easily eat up your remaining clearance.
 
#21 ·
Here is my .02. As a retired toolmaker and drag racer, I think I know a little
About machining and clearances. I built a few sbc that, when drag racing, spent
Time in each gear at 8,500 rpm and 7,500 rpm in 4 th thru the Quarter mile
For 2 years with no problems. To do this, i set the rod clearance at .0002 and
The mains at .0027. Also, have you mic'd each crank journal from one end to the other
To make sure it is straight and true. No taper? Builder beware. I' m not real fond of plasticgage.I prefer inside mic's. Good luck.
 
#28 ·
Here is my .02. As a retired toolmaker and drag racer, I think I know a little
About machining and clearances. I built a few sbc that, when drag racing, spent
Time in each gear at 8,500 rpm and 7,500 rpm in 4 th thru the Quarter mile
For 2 years with no problems. To do this, i set the rod clearance at .0002 and
The mains at .0027. Also, have you mic'd each crank journal from one end to the other
To make sure it is straight and true. No taper? Builder beware. I' m not real fond of plasticgage.I prefer inside mic's. Good luck.

Rods at .0002? Must be a typo. That is tight enough to lock up,
 
#22 ·
Bore gage and outside micrometer if you want to know what you have for sure.
 
#23 ·
I used to hang out at "Ball Automotive" as a 16 yr old kid. :D

Larry Ball was the owner and a famous engine builder around the area.

He told me .0025" on the rods and .003" on the mains is how he built his small and big-block chevys.

I've always followed his advice.
 
#26 ·
One thing that some of you folks are not thinking about. Plastigage is NOT the only means used to ensure clearances. The machinist that machines the parts and knows what parts you are installing is the primary means to achieve a specific customer clearance for the engine. As a 'customer/user', I use plastigage to verify the clearances and the consistency of the clearances on final assembly! It gives me that final degree of assurance that all the parts are right.. that the rods were resized properly, the crank was machined properly, the line honing was done and verified! IF you have absolute TRUST in whomever did the machining, whomever made the bearings, then bolt the thing together and *hopefully* you won't have an issue... but I like to verify! :) and a $1 strip of plastigage and a little time is very cheap assurance for me.. :)

and Yes, I too like build my performance engines on the loose edge, but if I'm building an engine for another person, then I explain the differences (one thing that the closer tolerances provide is longer life before the clearances are TOO large, which translates to mileage for most regular drivers). That all said, every engine that I've built over the past 39 yrs is still running AFAIK, and none have ever been 'blown up' or spun bearings, etc .. :)
 
#31 ·
I did get the new Clevite 'X' bearings in, and had the following clearances: #1 - .0026, #2 - .0027, #3 - .0027, #4 - .0028, and #5, .0029. About where I expected them to fall. Thanks again guys.
 
#34 ·
Hi guys,

I am trying to find the answer but no luck so far.

How do I know what size bearings I need for my crank and rods?
I am coming from the powersport industry by building race bikes for living, and we use different color coded bearings with corresponding sizes to compensate the factory manufacturing differences in the clearances. But for the SBC I see stock size and different oversizes if I want to cut the journal. Or GM use such a huge clearances that no matter few thousands of a milimeter? Sound weird. In a racebike few thousands of a milimeter can be catastrophic engine failure...
 
#35 ·
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.
HOW are you measuring? Plastigage is unacceptable.
2-3" OD MICs and a Dial bore gauge or telescoping gauge is the acceptable and accurate way to check your clearances.
 
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