ive noticed that the #8 main bearing is a little wiped on a 350 ive got.... i was wondering if a bearing is just a little spun if i could replace it by just popping a new one in, or evey time you replace main bearings you must machine. that goes for a cam bearing as well. if i want to put a meaner cam in...could i just slide a new one in if the bearings are good. ive got some plastigage so im gonna check the clearance if its ok to repalce the bearing without the machining

You don't have to machine th eblock if it's not damaged, if that's what you're asking. Although there are only 5 main bearings, so maybe you're talking about a rod bearing not a main, in which case the rod is probably damaged. But again if it's not, you don't have to have it machined.

However, you will certainly need to turn the crank, otherwise it will just trash a new bearing just like it did the old one.

yeah i was talking about the rod bearing....thanks

so how about the cam bearing...if the bearings are good can i just slide in a new cam?

Absolutely! Most people do re-use the cam bearings. Just be careful removing the old cam... It's very easy to nick them with a lobe. Get some very long 3/8" NFT bolts to use as a handle...

If the crank journal looks smooth and only spun inside the rod you can just change the bearing and check the clearance, but the reason the bearing spun in the first place is that the big end was too large so there was not enough bearing crush, so change the rod as well, which means you are going to disturb the rings so a complete refresh is a good idea. Do it right or do it twice.