entropy
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posted April 22, 2009 02:31 AM
 Edited By: entropy on 22 Apr 2009 10:54
main bearing clearance
I've been asking (and keeping track of opinions) for 5 years or so on the topic of optimum main bearing clearance for a race engine.
A & B camps are truly divided:
"A Camp" says run mains on the lower end of OEM spec (.0012-,0015), send as much oil as feasible to the rods for cooling, set rods up loose .0025-.0030.
Apparently some folks favor this set up especially for strokers.
"B Camp" says run mains looser(like .0025) for hp, and run rods looser yet (like .0028-.0030).
I have been running all blues for years as they are the tightest shells available for the 12, but i had been measuring mounted bearings w/bore gauge and THOUGHT that .0022-ish was as tight as i could get. Turns out that the bore gauge was digging and the .0022 was really more like .0014-ish
Early this morning i measured the
bare main journals ID,
crank main OD's,
bearing thickness,
and came up with:
22-Apr-09 "OLD" cases, DINGED crank, new coated BLUES, clearance;
#1 0.0014
#2 0.0015
#3 0.0013
#4 0.0012
#5 0.0012
Again, this is the sort of clearance on the mains i have been running since i started "measuring" 5 or so years ago. I just didn't know i was running em that tight.
Then i measured up a set of brown shells i had laying around, and came up:
22-Apr-09 "OLD" cases, DINGED crank, new uncoated BROWNS, clearance;
#1 0.0018
#2 0.0024
#3 0.0020
#4 0.0021
#5 0.0017
sooooooooo...
what say ye, Bikeland folk, run em tight w/blues like i have been doing since Christ was a kid?
or
change direction and run 'em loose with Browns???
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Y2KZX12R
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posted April 22, 2009 03:23 AM
Depending on how well the main bore alignment is and the taper on the cranks main journals and if its a street/drag/or LSR engine.....
If everything is perfect .002"-.0022" on the mains would be ok.
But these weldups are far from perfect and the main bore alignment is most likely off several tenths on a set of used cases. So .0024"-.0028" would be a better number with a weldup and used cases. And LSR racing creates a bunch of heat in the bearings, crank and rods. So oil flow is your friend. But oil flow creates windage, and windage is drag on the rotating assy and costs you HP so its a trade off.
The rods? Again the crank pins are far from perfect on weldups. A result of how well and often the guy dresses the stone on the crank grinder. Also the rod bearings develop more heat than mains and tighten up more under load. So .0025"-.003" would work well if the rods big ends are "round" with no taper.
The old bearings are the best indicator of how well the clearance you were running is working for the parts you are using and the conditions you are running it at.
Sounds simple but....
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LandspeedLarry
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posted April 22, 2009 04:42 AM
There is more good advice in Y2KZX12R 's post than most acquire in a lifetime. I learned something. Thanks. LSL
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TRNorBRN6001
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posted April 22, 2009 05:29 AM
I don't know Karl, I'm an idiot!........................I would stick with what has seemed to have worked very well for you. You have never really had any problems with bearings due to clearance. Most all your motors have been worked very hard on the Dyno (sustained high RPM) or LSR. With that high compression motor you have, it would seem to be a good thing to keep a good amount of oil pressure in the rods. I guess to a small degree it may also depend on what viscocity oil you use. If you do go to the larger main clearance you may consider keeping the main that does not feed a rod a little tighter so it does not bleed off as easy. Just my idiots two cents.
Gary
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Shane661
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posted April 22, 2009 05:36 AM
While it may be true that Karl has not yet had any bearing clearance issues...he often doesn't put even 100 miles on a motor before it is torn apart to inspect. Correct?
It seems, based on Jim's post, that he is running the clearances much tighter than suggested. Since he has the motor apart so often, I would suggest at least trying the looser clearances.
Obviously I'm not an engine builder, so consider this my 01. 
Shane
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TRNorBRN6001
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posted April 22, 2009 05:51 AM
I know the ZX-12 and Busa motors are completely different beasts, but similar in some aspects. The Busa motor runs those tighter type clearances. A question I have always wondered about.
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NOX
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posted April 22, 2009 06:44 AM
I am running browns, with 5w20 oil, no issues, and beat it up at 12,200 all day long at the track...... Forgot what clearance we were going for....., I think 2 thou is what we were going for????
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TRNorBRN6001
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posted April 22, 2009 07:27 AM
Well Karl, like you once told me, "as long as your in the ball park it really does not seem to matter much", lol.
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entropy
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posted April 22, 2009 08:20 AM
lotsa interesting comments.
at the beginning of 2008, TimChin gave me his ss for bearings, but i couldn't really get my arms around the total difference between my direct measurement and the calculated methods. I convinced myself that some of the difference was due to digging in of the bore gauge.
Now i am a 100% believer and will go back to calculating.
I will also attempt to find a bore gauge with big balls, cause like Jim says it is good to look for eccentricity, etc now & again
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tcchin
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posted April 22, 2009 10:56 AM
In addition to big balls, you may want to ensure that your indicator is inserted the minimum amount into the bore gauge stem. This will minimize the preload on the contact points (balls).
To measure eccentricity, I measure the bare rod with used bolts torqued to spec (probably a little light in overall preload, but pretty close) and compare this with my measurements of a spare, low-hours, reference rod that I also use as a setting ring for big-end ID measurement. If there is a big difference, then I replace them with new rods. I assume that the only eccentricity I need to worry about is in the rod, not the bearings, as the bearings are replaceable and I measure them for runout anyway. Is this not an approved method? I understand that it does not take the bearing "crush" into account.
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NOX
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posted April 22, 2009 02:15 PM
Big balls are very important in this sport.......
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KZScott
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posted April 23, 2009 05:14 PM
quote:
Depending on how well the main bore alignment is and the taper on the cranks main journals and if its a street/drag/or LSR engine.....
If everything is perfect .002"-.0022" on the mains would be ok.
But these weldups are far from perfect and the main bore alignment is most likely off several tenths on a set of used cases. So .0024"-.0028" would be a better number with a weldup and used cases. And LSR racing creates a bunch of heat in the bearings, crank and rods. So oil flow is your friend. But oil flow creates windage, and windage is drag on the rotating assy and costs you HP so its a trade off.
The rods? Again the crank pins are far from perfect on weldups. A result of how well and often the guy dresses the stone on the crank grinder. Also the rod bearings develop more heat than mains and tighten up more under load. So .0025"-.003" would work well if the rods big ends are "round" with no taper.
The old bearings are the best indicator of how well the clearance you were running is working for the parts you are using and the conditions you are running it at.
Sounds simple but....
sorry to hijack, but how would your clearances differ for a stock crank?
thx
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entropy
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posted April 23, 2009 09:36 PM
Scott,
the main crank journals are stock
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entropy
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posted April 23, 2009 09:40 PM
quote:
, I would suggest at least trying the looser clearances.
Shane
Shane,
that's exactly what i'm gonna do this build!
Karl
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Y2KZX12R
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posted April 24, 2009 04:22 AM
Calculating by measuring works and reduces the number of cycles on the rod bolts. Its just easier to make a mistake and as you guys were saying, where you measure the bearing insert makes a difference. One reason the bearings have a taper is because the rods go egg shape under high load. The highest tension load is at valve overlap, TDC when there's no compression. The sides of the rod pinch in at the parting line, tightening up.
I would imagine Carrillo's would be less prone to this.
The short extreme taper you see on some bearings right at the parting line is an oil reservoir for oil flow. Every time the tapers go past the oil hole in the crank journal it allows extra flow.
So there's two separate tapers going on in some bearings.
One other thing to check I should have said before, look for babbitt on the ends of the bearings. Many times there's a blob of it stuck on the end of the bearings where they meet. This blob should be scraped off if there is one otherwise it can effect the crush. There can be blobs on the back side or other edges as well but its not that common.
As far as the measured oil clearance, Its always safer to be on the loose side then tight. Tight grabs and loose gives more windage. Our engine assemblers examine, clean, scrape, scotchpad, measure, and clean again, every bearing.
It may take an hour to prep them on a V8 engine. But in a road race 289 SBF engine that spends its entire life at 5000-8000+ rpm its important. Drag race engines are MUCH more forgiving.
Our SB2.2 engines are in these Rocket Sports Racing Trans-AM cars. 364 CI making 800 HP all day long. http://www.rocketsportsracing.com/index.php?option=com_content&task=view&id=575&Itemid=120
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whitehendrix
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posted April 24, 2009 08:35 PM
i need to find a job doing that kinda stuff instead of wishing i did.
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Y2KZX12R
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posted April 25, 2009 03:13 AM
Edited By: Y2KZX12R on 25 Apr 2009 11:14
quote:
i need to find a job doing that kinda stuff instead of wishing i did.
Sorry Karl, a quick thread hijack....
I enjoy the engineering aspects of it very much. But there is pressure and deadlines like any other job. We try some very speculative ideas with head porting at times and then play on the dyno etc. Some times you get the predicted results and sometimes you create more questions.
But no matter what you always learn new things.
Were working on a very different big block ford for dirt circle track. Its a VERY short stroke and HUGE bore engine with some very different porting in the heads. The heads are highly modified Pro Stock drag race heads with half of the ports epoxied up with some other "twists" going on.
This team has always been one of the top teams in the north east and are the team that everyone else tries to beat. Now, after two years in development and about $60,000 the engine is ready for an actual race. The intake manifold is a "one off" that we hand made from 2 cast manifolds cut to pieces and welded up. The rules don't allow sheet metal intakes. It took several major modifications of the combo to get it to perform like it is now.
Its making significantly more HP than there best engine (BBC) to date and its a torque monster off the corners.
These guys are going to walk away from the field at every race this year at every track. Its gonna start an R&D frenzy at all the other teams.
Anyway, this is what makes all the hours of work and testing very satisfying. But there's many times that ain't so satisfying and I stand there scratching my head.
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