bearing clearences

[gse_turbo]

Hey guys I just noticed something in the spec sheets. I've been setting main and rod bearings to .00X, the spec sheets are saying you ned to go to .000X which is a rediculous tolerence!

what are you guys setting your bearings to?

Garrett

[PrecisionBoost]

I was thinking that 0.0015" to 0.0020" should give a decent oil pressure..

If anything perhaps go on the tight side for stock power and loosen up a bit for more power.

I don't remember what the GSi engine was set at... probably somewhere in this range I would imagine.

[gse_turbo]

right on, thanks for the reply.

I just had to do the bearings in the nubira and when I checked the specs in the tech section I that that was crazy.

I set the rods at .0015 but the mains came up high, almost .003. no big deal, motor is pretty much trash and I'll have drop a new motor in here in the next week. it's just been too abused.

cheers,
Garrett

[Efratech]

this is too much for me lol

[PrecisionBoost]

right on, thanks for the reply.

I just had to do the bearings in the nubira and when I checked the specs in the tech section I that that was crazy.

I set the rods at .0015 but the mains came up high, almost .003. no big deal, motor is pretty much trash and I'll have drop a new motor in here in the next week. it's just been too abused.

cheers,
Garrett

Hmmmm..... that sucks........ if it was 0.0022" to 0.0025" then it might be ok for a performance engine that's not designed for longevity.... but 0.0030" would definitly be on the sloppy side and give you crappy oil pressure.

So I guess it means regrind/micropolish the mains or get a different one.

Ya.... I'm sure those factory specs must be with brand new components.... it would definitly be hard to get 0.0005" .... and frankly that's a bit on the tight side for a powerfull engine.... wouldn't leave much room for higher power and higher RPM.

[PrecisionBoost]

this is too much for me lol

not sure what you mean.... but I will explain clearance a little bit for everyone.

So.... you allways have to have a tiny bit of space between your bearing surface and the part it's rotating on.

For example lets use the big end of the connecting rods.

If the space between the bearing in the connecting rod and crankshaft surface is very small ( say 0.0005" ) then there is quite a bit of resistance for oil passing through that small 0.0005" gap.

If on the other hand the space between the bearing and the crankshaft surface is larger ( say 0.0030" ) then the oil can flow freely.... which causes lots of it to poor through and it decreases the oil pressure.

Obviously if the oil pressure is too high there won't be enough flow.... but if the oil pressure is too low it can cause a situation where the oil pump can't keep up and oil stops flowing to certiain parts of the engine.


Next consideration..... a tight gap is good in some ways because it means you have lots of time before the surfaces wear enough that the oil pressure drops ( hundreds of thousands of miles )

If the gap is allready large then it means you might only get 50,000 miles before your oil pressure is too low.


Now you might say "why not try to make it a smaller gap every time? " and the answer is power and RPM.

The Oil is like a dampener..... when the cylinder fires that 0.0015" gap squishes down to 0.0012".... the more power you make per cylinder the more it squishes down.

So 0.0005" might be fine on stock power as it squishes down to 0.0003" but at higher power you may end up getting 0.0000" which is bad.

Bearings should in theory never touch the crankshaft.... otherwise you can get pitting and scorring between the metals.... and eventually sieze up the engine or break a connecting rod.

So.... think of it this way.... if 100hp squishes the gap by 0.0003" then on a stock engine with 130hp it will squish 0.0004" leaving a gap of 0.0001" which means the two surfaces will never touch.

But if you increase that power to 200 hp then that gap squish is 0.0006".... oh wait.... we only had 0.0005 in the first place!!! oh no.... the bearing smacks into the crankshaft surface and causes a scrape.

Run 200hp too long with tight bearing clearance and you will end up with a damaged cranshaft.

So....at say 0.0020" with a high power engine making 600hp the squish is 0.0018 ( 6000/100 X 0.0003" )

So with a bearing clearance of 0.0020" and 600hp were still fine because the bearing never hits the crankshaft surface


So..... what else can cause squish...... RPM.... the forces multiply with the square of the velocity..... in other words raising the RPM from 6500 to 8000 makes the forces on the piston quite a bit higher ( 50% higher in fact )

so again.... if you have more force.... you have more squish

If your bearing clearance is small ( 0.0005" ) then it's a bad idea to increase the redline..... the larger the bearing clearance the higher the RPM that can be achieved without running into a situation where the bearing hits the crankshaft.


It's a big trade off......

tighter clearnaces = lower power & RPM but higher oil pressure = long engine life

larger clearances = higher power & RPM but lower oil pressure = less engine life

0.0018" would probably be a decent performance clearnace for anything under 350hp... over that you'd need to add a bit..... say 0.0020" for 600hp


Don't quote me as these are just quick estimates from what I know about clearance..... the best thing is to talk to an engine builder and give him the RPM and power to see what he thinks is a good number for your particular engine.

Keep in mind that a V8 running 600hp is like a 4cyl running 300hp..... both will have about 75hp per cylinder... so if your engine builder is use to V8.... you need to put it into terms that he will be able to equate to..... so use horsepower per cylinder.... not total horsepower.

Hopefully that helps people to understand the conversation between Garrett and I.... his clearance is too big and would probably cause problems with oil pressure (among other things )

[gse_turbo]

VERY GOOD POST.

Chris is spot on, for example, when I start the car up it's reading at 60psi+. once the motor hit operational temps it sits at about 25psi but after the oils gotten hot or the cars been driven on the pressure is at 8psi or so, very bad!

I have to order some bearings from Calico tomorrow (if they're open) so when I get ahold of them I'll see what they recommend. I'll also see if they would have a chart of some sort that I can post in the tech section.

Garrett

[PrecisionBoost]

Calico's CT-1 dry film lubricant used on engine bearings, valve springs, oil pump gears, timing and transmission gears is 0.0002" to 0.0004" thick and generally speaking does not need extra clearances. A benefit of Calico Coated bearings is the allowance for tighter clearances. Keeping in mind that bearing clearances of .002" to .003" are typical for High Performance applications.

So.... the coating would drop the clearance slightly.... but not much.

Hmmm.... I thought I'd look around the clevite site to see what they had to say and found a decent article

http://www.clevite.com/techbulletins/CL77-1-205R.pdf

Interestingly enough their example is with a 2" diameter..... and they give 0.0015" to 0.0020" as a recommended level which is pretty much bang on what I figured they should be.

They also figure 0.0025" would be recommended for high performance engines....so your not that far off with the extra Calico coating.....if you used a thicker oil it might just work out fine.

[Efratech]

great read! i did understand the basics but this gets things more clear!!!

for gap measure i see my tuner/builder using platigage, i guess it the rule to use it.

efrain

[PrecisionBoost]

great read! i did understand the basics but this gets things more clear!!!

for gap measure i see my tuner/builder using platigage, i guess it the rule to use it.

efrain

Plasigauge works..... but it's not the most precise measurement.... it is however the easiest... throw it in there.... tighten up the bolts to squish it and measure it on the supplied paper gauge.

Quite often they go 0.010" 0.015" 0.020" 0.030" as the markers and you must guess what the actual number is if the size is between these.

So.... easy to use... but most of the time it won't tell you the actual clearance.... just an estimate

It is good for measuring distortion in some ways.... you will be able to visually see a change in diameter of 0.005" or more

[gse_turbo]

...anything else out there that you might know about and I don't? ...other than plastigauge that is?

[PrecisionBoost]

Most of the higher end shops do the work by hand measuring very carefully with a micrometer and bore gauge.

The two things most important are:

First is Temperature control..... even the heat from your hands can change the readings... ideally wear a pair or two of nitrile gloves and minimize the amount of time holding the measurement tools

Changing the temperature by a few degrees can create 0.0005" or more differences in the gauges!!

Second.... you need to use one tool for the measurement.

Yes... you need both a micrometer and bore gauge.... but use the micrometer to measure the bore gauge instead of looking at the reading on the bore gauge.

This is to minimize error in measurement.... if one is 2% high and the other is 3% low then that is not much individual error but quite a bit once you add the two ( since they will be 5% difference )

If the micrometer is 5% high..... then it doesn't matter... because your using the micrometer for both readings.... so they will both be 5% high and give a total of 0% error in the difference calculation ( the difference being the clearance )



The plastigauge is cheap and quick..... just fine for a rebuild shop pumping out a dozen engines a day for $1000 a piece.

Measuring by hand might take a whole day..... but it's worth knowing the exact dimensions when building a race engine.

That's why a good race engine is going to cost 6 to 10 times as much money as a regular rebuild.... it's all the time spent measuring... adjusting.... measuring.... adjusting.... general blueprinting of the engine to make sure the dimensions are identical.... ensuring squareness.... ensuring weights (physical as well as rotating) are identical.

The more time spent measuring and the more expensive the tools used to modify the metal the better the end result

[PrecisionBoost]

Personally... I would use the Plastigauge as a secondary reading.... just to give me a ballpark figure and ensure that I don't accidentally screw up with measurement with the micrometer and bore gauge.

That is to say if I guessed a clearance of 0.0020" in a particular area of the plastigauge I should be getting something pretty close to that when measuring with the micrometer and bore gauge.

So if I guess 0.0020" with the plasigauge I might measure and find 0.0018" in one area 0.0019" right next to it and 0.0020" right after that.

So then I would have to decide if that is too far out of round or not.

Perhaps a simple micropolish is all that is needed to clean that up and get everything identical dimensionally.