New transom motor mount holes technique?

Nearly finished with transom replacement. Details: 1-1/2 in. marine ply encapsulated, faired, covered w/6oz. cloth. I've read about drilling oversize motor bolt holes (this is a 50hp outboard), filling w/thickened epoxy, then re-drilling correct size holes—idea being water can never reach wood. Question is, how much oversize to drill initial holes? Also, should hole be drilled, filled and re-drilled BEFORE laying cloth, or after? Also planning on boring 2 splashwell drain holes w/same technique...thoughts or experiences?

 

Hi greenacre, If you are going to cover both sides it woud be easier drill the holes and then to cover one side first when it sets up you can then parge in your bog into the oversized hole and it wont goosh out the other side.

You could also glass on a heavier pad a bit larger than the washers to spread the loads

 

I am not familiar with the thickened epoxy method. The way I do it is when I use 12 mm bolt for the motor, I drill 16,5 mm hole and use 16 mm stainless steel pipe 1,5 mm wall thickness to line the hole. Cut the tube 2 mm short of total transom thickness for the motor mount and 10 mm longer for the splashwell. Smear Sikaflex 11fc on the pipe and inside the holes after brushing them out with pipe brush on drill. Then put the pipes through the holes and turn them while pushing through, to make sure that the Sikaflex is solid between the pipe and wall of the hole.

 

Your S.S. pipe thing works as a compression tube, but not so much at preventing moisture past the sealant and into the freshly replaced core.

Drill your over size holes, at least 1/3 bigger then the fastener shank diameter. Fill this hole with thickened epoxy, after wetting out the raw wood with neat epoxy. A piece of duct tape on one side will keep it from falling out as you pack it in. When cured, drill for the proper size fastener hole and that's it. Log onto westsystem.com and download their free user's guide, which explains this as well as other techniques. This is called fastener bonding.

 

Have also used dowel slightly longer . drill slightly bigger than the dowel and epoxy in place ,drill the dowl the correct size for the bolt and seal with sika ! easy to repair anytime down the track as its just wood !.plane them level when you finished . Have dome this a lot over the years where holes have been butchered and not good !!

 

This seems like the best technique to use in my situation. I presume the epoxy surrounding the motor bolts will be stronger (and more waterproof) than the plywood core. Thanks to all for the replies...all good ideas!