Crankshafts

[monkdafyed]

Hi Guys, in the process of building 421 SBC. looking at scat crankshaft 4340, noticed no crossed drilled mains, did not get any reply back from scat regarding this. I would expect they are concern about oil pressure bleeding off at the mains, if anyone has any ideas or opinons would like to hear about them. I always used cross drilled cranks and never had any problems, just worried that the rods may suffer with only 180 degree of oiling instead of 360 degrees.

Thanks Brad

[ADR]

On the contrary...depending on how high you spin it you are better off with out a cross drilled crank. Especially if the drillings go to the center of the main like they do on my Ford. Its hard to get the oil to the center of the main so that it can then travel to the rod at high rpm .... has something to do with centrifugal force I think.
Dale

[SStrokerAce]

What Ford is it?

Fords have always amazed me with the journal diameters they have, they have small blocks that are as bad as a Poncho. That's how Smokey got ponchos to twist rpm... Cut down the journals so you could get oil to the bearings at high rpm.

Bret

[ADR]

I'm working with the Ford modular right now. Its 5.4L 4V, the rod and main journals are surprisingly close to a 400 sbc and over all it is a really nice crankshaft, but in my opinion they could have left out the cross drilling.
Dale

[OldSStroker]

On the contrary...depending on how high you spin it you are better off with out a cross drilled crank. Especially if the drillings go to the center of the main like they do on my Ford. Its hard to get the oil to the center of the main so that it can then travel to the rod at high rpm .... has something to do with centrifugal force I think.
Dale

Good observation. Think of a column of oil from the center to the outside of a 3.00 (or 3.25) main bearing. Now spin the crank and that column of oil has inertial force from it's mass and radial acceleration ("centrifugal" force). That inertial force throws th oil back out to the main bearing surface.

With a little math you could calculate the force and then the pressure it exerts at the main bearing shell. That's the pressure you need to overcome just to get oil not to back out of the main. It takes additional pressure to move it to the center of the main. Of course once you get oil to the center of the main, the same inertial force due to rpm does a good job getting it to the rods. The problem might be getting a sufficient quantity of oil to the center of the mains so that the rods aren't starved.

That was one of the things SStrokerAce referred to. Of course bearing speed is proportional to rpm and bearing diameter, so a 3.25 421/428/455 Pontiac at 6000 has about the same main bearing speed as a 3.00 main Poncho at 6500 or a 2.45 SBC at 8000.

[Darin Morgan]

On the contrary...depending on how high you spin it you are better off with out a cross drilled crank. Especially if the drillings go to the center of the main like they do on my Ford. Its hard to get the oil to the center of the main so that it can then travel to the rod at high rpm .... has something to do with centrifugal force I think.
Dale

You hit 8000 to 8500 rpm with a cross drilled ( butterfly drilled) crank and you will run over the parts! A ( butterfly drilled) crank is great right up till you hit the rotational speed where the centrifugal force of the oil spins it away from the rods and BOOM instant carnage. More oil pressure wont do a thing for you either. We refuse to use cross drilled and especially butterfly drilled cranks in any of our engines.

A butterfly drilled crank is cross drilled and the rods are fed from the center of the main. A cross drilled crank has the rods fed from only one of the cross drilled holes and is offset half way on that hole. Its better but it still can cause problems in high rpm engines.

[ADR]

Well now I know there's a name for it and not just " piece of $&*#@%cross drilled "

Thanks Darin