sealing outboard mounting bolts on new transom

[tbird175]

I am preparing to replace the transom in my outboard boat. My transom rot appears to have started around the through the transom mounting bolts (175 HP Merc engine). I have gathered from this forum that this is a common problem. Is there any thing that I can incorporate in my new transom design to help eliminate this source of future problems? I am playing with a way to sleeve the hole, possible by utilizing a transom cap or metal plate with tubes welded to it flush with the exterior side. The tubes would need to extend slightly beyond the interior side of the transom. Over size washers could be placed over the tube to provide a surface for the mounting nuts to tighten against. This would also provide a means of caulking around the tube. It would be a simple matter to seal the cap with tubes to the outside of transom. This design along with the sealant normally used on the bolts should provide a better seal. Any water that makes it way through the sealant would follow the tube into the splashwell or bilge of the boat.

I welcome any comments on this possible procedure or any suggestions on a better way of doing this. Maybe I am being overly concerned with this problem, if you think so, tell me.

 

[Bondo]

Re: sealing outboard mounting bolts on new transom

Ayuh,... Ya could go through all that,...

I think a liberal bathing of the mountin' bolts with something like 4200 will do just as well though...

 

[ajgraz]

Re: sealing outboard mounting bolts on new transom

What you want to do sounds like a Cadillac solution, when the Chevy solution everyone's been using since time immemorial works just fine. In other words, I agree with Bond-o above; some 3M 4200 (or 5200) on the bolts will do the job.

If there is a problem with what you propose, it is this: what if you need to raise the motor on the transom (i.e., change the "z-axis")? Or what if you ever repower with a different brand of OB? Then your tubes are no longer in the right place. If you just use the old "holes-drilled-in-the-transom" technique, then you can just overdrill, dowel, and epoxy the old holes and simply drill new ones.

 

[tbird175]

Re: sealing outboard mounting bolts on new transom

I am known for overkills on my projects, retired engineer with two much time on my hands I guess. Thanks for the replies.

 

[Bob_VT]

Re: sealing outboard mounting bolts on new transom

PLEASE do not use 3m 5200 ............ you may want to remove the bolts or loosen them someday!!! Use a 3m 4000 series sealant......... do not use the polyurethane type.

 

[Yacht Dr.]

Re: sealing outboard mounting bolts on new transom

PLEASE do not use 3m 5200 ............ you may want to remove the bolts or loosen them someday!!! Use a 3m 4000 series sealant......... do not use the polyurethane type.

I dont normally disagree with you Bob..but in the cases of Trans bolts I would Strongly recommend 5200 for the bolt holes. You can get the bolts out ( they are threaded not mounted and can be removed in a pinch ).

Any Underwater transducer/bolt/screws should be beaded with 3m 5200..( fast cure wont drip like the regular 5200 ).

Of cource anything beaded in 5200 ( especially flat things ) are Permanant seals if done correctly.

Id rather have a pain in the butt trim tab screw or mount bolt to take out then a leak ( or possible there of )

Also..be very carefull about suggesting 3m 4000 series..There IS 3m 4000 ( some above deck hardware ) and 4200 which is more versitile then 4000 sealant but 4200 is more aggressive. They are not one and the same..

Long and short..5200 for underwater screws and transducers/mount holes but not the whole trans plate..use 3m 4200 for "caulk" underwater... kinda..

YD.

 

[Woodonglass]

Re: sealing outboard mounting bolts on new transom

Another iBoater, I don't remember who, posted that he oversized drilled his holes, mixed up some Thickened epoxy and used either aluminum or brass tube as sleeves for the bolts. He then Used Aluminum plate on the inside and out to fabricate mounting bracket for the motor, thus alleviating the possibility of EVER having any LEAKAGE around the thru bolt area.

I'm just sayin...

 

[barry harrison]

I am new to the forum but this is what I do. Countersink the holes on both sides of the transom. Buy eight half inch stainless fender washers. On the bolts put the standard ss washer,then the fender washer.Then put a generous bead of 5200 around the bolt creating a fillet from the bolt to the fender washer. Put the bolts in from the inside. On the outside put another generous bead of 5200 creating a fillet from the bolt to the transom and covering the chamfer. Then I put another ss fender washer, a piece pvc pipe cut to length, a washer, and a nut. Lubricate the threads and snug not tighten the assembly. Let the 5200 cure completely. Once cured, remove the nut, washer, and the pvc pipe. The bolts are sealed and the chamfer acts like an oring seal when you install the motor and tighten the bolts. If you need to move the motor up or down, it is not a problem the bolts do not need to be resealed. You do not have to worry about the slot at the bottom either. This slot has caused many transoms to rot. I have to replace the transom in my 23 Steiger Craft now due to someone filling the slot with silicone thinking they were sealing the bolts. This is a big problem with outboards even factory rigging. thanks for letting me reply. sorry about my typing.

 

[Bob_VT]

Please read the forum rules........ this post is 3 1/2 years old!!!!!!!!!!!!!!