i own a chevy 250 six cylinder i pulled out of my nova just siitng here (not going back in, putting a 400 in) any ways it was running about a month ago and i want to put it on my engine stand and get it running again. I want to take the heads off and clean them off and i want to take the pistons out, put new rings on and hone the cylinder walls. NOw my buddy says i can do what i want (he is a mechanic) but when i put it back together its gonna make a loud bang and stop running. He said anybody can bolt something together. What is he talking about?? Timing???? any help. thanks- CAleb  

Not a 100% sure maybe that you do not have any engine building experience not enough experience or maybe not you might want to ask him what exactly he mint by that.  

i tryed asking him but he said "im gonna let you find out and let you learn your lesson" .
 like an idiot he wont tell me.  
---Caleb  

First off, a real buddy would tell you what he's talking about. Sounds like this guys has no clue and won't admit it.

The big problem with disassembling an inline six is you have to be super careful when you loosed the head bolts, so the head doesn't bow and ruin itself. This requires loosening each head bolt in sequence (reverse of installation) a little at a time so you gradually release the torque and stress on the head.

Now, on honing the block, what you sometimes run into with I-6 engines is the front cylinder will warp a bit because of the uneven coolant flow because the water pump comes in right at the front of the motor, so that cylinder gets cooled more quickly than the back cylinders.

If the motor was running fine beofe you pulled it, I'd just pull the head (CAREFULLY!!!) clean the top of the pistons, clean the head, new cam and valvetrain parts, new head gasket, then put it back together. Throw a fresh coat of paint on the motor and sell it.  

Seems kind of strange he won't tell you the reason? Does he have some alterior motive for you not to rebuild the motor?  

No he has no motive not to. This is the guy(him and his dad) that is rebuilding my 400 for around $1200-1300.    thanks -caleb  

Well, my brother rebuilt a 250 once to running condition by redoing the vavetrain, but it does have a loud knock in the crank case from past abuse and a bad attempt at a rebuild, so if your not sure, DONT OPEN IT! but we are here to help, and like stated before, the valvetrain is a good place to start, along with the fuel pump and carburetor, small things to get you started that arent easily damaged, but as for the rings, if it isnt smoking bad, burning oil, or losing compression, they should be fine. (assuming low experience from your last post that someone else is redoing your 400)