Bearing clearance.

[KVacek]

I wanted to do a rebuild due to the high mileage. It started a top end tick and using a little oil. Putting a BTR trucknorris cam, springs, lifters, trays, pushrods. So I figured I’d do it all.

You've got the bearings out. You're spending all that money on the top end. You'll be buying gaskets anyway. I presume you're also checking piston fit, ring gap and fit in piston grooves, and then honing the cylinders since you're in there.

What can you save by not replacing the bearing shells at least, assuming the crank is still good without further work. False economy.

 

[Tom-M]

Hey everyone. My apologies if this is a stupid question but I’m rebuilding my LQ4 and I’m at a roadblock concerning the main and rod bearings. I’m not building a high HP/RPM motor, just rebuilding. Can I just use stock OE replacement bearings? The old bearings looked great and the crank looks the same. I’m trying to keep this as budget friendly as I can.

The tolerance with some brands of bearings can be as much as +/- .0005" which means you could have .0015" to .0035" of clearance. The only way to be sure of what you have is to measure the crank with a micrometer, install the bearings and tighten the bolts per spec and use a bore gage to measure the ID of the bearings. Platigage is not very accurate but can work on a grocery getter application and is better than installing and hoping you are OK.

 

[tom3]

I've always thought that plastigage checking is probably better than my measuring abilities at those small tolerances. Just me.

 

[nonickatall]

I've always thought that plastigage checking is probably better than my measuring abilities at those small tolerances. Just me.

Easier to use, more practical for the average mechanic, who doesn't have an extensive precise measuring tools and sufficient for engines like the LQ, if you don't want to turn it into a 1000hp racing engine. And it gives a good indication of whether the crankshaft bearings are worn or still acceptable. Even if measuring with a dial gauge is of course technically the right thing to do.

 

[RST Dana]

I've always thought that plastigage checking is probably better than my measuring abilities at those small tolerances. Just me.

Same. It worked for my 400 HP+ street rides in the 70’s.