Bearing Clearances

G

GFOLSOM

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Jan 3, 2011
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Spoken like a gasoline man... (sorry, I'm still learning here)
Gas engines start with about 1-1.5 mils of crankshaft bearing journal clearance and go up to about 3 mils for short lived fuelers (less drag, but more slamming around of the rod on the crankshaft).
Does the same apply for a diesel in general?
Since we are at lower RPM (in general) but much more compressive force on the connecting rod and therefore torque on the crank, should I/we always stay with the tighter bearing clearances? The lower crankshaft speed and therefore slower bearing speeds seem to not need the reduction in friction as much...
 
1.5-2 for lower rpm/power levels 2-2.5 for more rpm/power...15w40 or 20w50 oil not 5w30
 
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Great, Thanks for the input. I know I'll need to get used to higher viscosity oils.
 
good rule of thumb for a stocker is .001 per inch of journal dia. performance we run about .0012-.0014 per inch.
 
Less drag is just a potential fringe benefit of large bearing clearance - primary reason is to keep the rod/cap parting line from pinching the journal at RPM for a spun bearing.

We ovalize the big ends to get the best of both worlds.
 
i've run 5-30 through 20-50 oils if the motor is built right you wont have a issue so as long as the oil psi is right.
5-30 is fine for the avg street driver, while 20-50 is better suited to raceing conditions.
 
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