Evo 6 main bearing clearance

Hi,

I am building a 4g63t for my evo 6 to be used as a street car and roll race car. Aiming to make around 700hp at the wheels.

I am just trying to get my main bearing clearance correct. Engine had brand new nitto crankshaft and arp main studs. Machine shop told me that due to arp main studs distorting the tunnel that the block/brace had to be tunnel bored, so that was done.

Trying go get a bearing clearance of between 0.0022" and 0.0025" with standard size bearings I'm at 0.0017" and 1 thou over will obviously give me 0.0027". Iv been told it's possible to use a standard size bearing shell on one side and an oversize bearing shell on the other side which should split the difference and give me a 0.0022" clearance. Is this advisable or should I look at other options?

Hi. For 700 whp you will be just fine with 0.0027" clearance and thick oil.

The rule of thumb is that you need 0.001" of clearance per inch of journal diameter so you would need 0.0023" clearance to match it. But considering the power level you're aiming to get to additional 0.0004" of clearance wouldn't hurt at all. In fact it's your safety margin. If you think it makes any difference you can use a thicker oil to maintain oil pressure when engine is under very high load.

Shota has built a LOT of these engines, chap knows his stuff!

Splitting the bearing sizes isn't uncommon, and some prefer to do that because the "tighter" clearance can be used on the load bearing half - connecting rod, or main bearing cap - for more load capacity, and the "wider" used in the 'rod cap, or engine block, for the oil flow that's required to draw the heat from the bearing that developed from the viscous friction in the oil. That last is overlooked by many, but is CRITICAL to prevent the oil breaking down from heat and destroying the bearings - there's a reason high rpm engines use more clearance.

Have a read of this, it might make things clearer (or not...) - http://www.substech.com/dokuwiki/doku.php?id=effect_of_oil_viscosity_on_hydrodynamic_friction_of_engine_bearings

Iv now fitted up the 0.001" oversized bearings and have a clearance of 0.0027 as I expected. This is being measured perpendicular to the bearing parting line. However if I measure at a 45 degree angle from the parting line of the bearing I'm getting measurements anywhere between 0.0032 and 0.0039 which seems a little excessive to me. In the engine building course it says to only measure perpendicular to the bearing parting lines. But surely these other measurements must be taken into consideration. Any help would be appreciated.

The bearing shells are made in a slightly oval shape, one of the reasons is because the big end distorts slightly under the tensile stresses - the piston pulling on it around TDC, especially on overlap - and this can close the clearance to the point where physical contact might occur without the extra clearance.

It's normal, and not something you should be concerned about.

That's absolutely fine, that's the way it has to be.

https://www.kingbearings.com/wp-content/uploads/2014/10/Geometry-and-dimensional-tolerances-of-engine-bearings.pdf