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The Yamaha F50 is mounted all the way down on my transom on my 1992 (Joe - Notice the 4 digit year? =8-) ) 15' GLS. One of my winter projects is to pull the motor off, fill in the existing holes, and re-mount the motor higher.

The current bolts are 1/2-20 stainless steel. They are about an inch too long (that's how much thread is sticking out past the nuts). There's no source for 1/2" fine thread stainless steel bolts around here, so I will be getting them from McMaster Carr (thank goodness they sell small quantities).

I have a couple of questions:

Should I go with 316 or 18-8 stainless?

With regard to the locknuts (the boat currently has two nuts "locked" against each other), should I go with Nylock, Flex-Top expanding, or distorted thread?

Thanks

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If the extra bothers you, why not just saw off the bolts you have? Just keep both nuts on there, saw off half an inch, grind or file a bevel on the end. Then when you take the nuts off, you'll reform any damage to the threads. The double nut thing has worked fine for me, both from motor to jackplate and jackplate to transom.

If you do go new, there's not really any need to use a fine thread - a conventional 1/2-13 from your local HW store will be fine. However, if you really want top shelf, I'd vote for 316 with a conventional nut first and a nylock on top.

Tim

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Every engine mounting bolt I've ever seen that came with an engine was fine thread. They do have more holding power.

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It doesn't have anything to do with holding power. OMC used to use course thread mounting bolts. You can use either though fine thread is easier to torque accurately.

I'd just use the bolts you have and cut them down. The ends will need to be careful cleaned up on the work bench but that is a trivial matter.

If you buy new, I'd but 316 for no practical purpose in this application, they just look slightly nicer.

Yamaha's practice has been, and continues to be, to use double hex nuts on the mounting bolts. Mercury's practice has been, and continues to be, to use a brass hex nylok lock nuts. OMC and BRP uses a stainless steel nylok lock nut, which is what I prefer. The double nut idea is simple but looks rather amateurish in my opinion, though it works perfectly well.

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If those are the original Yamaha mounting bolts they are metric M12, not 1/2", almost the same but nuts will not interchange if you decide to keep the bolts or nuts and swap the other.

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In the Midwest, most Ace Hardware stores carry 1/2" fine thread bolts and SS matching locknuts. Mercury uses fine thread and brass nuts to elminate the risk of galling.

If you use course thread with locknuts, be sure you put a dab of grease in the nut before putting it on.

You can buy complete bolt sets, including washers, from Mercury, in the length you need. That is what i would do.

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I second the cut down of the lenght of the bolt, However I would fine some nylon (aircraft) locking nuts, then dab some 5200 on the exposed threads, this will not come off unless you take them off...

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None of the Ace's around me carry SS 1/2-20 bolts.

Yamaha has the worst SS hardware on their outboards known to man. This is a 2009 F50. If you looked at the hardware, you'd swear it was on a 30 year old outboard that was left in the water year round. Everything is heavily oxidized. My boat stays on dry land and I wash it off after every use.