first engine build, small end to pistonpen clearance?

i have cp pistons and manley rods is it critical how much clearance the pistonpen has in the conrod smallend? if so how do i approach this??

If you're talking about clearance between the wrist pin and the inside diameter of the bushing in the small end of the rod then yes, this clearance is important. In most instances you tend to develop a feel first and foremost for what this clearance should feel like. Obviously this comes with experience and won't benefit a novice though. The clearance here is much tighter than the bearing clearances and may be in the order of 0.0005" or similar. Of course the engine workshop manual will give you the specified clearance and range but you may need to get your machinist to confirm the clearance if you don't have the required measuring equipment.

Okay thankyou, i have a side question.

I was assembling the head and i accidently dropped a valve on the ground. There are no visible marks.

Is it safe to assemble this valve or could there be micro fractures???

It's hard to really answer that. It's possible that dropping the valve may have slightly bent it (depending on how it landed), or it may result in a stress raiser that may later be a failure point. It really all depends on how far it fell, how it landed and what type of surface it landed on.

It fell from waist height and on a concrete floor, not sure how it landed. But basicly you would replace it just to be sure??

If I had to advise someone else then yes, I'd suggest replacing it. Very small price to pay for peace of mind and much cheaper than a dropped valve.