Crank help
Turbo Mini Forums

As Vegard says, the logic about the main bearing shells is simple.....

If the main is not cross drilled and one half of the main bearing shell has no groove, then the big-end will only get a high pressure/high volume feed 50% of the time as the single drilling up to the big-end gets its feed from the groove(s) in the main bearing shell.

So theoretically the big-end should suffer whilst the main bearing is fine and indeed can carry a higher load without the groove in the bottom half as there is more load carrying area.

It seems in practice the big-ends do not suffer from less oil even though they should.....

Schrödinger's cat - so which one am I ???

Are we forgetting all about the shear resistance of the oil between the shell and the journal , at 7000 rpm a crank turns 116.6 times a second or it takes or if maths is correct 0.0086 secs per revolution, now for the oil film to shear down before it gets a new squirt only 8 thousands of a second later is not going to happen in a healthy bottom end,

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True, but remember oil is there for cooling, not just lubrication.
In fact it is probably more important for cooling, so although the film my not breakdown in the time you calculate, there is (theoretically) 50% less cooling.

But as you say, they seem to survive despite it.....

Schrödinger's cat - so which one am I ???

Its easy to convert a plain bottom main bearing to a groove bearing simply by grinding a new slot for the different locating tags