Outboard Mounting Bolts - No washers on Transom Bracket Side?

[Tin Man]

I noticed on my new 2023 Tracker Pro Guide V16SC that the OB mounting bolts do not use washers on the outside Transom Bracket side. The inside (inside transom) uses a transom support plate at top with washers and large washers on bottom bolts.

I looked at outside transom bracket mounting bolts on:

1. A newer (2018+) 1650 Lund Rebel with a Merc 60hp and it also did not have washers on outside Transom bracket side
2. My same Tracker at Bass Pro and one at the Tracker Service Center....they also did not have washers on outside Transom bracket side.

I also looked up Mercury Parts Catalog and it only showed washers on inside....none are shown for transom bracket side.

It seems weird to me....as I always used washers on all my outboards on the outside transom bracket. The lack of washer seems wrong to me , but I'm no mechanical engineer!

Anyone have similar experience on newer outboards of similar HP?

Thanks

 

[Al U Minium]

My thought would be large washer on the inside to distribute force as the inside is likely a wood filled space or something. If it was just a nut against a small washer, I would expect the bolt to work right through the inside stern leaving the motor teetering. The outboard mounting bracket is quite strong and a washer isn't going to do anything to make a stronger tie to the stern. The top bolt has a locking nut but the bottom shows the head of a cap screw. That would bother me why it is like that. I would want the lock nuts all like the top one shown in the picture.

 

[eeshaw]

I just mounte a 125 mercury to my boat and I didn't use any washers on the bracket side of the transom. Big washers are all on the inside. I also have a reinforcement bar on the top inside of my transom to help distribute the load too.

 

[MrGiggles]

Those holes are pretty precise and the casting is fairly strong, really no need for a washer.

On some motors the lower mounting holes are slots for ease of adjustment, in that case you'd want a washer, otherwise there's really no need.

 

[FuzzyGrub]

G3/Yamaha: All are bolt thru from transom, washers on both sides, each bolt has two nyloc nuts

SunTracker/Merc: Top two bolts thru from transom, washers both sides, single Nyloc nuts, bottom bolts thru from engine side, with washer. Not sure what is on the inside.

 

[Tin Man]

My thought would be large washer on the inside to distribute force as the inside is likely a wood filled space or something. If it was just a nut against a small washer, I would expect the bolt to work right through the inside stern leaving the motor teetering. The outboard mounting bracket is quite strong and a washer isn't going to do anything to make a stronger tie to the stern. The top bolt has a locking nut but the bottom shows the head of a cap screw. That would bother me why it is like that. I would want the lock nuts all like the top one shown in the picture.

There are brass/bronze nylocks on all 4 bolts. Two bolts go from inside transom, out, two go from outside, in.

 

[Tin Man]

G3/Yamaha: All are bolt thru from transom, washers on both sides, each bolt has two nyloc nuts

SunTracker/Merc: Top two bolts thru from transom, washers both sides, single Nyloc nuts, bottom bolts thru from engine side, with washer. Not sure what is on the inside.

I read somewhere years ago.....
the bottom mounting bolts go from outside in, as they are underwater and threadless shaft of bolt is preferred with threads only inside of transom......to prevent water entering via threads.

 

[Tin Man]

G3/Yamaha: All are bolt thru from transom, washers on both sides, each bolt has two nyloc nuts

SunTracker/Merc: Top two bolts thru from transom, washers both sides, single Nyloc nuts, bottom bolts thru from engine side, with washer. Not sure what is on the inside.

I checked my friend's 2023 Suntracker Bass Buggy 18 with 75hp Merc....washers on all 4 bolts outside on transom bracket. Of course its not mounted to a transom, but that should not make a difference on transom bracket side.

 

[LDUBS]

G3/Yamaha: All are bolt thru from transom, washers on both sides, each bolt has two nyloc nuts

SunTracker/Merc: Top two bolts thru from transom, washers both sides, single Nyloc nuts, bottom bolts thru from engine side, with washer. Not sure what is on the inside.

Same with my Honda outboard. Small washers on outside. Big washers on inside. Two nuts per each.

 

[JL8Jeff]

I found some pictures of my boat (it's currently in the water) and it has small washers on the motor/bracket side and I know it has big washers inside. The lower bolts are inward and the upper bolts are outward.

 

[airshot]

My Sylvan has aluminum both inside and outside the transom so no washers. On my previous boats there were washers on the inside against the wood, and some washers outside as the aluminum on the transom was not that heavy. My Sylvan transom is much thicker/ heavier than previous boats.

 

[sonny.barile]

Mine has hex head bolts on the inside with large flat washers. There is silicone sealant under the washers. The motor bracket side has locking nuts and small flat washers. It came set up from the dealer.

 

[LDUBS]

I read somewhere years ago.....
the bottom mounting bolts go from outside in, as they are underwater and threadless shaft of bolt is preferred with threads only inside of transom......to prevent water entering via threads.

I would have never thought of that. It makes sense. On my outboard the mounting bolts are from inside out, so the threads are on the outside. But they would be above the water line (I think?).

 

[DaleH]

1st off, recall that from an engineering standpoint, there are 4 reasons to use a washer - load distribution - spacing - vibration absorption - & liquid prevention.

Yes, a large diameter 'fender' washer will spread the load over a larger area - that's a given, But another purpose (not stated in engineering manuals, LOL!) is to protect the material surface or finish that is being bolted together. One way to think if it, is that you'd want the washer against the nut that rotates, where sometimes you may notice the washer SPINS. In this case from the photo shown, it looks like there may not even be enough room to add a standard size washer under the bolt head. When torqueing that assembly, if you cranked on the bolt head, you rotate the bolt head against the OB bracket and would scuff the finish.

But be that as it may ... the washer under the nut also allows the nut to be properly torqued up and if needed, the washer can rotate against the parent material it is being affixed to. This is a common assembly practice. In fact, when dealing with marine vinyl, one uses 2 of the special cupped washers (see photo) under the screw or bolt head ... the 1st is to hold the material in place and the 2nd (next to the head) allows it to 'spin' if needed as the fastener is tightened. If there was only 1 washer ... and it spun ... it could tear or cut through the material.

 

[FuzzyGrub]

I checked my friend's 2023 Suntracker Bass Buggy 18 with 75hp Merc....washers on all 4 bolts outside on transom bracket. Of course its not mounted to a transom, but that should not make a difference on transom bracket side.

Does your 60hp Merc have slots or holes for the lower two bolts? Both of mine are slotted, but have seen a number of Merc that have individual holes. Makes raising or lowering more “fun”.

 

[DaleH]

True, if slotted ... definitely add a washer to help 'slide' the OB up or down. Easily accomplished with a car jack under the skeg (keep as vertical as can be). Or, lower trailer tongue, block up the skeg, then raise the trailer to slide the OB up. Removing the top 2 fixed bolts 1st, whilst loosening the bottom 2 'a bit', haha!

Doing so, I've moved my big V6 all by myself on my offshore boat, to raise the OB a hole or 2. One just must go slow and deliberate and NOT allow the OB to tilt - yikes!

 

[Beret Bolstad]

True, if slotted ... definitely add a washer to help 'slide' the OB up or down. Easily accomplished with a car jack under the skeg (keep as vertical as can be). Or, lower trailer tongue, block up the skeg, then raise the trailer to slide the OB up. Removing the top 2 fixed bolts 1st, whilst loosening the bottom 2 'a bit', haha!

Doing so, I've moved my big V6 all by myself on my offshore boat, to raise the OB a hole or 2. One just must go slow and deliberate and NOT allow the OB to tilt - yikes!

This is ingenious, I was calling the neighbor to help me.

 

[FuzzyGrub]

True, if slotted ... definitely add a washer to help 'slide' the OB up or down. Easily accomplished with a car jack under the skeg (keep as vertical as can be). Or, lower trailer tongue, block up the skeg, then raise the trailer to slide the OB up. Removing the top 2 fixed bolts 1st, whilst loosening the bottom 2 'a bit', haha!

Doing so, I've moved my big V6 all by myself on my offshore boat, to raise the OB a hole or 2. One just must go slow and deliberate and NOT allow the OB to tilt - yikes!

Yes, I used your method on the G3/Yamaha. Raised it one hole up.

 

[jethro]

Just looked at my Merc 150 Saltwater that was professionally installed by an authorized Merc dealer, washers and nylock nuts on all 4 bolts.

 

[DaleH]

Just looked at my Merc 150 Saltwater that was professionally installed by an authorized Merc dealer, washers and nylock nuts on all 4 bolts.

I’ll do you one better … I put nylon washers between anything stainless and anything aluminum on saltwater boats.