SBC - main bearing clearance question

[Belgian1979]

Hi,

Clutch car, 350 SBC, oil is 10W40. What kind of oil clearance would you recommend on the main rear bearing? Currently have .00275".

[RCJ]

. 001 x main bearing diameter

[Belgian1979]

Also for the rear? Thought it needed more. If I use one .001 smaller shell, it will become .00225"

[RCJ]

Yes on the rear. Would add .0005 ,for me it is rare for the rear not to come in with more clearance . The last 2 I put together were at .0022 . 350hp daily drivers 30 to 40psi at idle with stock pump ,10w30 Castrol

[Belgian1979]

So, .00245+.0005= .00295"
I don't think I can get there. These are std's. So If I use 1/2 X-bearing, I would add another .0005" and that would put me at .000325". Looks like too much.

[mag2555]

This is the clearance range my Bearing book calls for with there race main bearings and a steel crank , so .0028” is perfect and even .003” works.

[RCJ]

I would go with .0029. We are having this discussion at work. If you are getting down to .0005 the kind of tools , quality , calibration, and skill of operator all come in to play. All you can do is be consistent in how you do things and find what works for you. I'm a sbc oval track racer those are the numbers that work for me. Once you start trying to perfect those clearances you will have 100s of dollars in incomplete bearing sets

[mag2555]

If your current bearing at .0027” has a gray break in coating then I would remove it from both bearing half’s with fine scotch brite and then assemble and run it!

Calculating bearing clearances out to 5 decimal
Places is just plain insane to me!

[novafornow]

I always run the rear main +.0005. Been doing this for years.

[rebelyell]

(not My) Rules for Significant Figures (but they are & have been gospel)

To determine the number of significant figures in a number use the following 3 rules:

(1) Non-zero digits are always significant
(2) Any zeros between two significant digits are significant
(3) A final zero or trailing zeros in the decimal portion ONLY are significant
Example: .500 or .632000 the zeros are significant

.006 or .000968 the zeros are NOT significant

For addition and subtraction use the following rules:

Count the number of significant figures in the decimal portion ONLY of each number in the problem
Add or subtract in the normal fashion
Your final answer may have no more significant figures to the right of the decimal than the LEAST number of significant figures in any number in the problem.




For multiplication and division use the following rule:

The LEAST number of significant figures in any number of the problem determines the number of significant figures in the answer. (You are now looking at the entire number, not just the decimal portion)
*This means you have to be able to recognize significant figures in order to use this rule*

Example: 5.26 has 3 significant figures

6.1 has 2 significant figures

[panic]

When did bearings clearance stabilize at .001" per 1.000" of journal diameter?
WW1

[Mark O'Neal]

Mark's Second Rule of Engine Building…."95% of engine building is opinion."

[V12MECH]

Street car?! I would be happy at.0027" just make sure the thrust is around.006", really splitting hairs over .0005" keep in mind it will pickup a bit more after it breaks in.

[V12MECH]

For street hot rod I would not scuff bearings with anything, for SBC rods we run .0022" and .0025" mains, for lots runs at the drag strip, we would add .0005" additional clearance, but then you might do a tear down mid season for inspection, 40 years ago I remember my boss at speed shop giving super modified BBC 's bearings the scotchbrite treatment, haven't seen that done much anymore, unless this is something requiring special procedures for racing only put it together and go!

[Belgian1979]

When did bearings clearance stabilize at .001" per 1.000" of journal diameter?
WW1

Clevite states in their manual .00075" to .001" per inch of crank diameter. So this would turn out around .00186/.00245 for the mains and .00157/.0021 for the rods. The lowest value seems to be used for stock engines with light weight oils.
My experience is that it is rarely possible to get exact, even when mixing bearings of different sizes as at the very least you would go +-.0005.

That said I had:
- std bearing on the rear: .00275", which seems ok.
hx on everything else, which gives:
- number 4: .00275, bit large but acceptable since the trust face needs extra oil.
- number 3: .00215, too tight
- number 2: .00215, too tight
- number 1: .00235, a bit tight.

The original bearing set that came out, had marks on the number 3-2 and a little on the number 1 (marks in the location where the journal is the smallest). And yes I used bearing lube at the time and prelubed the engine before starting.
I would think that I need to go bigger especially on number 3 and 2 and maybe a little on 1. So, I was thinking of putting 1/2 hxx's in the number 3,2 and 1. Assuming that would increase it with another .0005" it would become .00266 and .00285 respectively.

Oil pressure was around 75 psi cold and 45 psi hot. HV/HP pump. So I have more than enough head room for larger clearances.

Have a set of HXX's coming my way. Yes, expensive to get it right, but didn't like what I saw after the original set came out and even that was a mix (std rear, HX on the rest)

I have an original set of a factory engine and it had bearings in the .0006" oversize for example. So they must have had quite a range to choose from.