im in the stages of building mine up and i just put plastic bolts through my u/c as a mates u/c also done the same breaking wood out of the bottom...it dont matter if your a good pilot or not sometimes it may come down a little hard...Ive not broke a U/C as yet lol but i always usses plastic bolts cause if i do it just snaps the bolts and dont break the wood out of the bottom...

anyway try pladtic bolts...

thanks Dean