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Aluminum boat transom bolt torque?

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I just installed a jack plate on my Alumacraft which has a 20hp outboard; however, I could only get the 3/8 stainless bolts torqued to 22 Foot Pounds before the transom started bending inwards a little at the area of the bolts washer; my transom is hollow and has two .125 aluminum layers on each side - so 4 total. I have read about guys torquing transom bolts to 90 pounds and crazy stuff - although I think those are 1/2 bolts and on fiberglass transoms. If someone has some knowledge on aluminum boats, will 22 pounds work? It seems tight, have locking washers and nylon nuts plus sealer got all over threads, so hoping they're not going anywhere. 

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That should be okay. The standard torque for a 3/8 threaded bolt is 35 ft/pounds. However most 20 hp motors use two bolts and two hand screw bolts so i would think you are good. If you wanted too you could have cut some pipe the thickness of your transom and then slid them down the bolts (used as spacers)  and then added your washers, locktite on the nuts and tighten them down. You are probably okay though.

 

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I would think that on a transom like that a metal plate on the inside and outside through which to bolt would be used. Just an idea.

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I think you'll be fine as long as you used nylock nuts.

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You'll be fine 😉

 

My 40 hp Tohatsu is bolted with 1/2" bolts torqued 30# with Loctite. 

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Add 1 1/2" dia .39 id X.125 thick fender SST washers to spread out the torque load, use self locking nuts. You should ok.

Tom

PS, use 3M 5200 marine sealant on the bolts and washers.

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On 7/26/2019 at 1:49 PM, OnthePotomac said:

I would think that on a transom like that a metal plate on the inside and outside through which to bolt would be used. Just an idea.

Thanks, the exterior has the plate of the jack plate distributing weight - the interior not so much - have two larger fender type washers on top but had to use smaller washers on the bottom bolts due to space issues as there are transom braces and a plate welded in there which are higher areas, so had to fit the bolts sort of in-between these welds.

On 7/26/2019 at 11:28 PM, WRB said:

Add 1 1/2" dia .39 id X.125 thick fender SST washers to spread out the torque load, use self locking nuts. You should ok.

Tom

PS, use 3M 5200 marine sealant on the bolts and washers.

I have two larger stainless washers on the top bolts - almost size of fender washers, but washers themselves started bending there; would have liked to buy grade 8 washers but they're not stainless, no good options in finding strong washers - locally at least. Did use 3M 4200 on everything, it got all over the threads as its a mess, but maybe that will act as a loctite. The bottom four bolts have smaller washers and yes this is where bending is more evident unfortunately; it looks fine, but my main concern is that trailering it or on the water will not stress these areas and cause more bending? Guess we'll see; at least it has larger washers on the very top bolts and those will probably take the most stress.

On 7/26/2019 at 6:12 PM, Catt said:

You'll be fine 😉

 

My 40 hp Tohatsu is bolted with 1/2" bolts torqued 30# with Loctite. 

Thanks, makes me feel better! My main concern is just having washers on the inside of my transom - would have built a larger aluminum strip to distribute weight but no room; from videos I have seen lots of guys do it this way, so hopefully it wont bend from use in there?!

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On 7/26/2019 at 12:05 AM, fishnkamp said:

That should be okay. The standard torque for a 3/8 threaded bolt is 35 ft/pounds. However most 20 hp motors use two bolts and two hand screw bolts so i would think you are good. If you wanted too you could have cut some pipe the thickness of your transom and then slid them down the bolts (used as spacers)  and then added your washers, locktite on the nuts and tighten them down. You are probably okay though.

 

Yes, have four bolts on the lower part of outboard and then two hand clamps wired together, so outboard itself should be solid; main weak point from what I can see is that stress will fall on the washers for the bolts on the inner transom area - but hopefully it can handle? It does have 4 layers of .125 aluminum in total. I wish I could have built a metal strip to distribute weight more there, but there are braces and plates welded all in that inner transom area causing high spots - so there was no way to lay a plate there without it being half inch higher than parts of the transom. After sealing it all with 3M 4200 dont think Ill be going back in there for a while, so just keeping fingers crossed that those washers hold on the inside and dont cause bending under stress.

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43 minutes ago, rustybass said:

My main concern is just having washers on the inside of my transom - would have built a larger aluminum strip to distribute weight but no room; from videos I have seen lots of guys do it this way, so hopefully it wont bend from use in there?!

 

You could simply build a strip out of aluminum, .250 thick X 2.0 wide X whatever the distance is between the bolts plus 1/2" on each side. 

 

Personally I don't see an issue with the way you have installed it. With lock nuts & Loctite the nuts will not back off. Unless you hit something extremely hard the bolts will not pull through.

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1 hour ago, rustybass said:

Thanks, the exterior has the plate of the jack plate distributing weight - the interior not so much - have two larger fender type washers on top but had to use smaller washers on the bottom bolts due to space issues as there are transom braces and a plate welded in there which are higher areas, so had to fit the bolts sort of in-between these welds.

I have two larger stainless washers on the top bolts - almost size of fender washers, but washers themselves started bending there; would have liked to buy grade 8 washers but they're not stainless, no good options in finding strong washers - locally at least. Did use 3M 4200 on everything, it got all over the threads as its a mess, but maybe that will act as a loctite. The bottom four bolts have smaller washers and yes this is where bending is more evident unfortunately; it looks fine, but my main concern is that trailering it or on the water will not stress these areas and cause more bending? Guess we'll see; at least it has larger washers on the very top bolts and those will probably take the most stress.

Thanks, makes me feel better! My main concern is just having washers on the inside of my transom - would have built a larger aluminum strip to distribute weight but no room; from videos I have seen lots of guys do it this way, so hopefully it wont bend from use in there?!

Use 2 washers to double the thickness or a plate or plates as Catt mentioned if the bolts are long enough, you need about 2 threads minimum through the lock nut.

Tom

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3 hours ago, Catt said:

 

You could simply build a strip out of aluminum, .250 thick X 2.0 wide X whatever the distance is between the bolts plus 1/2" on each side. 

 

Personally I don't see an issue with the way you have installed it. With lock nuts & Loctite the nuts will not back off. Unless you hit something extremely hard the bolts will not pull through.

 

Did not even consider .250, that would be good to buy - I have some .125 now; really don't want to mess with it for at least a year if possible though as that 3M 4200 is all over everything - sure it would take a day of wire brushing to get it off. My main concern with the metal strip idea to distribute weight was that it would end up high over top of the side of a welded plate or bracket with a low point around the bolt itself if that makes sense; looking back on it now guess I could have just caulked that entire low area with 5200 or something.

 

My main concern is probably stress while trailering, we'll just have to see how it goes. I really really don't want to use a transom saver as its a complete pain in the butt with a manual tilt motor - I've been driving with outboard down no problem and launching and landing boat with motor down too, but with the jack plate I can see stress will be put on those two top bolts of the jack plate, so will just have to see how it goes - luckily those two top bolts have the larger fender type washers; the other good thing is that this jack plate has 6-total transom bolts - rather than 4, so that should hopefully help.

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