bearing clearances in relation to oil vis.
- Thread starter James408
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Hey I gotta get my $0.02 in here too...What's the difference between a piston pin and a connecting rod journal?
Piston pin might have 0.001" clearance and is not forced oil fed, and around a 0.75~1.0" diameter. Rod journal usually around 0.002~0.003", is forced oil fed through passages in the crank which itself feeds from passages in the main webbing, and they (rod bearings) are usually around 1.5"~2.5" diameter.
Another issue on rods is the side clearance in the crank, you get large here and the system oil pressure goes down from the larger "leak" Alot of aftermarket American V8 stroker combos have this issue as they are putting Chevy spec rods about everywhere. A typically used bandaid here is a high volume oil pump and/or thicker oil to keep the system pressure up.
Another issue that NOBODY finds unless they did it themselves is the leaking around the cam bearings. After installing a few cam bearings, this is probably the biggest potential hit or miss leak zone in an engine. I started using smaller diameter restrictor holes to feed the cam bearings rather than the large potential bleeders they have stock. Especially since many cam in block V engines the same passage that feeds the cam bearing also feeds the main bearing.
My Mopar 440 currently does 20 PSI hot with 4.5 quarts of 5W20 and a bottle of VSOT (equals about a 30W), at a 600RPM idle and goes up to 40 PSI at 1500RPM and maxes at 60 PSI at a 5000RPM redline. Plently enough pressure to have gone the last 20K miles with my foot often on the floor.
Geometry can be dead on, and you still have to deal with surface finish. They're both important, along with all the other variables.
Look at how a SBC feeds the mains and you'll cringe.
Man you would hate to see some of the crank finishes I have seen on fresh rebiulds. Most are done with a high speed 400 grit cloth if they are lucky. For my own personal I take it another notch to 600 grit.
Another important difference is that the connecting rod journal is rotating 360 degrees continuously in the same direction, whereas the piston pin is alternating direction of rotation about every 45 ( I guess) degrees of rotation. So it is a very different situation.
When parts are being made the first concern is typically holding tolerances of location, such as position, profile, or just a dimension related to a datum, and staying within the required envelopes of maximum or minimum material. Next is controlling feature size, which can be interrelated with tolerances of location. Tolerances for geometry are refinements to tolerances of location and size, as unless otherwise specified location and size determines geometry. Last are finish tolerances and other tolerances, although finish is often determined in the initial fabrication process.
One more thing - the expected temperatures of the oil are another parameter, along with the bearing clearances.
Oil pumps can be designed for high flow [not necessarily high pressure]. This also can be an influence on the oil viscosity requirements.
A straight six vs. a V6 would leak differently.
Most worn bearings we see are the the farthest from the pump.
Oil pumps can be designed for high flow [not necessarily high pressure]. This also can be an influence on the oil viscosity requirements.
A straight six vs. a V6 would leak differently.
Most worn bearings we see are the the farthest from the pump.
That's why old time engine hackers swear by SAE 40-50s. Between clearances and surface finishes they normally see, they're not lying when they swear by those grades.
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