"Acceptable" bearing clearance

Posted by Ken_P On 2015/4/26 20:50:21

So, after installing new main bearings, I am in a bit of rough spot. I measured main clearance using plastigage, and got:

#1: .006
#2: .005
#3: .006
#4: .0055

The book says I should have .0005 - 0.0015. The crank was polished, not ground, but I don't think the machinist took much, if anything, off, because it doesn't look like the finish even changed.

I have remeasured the crank several times, and my results indicate I should be about .002 to .004 under nominal of 2.7465, so I should have .0025 to 0.0055 clearance. It seems odd, because I checked clearance with my old bearings, and got similar results, but I had 20-25 psi with the engine warm at idle, which seems high for that much bearing clearance. (At least that is what the dash gage read)

Should I spec out 0.010" over bearings and grind the crank, or just run what I have?

The genesis of all of this work was a "simple" ring and valve job. Valves are great, now, but haven't gotten to the piston rings yet. Engine ran ok before I took it all apart, but smoked a little, which is why I started in on the ring and valve job.

I'm interested in what the experts think.