everything new in motor  mechine work done  balanced every thing checked out good put it together an it was running good think i drove it somewhere  around 4000 miles then started bulding up pressure in radiator  every one said its prob ur head gasket so i took it off an it looked good

Is there a question here?

Who is every one that told you it's probably the head gasket?  Any experienced mechanics amongst them? Did they also tell you that it is normal for an engine to "build pressure" in the cooling system up to a point - the cooling system is designed to run at approximately 13 psi over atmospheric pressure?

What is the "everything" that checked out good?  Did you check the head & block surfaces for warping?

Now that you have taken the head off and disturbed the gasket, you're going to need to fit a new one and start over again - this time don't forget to retorque the head bolts after the first few hundred miles.

We need a little more than you have provided if we are to assist, information like what engine are we dealing with, are there signs of overheating, how long it takes before it "builds pressure", and please - punctuation makes a difference.