View Full Version : Bearing clearances


LilJayV10
04-28-2011, 08:28 PM
One thing I have always wondered is how do bearing clearances effect an engines performance/longevity?

I would guess that tighter clearances would net more oil pressure. Are there any other benefits?

How about benefits to running looser clearances?

I hope to learn something.

J

mark21742
04-28-2011, 09:50 PM
Im subscribed...also would tighter clearances make any difference in higher rpm stability

JS01
04-29-2011, 12:28 AM
The general rule of thumb is .0001" bearing clearance per inch of journal diameter. Which on a LS1 rod journal (2.100") nets .0021" clearance. For any build with significant horsepower you can add .0005" to that i.e. .0026" clearance. That is not an exact rule but it will get you in the ballapark as every engine/application is different. Unless you have build the same combo a few times and ran it, which is the best way to find proper clearance from observing the wear. The crank journals ride on a film of oil created by hydrodynamic pressure which you want to achieve as soon as possible on start up. If you have too loose of a clearance there isn't as much area to distribute the load of the journal on the bearing and you will have a longer period waiting for the journal to achieve this 'hydroplaning' effect of the journal gliding on the oil wedge. Most of your bearing wear occurs on this start up time period before the crank is gliding on the oil. Also the another critical time where bearing clearances come into play is when the piston/rod start to load the crank on the power stroke, the upper rod bearing and lower main bearing will see their highest loads here and the more surface area you have to distribute that load the better off your bearings will be.

This is as best as I understand it and there is far more to it I am sure.

JS01
04-29-2011, 12:39 AM
Here is an image I thought may be useful:

http://upload.wikimedia.org/wikipedia/commons/thumb/b/ba/Hydrodynamic_lubrication_attitude_angle.svg/547px-Hydrodynamic_lubrication_attitude_angle.svg.png

LilJayV10
04-29-2011, 03:08 AM
Thanks for the posts!

KCS
04-29-2011, 09:49 AM
There is some great info in Clevite's catalog, which you can find online. It talks aboutthe basics of how bearings work, what characteristics different bearings exhibit, general rules for bearing clearances, and even data on the effects of grooved bearings on oil flow and oil fil integrity.

94mustang
06-14-2011, 11:06 PM
with looser clearances the engine will make more torque. we have seen 11ftlbs on a LS3 at 416"

SlIm95
06-16-2011, 04:00 PM
Looser is safer. to an extent. I dont see 11ftlbs of torque in bearing clearances though.

SlIm95
06-22-2011, 02:13 PM
I understand the total clearance meaning all clearances, but thats still not going to give you more power. Ring cleance will have an effect on power, but that can go so many diff ways. I installed valves in guides with .0005 or less in NASCAR heads. I have honed pistons to straighten out the bores just enough to slid the pin in. I do not believe more clearance will make more power, but this will change from one engine builder to the next.

Old SStroker
06-23-2011, 05:37 PM
Also the another critical time where bearing clearances come into play is when the piston/rod start to load the crank on the power stroke, the upper rod bearing and lower main bearing will see their highest loads here This is as best as I understand it and there is far more to it I am sure.


FWIW: In an NA engine and moderately blown engines (Top Fuel excepted!), maximum rod bearing load is at TDC on the exhaust stroke at max rpm (lower rod bearing), not the power stroke. It's all about inertia of the moving parts and Newton's Second Law [F=Ma].

In a 90 V8 are not the power stroke loads as well as the inetia loads mentioned above at ~45 to the mains and spread out over at least two mains?

No mention so far of oil viscosity and its relationship to bearing clearances. Why? It seems to me that would be important. Ask your engine builder what oil viscosity he designed the bearing clearances for. "Huh?" or "It doesn't matter." are not good answers.

My $.02