Need help with drilling a hole in my wall to install a radon system

Hi, this is my first time posting. Wasn't quite sure which forum to post in so I hope this is ok. My question is that I'm installing a radon vent pipe and fan to run from my sump pit (which I'm going to seal) to outside the house. I'm drilling through cinder block and then through brick to the outside of the house. I rented a hammer drill and a 4.5 inch drill core bit from a local hardware store. The pvc pipe I purchased to run through the hole has an external diameter of 4.5 inches. I asked the guy if that would be too tight a fit, and he said no, it should be fine, since he had recently done it himself with the same size pipe. Well, it didn't work. I drilled the hole, but I can't even get the pipe in the hole.
Fortunately he refunded me, but now I need to figure out how to get the hole bigger. Because of various issues in my basement, this one spot really is the only place I can run this pipe without running into complications. If I rent another hammer drill with a five inch core bit, will that work to enlarge the hole? I'm concerned because there won't be anything for the guide bit to bite into, since I'm enlarging a hole I've already drilled. Think I'll be ok?
Thanks!

 

Hi, welcome to the forums here!

No, it won't be okay without the guide bit. What I would probably do in your case, is take a piece of 3/4" plywood and cut a 5" hole in it with a jigsaw so that the core bit fits exactly inside the hole in the plywood (your core bit probably won't want to cut the wood...) Then tapcon the plywood to the cinder block wall with a couple 1/4 x 2 3/4" tapcon screws (use a 3/16"concrete bit in a hammer drill) so that the plywood can't move. The 5" hole you cut in the plywood will act as the guide for the outside of your core bit. If your wall is thicker than your core bit is long, you "might" find that you need to come at it from both sides, using the plywood as the guide.

 

Hey thanks for the reply! When cutting the 5" hole in the 3/4" plywood, I'm assuming that should be just a bit larger than 5" correct, so the 5" core bit doesn't seize up? Would you suggest something like 5 and 1/16 or would you go with 5 and 1/8? Not sure how big is too big.

 

You need the hole in the wood to be as close as possible to the size of the hole saw so that the bit doesn't wobble or walk.

I've done something similar where I mount a board to the wall and drill a 1/4" hole exactly in the center to hold the starter bit. With this method you need to make sure before hand that the core bit you use will cut thru plywood. Many won't.