Connectiong rod side clearance

[Drewster]

How critical is connecting rod side clearance?
I am thinking of running scat h beam rods on a stock crank. I had concerns about the side clearance. The scat rods are .844 wide. Quick checks put me in around .021 clearance. Should i go with K1 which is .855 wide and would give me around .011 side clearance?
The Scats are better for my budget but making a motor that lasts is even better.
Andrew

 

[Chris Hogeland]

How critical is connecting rod side clearance?
I am thinking of running scat h beam rods on a stock crank. I had concerns about the side clearance. The scat rods are .844 wide. Quick checks put me in around .021 clearance. Should i go with K1 which is .855 wide and would give me around .011 side clearance?
The Scats are better for my budget but making a motor that lasts is even better.
Andrew

The scat rod will be fine.

 

[m233roller]

Yup as Chris says. Iv'e ran rods with .022 side clearance and no problems at all and had good oil pressure.

 

[Buick Mark]

Rod side clearance has more to do with oil control than anything else. If your a bit wide, not a big deal mechanically, you'll just have a bit more oil pissing from between the rods.

 

[Drewster]

Ok thanks for the info
Andrew

 

[PHANTASM]

Excessive side clearence can make noise off idle similar to lifter tick.

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[earlbrown]

Rod side clearance has more to do with oil control than anything else. If your a bit wide, not a big deal mechanically, you'll just have a bit more oil pissing from between the rods.

Actually that's not true. The only way side clearance can do oil control is is the curtain area of the clearance is tighter than actual oil clearance.

Lets say one has a loose engine with .002" oil clearance on the rods with a 10 under rod journal (middle of spec)......

There's 3.945 sq" of space in the rod journal with a 3.939 sq" crank journal in it for a total of .007 sq" of clearance area (X2). To end up with with side clearances the same as the 'loose' engine, they would have to be .0019877" per side for a total of .003975". Once the side clearance gets greater than .004" there's no more flow restriction brought to the table. It'll hemorrhage whatever the bearing clearance provides.

On my 10 under engine with .001" oil clearance(.0035 sq" X2) I'd have to have side clearances of .0009" per side or .0019" total side clearance to offer any flow restriction.

That's why our engine can maintain good oil pressure with up to .065" in rod side clearance. (and if it's crappy, it's not the rod width's fault)


Throw in some centrifugal force acting on the side clearance's oil and it's actually pulling on the hydrodynamic wedge's oil. (but that's REALLY nitpicking!)