[ERIC17621]

OK, I'm putting together my first engine, and have a question. Maybe someone could help me out with opinions. This is a 1965 389 Tripower engine, close to stock engine. Stock/Standard crank, stock rods w/ARP bolts, TRW forged +.040 and 12cc dished pistons (9.3:1), Comp XE262-H cam, 77 heads, hardened seats, screw in studs, new guides, etc. On the crank, I was checking the bearing clearances with some plastigage, and think that they may be a bit too big. From #5 to #1, I am showing .003", .003", .0025, .0020, and .0020 with the plasigage. But, when I mic the journals, they all appear to be right at 2.9980. Is this two much clearance? I don’t understand the different readings from the plasigage when all the journals appear to be the same 2.998” on the mic. Does 2.998” need larger bearings, or is that acceptable? I just have standard bearings right now as my machinist told be crank was fine to stay standard, and said nothing about the clearances being a bit on the large side. Before I pulled the motor apart, I ran 10-40, and always had good oil pressure (20-60lbs.) Please help.

[Jack Gifford]

As others have said, Plastigage only provides a ballpark figure. Actual clearance must be determined by micrometer measurement of journals, and dial-bore guage (3-legged) measurement of installed bearings.
That 2.9980" measurement is puzzling. Spec for a new crank is 3.000" +/-.00025", and .002" main journal wear is hard to believe- I've never seen that much. Without meaning to doubt your figures, is it possible you meant to say 2.9998" (.0002" wear, not .002")?
On another note- are you aware that rods need to be resized after pressing in new rod bolts?

[ERIC17621]

I figured that the plastigauge wasn't the most reliable. I will double check that 2.998 measurement tonight. The rods were resized when the bolts were installed. So I shouldn't worry too much that the plastigauge says the clearance is a little off if the Machinest says it was good? will the motor live if it really is around .0030?

[Jack Gifford]

The Pontiac main bearing clearance spec on a brand new engine was .0005"-.0030" (except front main was .0005"-.0025"). Opinions vary, but I think .0030" is fine.