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I recently purchased a 1998 Mercury 30hp ELPTO outboard for my 1963 13ft whaler, it did not come with the thumbscrews for mounting to transom. The OEM thumbscrews cost around $100. I would like to save a few bucks and replace with common bolts. I'm wondering if anyone knows what type of threads are used on these motors? A friend thought they used acme threads. Any ideas on this? Or perhaps I should just buck up and pay to play.
What boat will you be mounting the engine too? Sport 13? Have you considered using bolts to mount the motor to the transom rather than use the clamp mount?
Being fairly familiar with Mercury outboards, they always have furnished 1/2" FINE thread bolts, with brass nuts (to avoid SS galling) for engine installation. So I would guess that the threads in that bracket are also fine thread, maybe also 1/2".
You should be able to tell diameter by looking at it. Then go over to your local ACE hardware and pickup a fine thread bolt to test, in cheap cad plate. If it works, then buy the SS versions, which are usually found in those pull out trays.
But I am confused, once found, it is your thought to tighten the bare end of the bolt to the fiberglass transom. Seems like it would damage it. Don't most of those thumb screw bolts have a large diameter plate on them?
But along the lines of what has been suggested above, and looking at that transom bracket configuration for your engine, I see a top set of 3 mounting holes for through bolting. I think that is what should be done, and is probably the reason you don't have the thumb screws - they are not needed nor recommended.
Thank you so much for the replys I am somewhat of a newbie in the boating world so thanks to all that make the experience fun. I guess I left out some specifics on the method of my madness. I am mounting the Mercury 30 hp to a 1963 13ft Classic which has a raised metal fabricated mounting plate. Before doing any permanent drilling through the mounting plate/transom I would like to temporarily secure the motor to the transom plate with the thumbscrews and lightly lake test the configuration to be sure the trim/mechanics are good before the drilling. The Fin plate above the propeller is just about even with the keel but due to the height of the raised mounting plate the engine can not be moved any lower which would require modifying/lowering the mounting plate. My goal would be to find a couple of bolts from Ace is the place and temporarily secure motor to metal plate, test then drill baby drill. Thanks Again
