hi guys we are rebuilding a 350, just dissambled the whole engine.
i picked up a plasti gauge from auto zone.
the piece of plastic is very small.
please explain how it works?
do you leave the old bearing in place when you use the gauge?
or take out the old ones?

Put a strip of that little dental floss looking stuff between the bearing and the journal you want to measure and torque it to spec. Don't rotate anything!. Then unbolt it carefully and compare the compressed thickness of it to the little gauge chart that comes with the stuff and it will show you how much clearance you have.

If you are replacing the bearings no real reason to measure the old ones.
When you use the platiguage do not put any oil on the assembly you are measuring, it takes up part of the clearance & gives you a false reading.
Plastiguage is definitly better than not checking but the only accurate way is to mike the crank & use a dial bore guage to measure the bearing bore. It is also best to use the mikes you measured the crank with to set the bore guage. Then even if the mikes are off a little it will make no difference in the clearance measurement ;)

As Wolf said, have it checked with a bore gage. You'd be surprised at what you might find. I checked a guy's block that had the mains .003" out of round. Half the diameter was right where you would want it, the other half who knows what happened. This was right after it was align bored.

Thank you for all the info.