I also bought a drill guide,made by General.We used it to drill all the holes in my new transom to mount the outdrive transom assembly.
The mounting holes have to be at the angle of the transom,and it's important that they be right on.The guide was a big help to keep the drill at the proper angle.It's not NASA quality,but it's far superior than doing it by eye.
We also coated the inside of each hole with two coats of epoxy resin,to protect the core from any possible water saturation.
http://www.sears.com/shc/s/p_10153_1...2&blockType=G2