Without seeing the dome configuration and chamber design this is a shot in the dark but it could this be a quench issue. How far was the piston down in the hole? Were the heads planed? Did you degree in the cam properly? No offence intended but W/O knowing your mechanical ability I have to ask. If I were you I'd call the piston manufacturer and explain your problem but be sure to have cam grind sheet, head numbers and head gasket manufactuer name and part number. ( yes that can make a difference) Best of luck. 

Sorry To Interrupt:But,I Have A Question I Rebuilt My 305Chevy Motor My First Rebuild.I Used Flattop Pistons Standard Bore 58cc Heads Shaved  10 Thou.  260/270 dur. 394/414 Cam W/QjetCarb.It Rattles/Sounds Like A BB IN A JAR But I Can Back Timming A Little And No Sound.Oil Pressure Runs 40 Pounds @  65mph. I Used Mach.Shop And Spent 1300 Bucks. It Has Run This Way For 75,000 Hard Run Miles And Is Still Running Good And Strong My Onley Ever Rebuild  

I have a question I rebuilt my 305 chevy motor 11/15/01.My eng. rattles like a bb inside a glass tube.It has ever since break in. However;I can back my timing off and it is quiet(dont rattle when racing motor and i have run it that way since first discovering cond.) I run standard bore,flattop pistons,58cc heads shaved.010,and mycam is 260/270 duration;with 394/414 lift still smooth idle. 83eng.I run 40 psi. at65 mph 350turbo tranny and. 2.41 rear. Motor has 75,000 hard run miles and still running good. I would like to be able to advance my timing if i knew what was causing my bb rattle.I think my compression may cause valves to rattle in guides.unsure.Your reply or opinion from someone with more experience than i have will be most helpful and greatly appreciated.Thanks