CONTINUOUSWAVE

I am temporarily changing outboard engines. The mounting bolt pattern on the temporary engine is different from the mounting pattern on the engine already on the transom.

Q: What is the best method to fill the transom mounting holes so that I can re-use them later?

I was thinking of using wood dowels to fill the space in the transom wood while leaving a 0.125-inch to 0.250-inch depression below the gel coat. Then fill the depression with epoxy so it would be waterproof.

Oldslowandugly wrote:I am temporarily changing outboard engines. The mounting bolt patterns on the temporary engine is different from the mounting pattern on the engine already on the transom.

Of the two engines involved, which engine has the mounting holes in position according to the BIA standard: the temporary replacement engine or the permanent engine?

What is the duration of the use of the temporary engine?

Oldslowandugly wrote:I was thinking of using wood dowels to fill the space in the transom wood...

Don't use a dowel. Cut a plug from marine plywood to fill a large diameter hole in the transom as would occur with an old engine mounting bolt hole, and avoid the mismatch in the wood grain orientation.

jimh wrote:Of the two engines involved, which engine has the mounting holes in position according to the BIA standard: the temporary replacement engine or the permanent engine?

The 48-HP engine has the BIA standard mounting hole locations.

The temporary 30-HP engine has two of the old-time thumbscrew clamps as well as a couple of bolt mounting holes farther down the engine mounting bracket.

The bolt holes are one half inch diameter right now. To match the thickness of the transom I guess I would need several half inch plugs of plywood to fill the holes with. The thickest marine plywood I have is only one half inch thick.

jimh wrote:What is the duration of the use of the temporary engine?

The temporary motor will probably be on for the entire 2021 boating season. After that then those holes will need to be filled as well.

I don't understand what you said about the grain orientation.

Would that cause a dowel to be stressed?

Cover the existing BIA-located holes with duct tape.

Clamp the 30-HP on the transom. Drill the minimum diameter hole for any new holes. You don't need a 0.5-inch diameter bolt for a smaller engine.

If you insert a dowel into a transom bolt hole and then cover it with epoxy, it will crack due to the shrinkage of the dowel. The grain of the dowel is going perpendicular to the transom face.

Hardwood plugs are recommended. The grain of the plug runs parallel to the transom.

I removed the 48hp motor today. The bolts were one half inch but the holes were slightly larger. Phil, I had a 9/16" hardwood rod so I used that. The transom measured two inches thick so I cut the rod into 1.5" lengths. The 9/16" wood plugs fit very tightly into the holes. I left about 1/4" open at each end. That worked well as the wood filled where there was wood, and I used epoxy to fill the 1/4" where the gel-coat layer was. I am confident no water will get in. When I need them again, I can just drill the epoxy and wood filler and have my holes back. Tomorrow I will hang the 30hp motor and make new holes for that mounting pattern.