motor wont turn over?

well, I figured it out. the hard way. it was the heads. when I tore the engine down to inspect the top end I used a different setof heads. I had a set on the shelf, shaved ten thousanths. I guess between the heads and the flat top pistons there was to much compression with the plugs in. after tearing it back down for the 3rd time I swapped heads and she turns over just fine. I just can't beleive that ten-thousanths would make that big of a difference. Like I said, this is the first time I have ever had this problem, and it seems like I have run these heads on a flat top motor before but I just can't remember for sure. but any way, thank you guys for the help