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I am trying to determine what is the correct size (diameter wise) for an Outboard install.
In a previous thread I mentioned that I was fabricating custom transom saver plates because of a situation that was discovered with my Dauntless 15. I assumed the hardware used to secure the Yamaha 90 were 1/2-20 S.S. bolts. Upon inspection they measure 7/16-20. Does any W/C member have an idea what bolt size Boston Whaler may have used originally for motor mounting on a 1996 Dauntless 15 or similiar sized Whaler? Any and all help will be greatly appreciated before I attempt drilling out the existing holes for 1/2 hardware.

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All mid size to large size engines I have seen use 1/2" bolts.

Will a 1/2" bolt fit through the holes in your engine bracket?
If these holes are only for 7/16" bolts then drilling 1/2" holes in the transom won't do you any good.

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At the OEM level, rarely are the bolts shipped in the engine rigging kit used. Most are substituted with 1/2"-13 SS hex-heads with Nylock nuts. The reason being as transom thickness's vary, and one length does not work for all transoms.

Another thing most builders do is drill out the transom mounting holes to 9/16". It's very difficult to get all 4 holes perfectly lined up and perpendicular to the transom, drill fixture or not.

I've hung and rigged a few motors in my life. This method works...

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Hello Joe and Bob, thanks for the replies. To answer your question Joe, yes the P/T bracket mounting holes are clearanced for 1/2 Dia. hardware. I will be un-bolting the Yamaha in the next week or so for the install of the transom saver plates I made and will switch over to 1/2-20 S.S. bolts and Ny-Loc nuts for a sturdier assembly. Thanks again for the info...

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For my money, the finest engine mounting bolt kits come with new Mercury outboards, as part of your engine purchase.

These bolt kits can also be ordered from a Merc dealership, in optional lengths.

Furnished with the engines, 40 hp and higher, are 4-1/2" x 1/2" FINE thread bolts, with a small outer diameter "ring" washer that goes against the engine, and large, thick fender washer for the boat side, and a BRONZE or BRASS (not sure which) fine thread nylock nut, to prevent galling. On all engine installations, Mercury recommends the bottom bolts go in from the outside, and the top bolts from the inside. This provides for the best caulking seal on the lower bolts.

If you don't want to buy the Merc bolt kits, I have found fine thread SS 1/2" bolts and nylock nuts at Ace hardware. Fine thread is highly recommended, but be sure to use a dab of grease or Anti-Sieze on the threads of the SS nut is used..

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Thanks Finnegan for the info. I am not sure what length bolt I will need yet, will calculate soon. I had read somewhere about the Mercury install technique you described, sounds logical. Thanks again all for the help.

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The "ring" washers that Larry refers to are known as SAE Washers (Society of Automotive Engineers Specification) and have a 1-1/16" outside diameter for the 1/2" size. Stainless steel ones are common and available at any good hardware store.

The lock nuts supplied by Mercury are brass, not bronze and these too are available at good hardware stores. Stainless steel lock nuts work just as well so long as you tighten them by hand and use some anti-seize or grease on the threads.

I recommend you insert both the upper and lower bolts from the inside. It gives a cleaner appearance from the inside of the boat and you must seal the lower bolts no matter which way you insert the bolts.

If you do use brass nuts and boat in saltwater, then you would want to reverse the lower bolts as Larry suggests to reduce the electrolysis.

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In reference to my above post, here is a link to some pages out of Mercury's engine rigging manual that is of value and interest, regardless of your engine brand.

http://www.marinepartsman.com/install...-boat.html

Note that they recommend drilling the transom bolt holes 17/32".

Tom, I have had a hard time finding the SAE small washers, and also the brass fine thread nylock nuts. Even the 1/2" fine thread bolts are somewhat scarce.

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Fine threads are not a necessity and the last three (all 2009 production) motors I've hung with the Mercury Kit had all stainless hardware and bolts were coarse threads. The 9/16th drill bit is fine if you dont have access to the motor template, just be generous with the life caulk. Don't use cheap silicone, and don't get the life caulk on the threads, you'll stand a reasonable chance at galling the threads.

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Hello W/C and members and thanks for the inputs. In responding to Tom's install suggestion, on the '96 Dauntless 15 the lower bolts sit in a well just below the splash well and are hidden by a cover that fits over this opening. Its secured with sealant and fasteners. For these 2 bolts I could install from the outside feeding in with sealant. In this manner the threads and nuts will always be clean of salt corrosion. I only use my boat in salt water Tom, fresh water access down here in So. Florida has gotten a bit touchy in the last few years (unless I head out towards the Fla. Everglades National Park) thanks in part to the many irresponsible jet ski and wave runner owners. Again thanks for all the inputs.


Angel

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I just got the chance to check the bolt size that was used to secure my outboard. They used a metric thread size M12 X 1.25 X 4.0 L not the 7/16-20 I had thought and they are hex head. Did B/W ever use metric hardware for motor mounting? I've got some re-configuring to do to make this upgrade standard size. Thanks again for everyones help.

Angel

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12 mm is almost 1/2". It is actually between 15/32" and 31/64". Those are Yamaha mounting bolts, Mercury does not use those.

Larry, I went to he hardware store yesterday. They had stainless steel SAE washers in both type 304 (18-8) and type 316. How many do you need? I'll send you some.

For the rest of you, be sure to check out McMaster-Carr if you cannot find obscure hardware locally:

http://www.mcmaster.com/

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Thanks again Tom for the input. I just had the Aluminum hardware that I fabricated powder coated in semi-gloss white, should compliment the transom area well. Will post some pic's soon.

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I installed my engine last summer and then decided to move it up one hole. I had used SS bolts originally since that was all that was available locally. I had to break two of the bolts to get them off since they threads had galled. I replaced them with Silicon Bronze nuts and bolts from here: http://www.boltdepot.com/Default.aspxfault.aspx

Bolt Depot used to be a small family run hardware store south of Boston. When Home Depot moved in down the street they were forced to close and become Bolt Depot. I used to go in there when I was a kid with my dad. They had absolutely every kind of fastener that you could imagine in any type of material you could ever want. They have a lot of strange stuff that isn't even on the website.

I always had great service in that store.

Kevin