bearing clearances and oil weight

[lvporschepilot]

This question came up the other day and thought I would bound it off the heads here.

For a motor running .0025" (.0635mm) rod bearing clearance, is it unwise to run lower than a 50w oil or is 40w going to be ok? This clearance is a bit on the high side of things but not too high IMO. Personally, I believe a good 40w Motul or the like would be just fine although I'd be interested in hearing what the experts here have to say about it.

Best,
john

[HawgRyder]

If your clearances are on the high side...it is the pressure that counts.
So...if you need to go to heavier oil to maintain the correct pressure...you have your answer.
When an engine is running the bearings do not contact the crank if the pressure is correct....the rods and crank bearings ride on a thin coating of oil.
The reason for the wear marks on the bearings is from starting and stopping the engine (when oil pressure goes to zero).
I have seen truck engines with well over a million miles at teardown and thebearings look like brand new (some trucks are almost never turned off).
Bob

[tadd]

Oil weight (viscosity) was orginally the only realy way to change the film strength (shear) of the oil. Its the film shear that keeps metal from touching metal. The stronger the molecules interact, the thicker the film or the larger the clearence you can run.

This film literaly tears appart the way the molecules stack under the pressure of being squeezed between the bearing and the journal. This generates heat, so to combat that you add new oil from the pressure and flow of the system. Hence larger clearences to aid in 'oil exchange' so you dont get coke (carbonized oil).

So in the day when the organic chemistry wasn't so interesting, you needed to up the weight (viscosity) to span a larger clearance for oil removal.

Then came chemistry . Mmmmm, chemistry . First came synthetics. This helped greatly with heat resistance. So clearences could be closed up and lighter weights used.

Now enter multi weights and molecules that expand or contract under heat and stress. An outgrowth of liquid crystal work for displays. One oil can now do a varity of tasks.

t