Motor bolt hole question
 

I'm reading in multiple places people talking about drilling oversize holes for mounting the motor to the transom and then coating the holes with west system to prevent any water that makes it pass the LifeCaulk from getting into the wood. Anyone done this? Wondering how much bigger of a hole. 1/2" bolts so 17/32 or maybe 9/16? I'd like to do this to the blue boat before I hang the motor which is going to happen very shortly.

I don't know about anyone else, BUT I DO know tolerances etc. I know that the larger the hole, the greater the surface area and to me, the greater the surface area, the more likelyhood of water intrusion.

To my way of thinking the engine vibrates when it is running. No matter how smooth and quiet, no matter how great the anti-vibration system (if installed) it still vibrates. So if you have oversize holes that vibration is going to transfer from the engine, thru the bolts that the engine is hanging on and into the transom. If there is an oversized hole then the vibration must transfer thru the sealing medium first before entering the transom. So the larger the hole, the greater the chance of the vibrations, over time, destroying the sealing medium that it's passing through. It just doesn't sound like a good situation. Maybe it will last forever, maybe not.

Here's what I do. (Understand this is just my opinion, based on life experience and not some scientific answer)..... I take the holes that are there and I give them a very liberal coat of 3M 5200 sealant, inside and around the edges.. and I rub the stuff into the wood with a dowel, piece of scrap wood, whatever. Then I coat the underside of the bolt head and the shank of the bolt with the same sealant, slide the bolts through the holes, coat the area right around the bolts where they peek through the transom, touching the bolts with a continuous bead around the entire bolt, let it dry and then I hang the engine. This process insures that the bolt holes are 100% sealed against water intrusion.
If you feel that's not enough, then I would use some kind of epoxy liquid wood sealer https://www.jamestowndistributors.co...Fd5WDQodZ3oBfA that will be absorbed into the wood first, then after it dries do the process I described above.

I've always used large amounts of 5200 myself. I do my best to cover the inside of the hole before inserting the bolt, then I put a coat on the shoulder of the bolt before inserting it, the excess builds up under the bolt head sealing it water tight, then I put some around where the bolt exits the transom, and bolt the engine on. Has worked well for me so far.

That sounds like a very bad idea to me. I coat the hole good with 5200, let it dry, then 5200 again when I mount it. Epoxy would work well for the first step of sealing the hole, but the 5200 is flexible which is important.

squid, that transom looked like it had some work done to it. whats in the holes now. if its just bare wood. i'd probably take some epoxy and coat the wood, then 5200.

on smaller stuff like transducers, swim platforms, etc. I've done the over-drill, then completely fill hole with epoxy. after curing, drill pilot hole and mount screw.

I charge a whole lot of money to take motors off that people used 5200 to mount them. You should have to have a permit to buy that stuff. Life caulk works fine and isn't permanent. If you want to do it right, drill the hole over sized to fit a piece of 1/2 ID PVC, CPVC or PEX pipe. Coat the inside of the hole with epoxy, making sure to cover all exposed wood, coat the outside of teh tubing and insert it in the hole, wait till it cures and cut it off flush. Now when you mount the engine, only use regular silicone

Thanks for the feedback. The transom has bare wood in the hole now. But on a good note it's dry wood. Think I'll just get some 5200 and use an acid brush and brush it in the holes for a couple coats then just use LifeCaulk to seal the motor on. I've read horror stories about using 5200 to actually seal the motor on. And as much as I pull motors off that's the last thing I'll need.

I don't know to much about that but I do know that 5200 3 m is supposed to be permanent If and when you have to remove the engine it's going to be a real hard time your going to have I would not go by what I said but I would ask the heavy hitters and see what they suggest you to use

I'm thinking about putting a couple coats of 5200 in the bolt holes and letting it dry completely then seal the bolts with LifeCaulk.

I drilled my holes a 1/2 inch larger than my motors bolts. I then hotcoated the holes and taped one side closed. I then filled the hole with thickened resin and then glassed 2 patches of 1.5 csm on either side. The next day i drilled the holes to the actuall bolts size leaving 1/4 ring of glass around the bolt hole. Now this is the extreme end of the spectrum! And almost a little radical. Hot coat with West Systems 105 then use 3m 4200. I used 5200 on my first motor i mounted on my boat....when i took it off to put my suzuki on it pulled the paint and some gelcoat right off. I would west systems and 4200 if i did it again. If yo dont have west systems use a vinylester resin or a thinned polyester resin