Bolt Orientation in Engine Mounting

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
rnln
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Bolt Orientation in Engine Mounting

Postby rnln » Tue Sep 06, 2022 7:46 am

Q1: what orientation of the mounting bolts for an outboard engine is preferred?

In my experience I see most engines installed with the nuts on the outside of the boat, and my boat’s engine is mounted that way.

A service manual indicates to have the nuts in the transom engine splash well.

My boat is 1987 REVENGE 22

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Phil T
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Re: Bolt Orientation in Engine Mounting

Postby Phil T » Tue Sep 06, 2022 10:46 am

I have only seen one transom with engine bolts on the inside and that was done by an insufficiently trained person who described themselves as a mechanic.

Then again, if the OEM service manual says so, I may be incorrect.

What year, brand and model of outboard do you have?
1992 Outrage 17
2019 E-TEC 90
2018 LoadRite 18280096VT
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jimh
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Re: Bolt Orientation in Engine Mounting

Postby jimh » Tue Sep 06, 2022 11:03 am

My boat is a 1990 REVENGE 22 Walk-Through Whaler Drive—just a coincidence and the make or year or model probably does not matter. The engine on my boat is an Evinrude. The engine was installed by an Evinrude authorized dealer who specializes in selling loose engines and installing them on customers’ boats, and has been doing this for many decades. I mention this as a background to my answer to your question.

rnln wrote:Q1: what orientation of the mounting bolts for an outboard engine is preferred?
The nuts are on the outboard side of the transom on my boat. The ends of the bolts are covered by white translucent caps that are long enough that almost all the exposed threads of the mounting bolt are covered.

My inference for this preference:
  • having only the bolt heads on the inboard side removes any hazard that an engine rigging cable or a fuel hose or an oil hose or a battery cable could snag on a protruding bolt end and be cut or abraded or worn and cause a leak of fuel or a leak of oil or a cut in an electrical cable or cause a short circuit;
  • having the bolt threads projecting outward on the outboard side does not cause any interference to engine operation nor create any potential for snagging; and
  • with bolts projecting outward on the transom and fit snugly in the mounting holes, alignment of the engine to the bolt during initial installation may be easier.

rnln
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Re: Bolt Orientation in Engine Mounting

Postby rnln » Tue Sep 06, 2022 1:29 pm

My [outboard engines are twin] Yamaha 70-HP two-stroke-power-cycle engines.

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Phil T
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Re: Bolt Orientation in Engine Mounting

Postby Phil T » Tue Sep 06, 2022 3:09 pm

I would go with nuts on the outside. Both of my Yamaha engines were mounted that way.
1992 Outrage 17
2019 E-TEC 90
2018 LoadRite 18280096VT
Member since 2003

rnln
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Re: Bolt Orientation in Engine Mounting

Postby rnln » Wed Sep 07, 2022 7:14 pm

Are the mounting bolts tightened to a specific torque, or just to be "tight"?

My older Yamaha outboard engines on my older boat right now [have the engine mounting nuts on on the outboard side of the transom] too. The only literature I have seen which instructs to have the nuts on the inboard side of the transom is [some unidentified] service manual.

Thanks.

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Re: Bolt Orientation in Engine Mounting

Postby jimh » Thu Sep 08, 2022 9:33 am

Re a torque specification: check the manufacturer literature, but don’t tighten to the point of crushing or indenting the gel coat on the inboard face of the transom. In some installations a large and thick washer is used under the bolt head to spread the load onto a larger surface area of the transom, or even a long plate spanning the width of the upper bolt holes.

Also, if you choose the length of the bolts properly, they will not protrude excessively on the outboard face of the transom. The thread of the mounting bolts should be the FINE thread pitch instead of the coarse pitch. A 0.5-inch bolt is common for larger engines.

Who is the publisher of “the service manual” you refer to in your posts?

rnln
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Re: Bolt Orientation in Engine Mounting

Postby rnln » Sat Oct 15, 2022 5:33 am

My boat has twin Yamaha 70-HP engines.

I have two [engine service] manuals:
  • a "Yamaha Outboard Shop Manual" by Clymer, and
  • a service manual denoted as "Genuine Out Board" By Yamaha.

Moderator's note: DISREGARD THE SPECIFICATION GIVEN BELOW; SEE RETRACTION OF THIS SPECIFICATION IN A FOLLOW UP POST
[In the engine service manual denoted as the "Genuine Out Board" By Yamaha] on the page marked as PAGE 8-9, and on the lower half of that page--which is one of the last pages of the manual--I read that the engine mounting bolts are to be [Grade 3]. In that same place I read that the amount of torque applied when tightening the engine mounting bolts is be limited to 4.3-lbs-feet.

Earlier when I was loosening the four engine mounting bolts, I recall the force needed to loosen the engine mounting bolts was very light. I was very surprised when later I read in the manual the torque specification for the engine mounting bolts was only 4.3-lbs-feet. A fastener tightened to only 4.3-lbs-feet seems less than fingers tight.

Q2: does a factory manual specification that calls for engine mounting bolts to only be tightened to 4.3-lbs-feet make sense?

ASIDE: a while ago, I took out some of the engine mounting bolts to refresh the caulking, but I not yet put any of those removed bolts back in place. One reason for the delay in reinstallation of the engine mounting bolts is I am not sure if the torque specified to be used in tightening that I read [in the engine service manual denoted as the "Genuine Out Board" By Yamaha] is correct. I made an internet search and could not find an answer.

Tom Hemphill
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Re: Bolt Orientation in Engine Mounting

Postby Tom Hemphill » Sat Oct 15, 2022 4:12 pm

rnln wrote:Q1: what orientation of the mounting bolts for an outboard engine is preferred?
Q2: does a factory manual specification that calls for engine mounting bolts to only be tightened to 4.3-lbs-feet make sense?
My Evinrude E-TEC Installation Manual (75- to 300 HP, All Models) says to "install the mounting bolts through the transom from the inside of the boat," and to "tighten nuts and bolts to a torque of 40 ft-lbs (54 N·m)."
Last edited by Tom Hemphill on Sun Oct 16, 2022 5:16 pm, edited 1 time in total.

jimh
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Re: Bolt Orientation in Engine Mounting

Postby jimh » Sun Oct 16, 2022 11:30 am

rnln wrote:Q2: does a factory manual specification that calls for engine mounting bolts to only be tightened to 4.3-lbs-feet make sense?
That specified value for tightening the engine mounting bolts to only 4.3-lbs-feet of torque seems very low to me.

If the publication that gives that value is the official service or installation manual for the Yamaha 70-HP engine, then there seems to me to be only two conclusions:
  • the value of 4.3-lbs-feet is an error, and perhaps the value was supposed to be 43-Newton-Meters, or
  • the value is not in error and there is some reason the manufacturer thinks the engine mounting bolts should be only tightened very lightly.

Note that 43-Newton-Meters converts to about 32-lbs-feet.

Derivation:

1-lbs = 4.448221-Newton
1-Foot = 0.3048-Meter
1-Newton = 0.2248090795-lbs
1-Meter = 3.28084-feet

Conversions

1-lbs-foot x (4.448221-Newton/1-lbs) x (0.3048 Meter/1-foot) = 1.3558-Newton-Meters
1-Newton-meter x (0.2248090795-lbs/1-Newton) x (3.28084-feet/1-meter) = 0.73756-lbs-feet

rnln
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Re: Bolt Orientation in Engine Mounting

Postby rnln » Mon Oct 17, 2022 6:11 am

Let me retract what I said about the torque specification for the engine mounting bolt fasteners [that was cited above as originating from the factory service manual]. I must be reading at the wrong section [of the factory service manual].

In another section [of the factory service manual] there is [an illustration of engine mounting bolts with a caption that instructs] to tighten the engine mounting bolts so they will not become loose, and no torque specification is provided.

I [will now try to tighten the engine mounting bolts to a torque] between 32 to 40-lbs-feet.

Thanks [for the advice given above].

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Re: Bolt Orientation in Engine Mounting

Postby jimh » Mon Oct 17, 2022 9:28 am

The engine mounting bolts should not be tightened to the point where the bolts begin to crush the gel coat laminate of the inboard face of the transom of the Unibond hull. A good plan to avoid this is to use larger and thicker-than-usual washers to help spread the forces on the gel coat laminate over a wider area of the transom. On my boat the engine mounting bolts are fastened under 0.25-inch-thick galvanized steel plates that are about 2 x 2-inches square.

rnln
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Re: Bolt Orientation in Engine Mounting

Postby rnln » Tue Oct 18, 2022 1:47 pm

Thanks Jim

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Re: Bolt Orientation in Engine Mounting

Postby rnln » Mon Oct 31, 2022 5:55 pm

I found the torque specfications the hard way: by increasing the torque on my wrench from 10 to 20-lbs-feet step by step. I found that some of the nuts are between 16 to 18-lbs-feet. So, I set my mind at 20 to 22-lbs-feet.

On reinstalling, I started at 15-lbs-feet then increase to 18-lbs-feet, some times directly to 20-lbs-feet. To me, 20-lbs-feet on that bolt is nothing--but funny that one of the bolts snapped at 18-lbs-feet.

Q3: can I just go to a hardware store and get a same-size galvanized bolt?

Q4: do I have to order an engine mounting bolt from a marine store?

Q5: are engine mounting bolts normal-sized?

Q6: are engine mounting bolds odd-sized?

Q7: will I be unable to find engine mounting bolts?



Thanks

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Re: Bolt Orientation in Engine Mounting

Postby jimh » Wed Nov 02, 2022 7:19 am

rnln wrote:Q3: can I just go to a hardware store and get a same-size galvanized bolt?
Yes

rnln wrote:Q4: do I have to order an engine mounting bolt from a marine store?
See answer to Q3.

rnln wrote:Q5: are engine mounting bolts normal-sized?
Yes.

rnln wrote:Q6: are engine mounting bolds odd-sized?
See answer to Q5.

rnln wrote:Q7: will I be unable to find engine mounting bolts?
Typically an engine mounting bolt will not be threaded over the entire length. Whether or not your local hardware store will have these bolts in stock at the desired diameter, thread, length, and grade of steel is unknown to me. Perhaps the most reliable source will be to get an OEM engine mounting bolt from a local authorized dealer of the engine brand.

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Re: Bolt Orientation in Engine Mounting

Postby msteinkampf » Sat Dec 03, 2022 1:00 pm

The OEM service manual for my 1990 Mercury 90-HP outboard engine is quite specific regarding the orientation of transom mounting bolts. Here is the diagram on page 7A-2:

Mercury outboard manual pic 2.jpg
Recommended engine mounting bolt orientation for Mercury outboards 70-100 HP, from the OEM service manual copyrighted 1989.
Mercury outboard manual pic 2.jpg (55.87 KiB) Viewed 248 times

Note that in the diagram (and in numerous photos throughout the manual), the lower bolts are oriented with the nuts inside the boat. The manual does not comment on the reason for this, but I suspect it was to reduce the risk of interference between the engine mounting bolt and the bolts that secure the transom bracket to the power trim assembly (especially the top power trim bolt, which is quite close to the engine mounting bolt, as shown on the diagram) if the top hole of the mounting bracket is used. On my own boat, the dealer did use the top hole in the lower part of the bracket, and he oriented the nut to the OUTSIDE of the transom. When I went to remove this bolt in preparation for installing a jack plate, the proximity of the head of the upper power trim assembly bolt prevented me from using a socket wrench on the engine mounting bolt nut. (When I installed the motor to the jack plate, I used the hole one below the previously used one so that this wouldn't be a problem.)

Interestingly, the installation instructions for my Atlas Micro hydraulic jack plate says the following:

"It is recommended when possible to insert bolts from inside the Atlas Micro plate through the transom...".

I am sure this recommendation is to minimize the risk of contact between the mounting bolts and the mechanism of the jack plate. However, I suspect someone in the company recognizes that customers are used to seeing bolt heads and not nuts inside the transom, so a photo in the instruction sheet shows an alternative mounting scheme:

Jack plate pic 1.jpg
Photo taken from the Atlas Micro Jacker installation instructions. Note the orientation of the transom mounting bolts. The lower bolt (circled in yellow) appears to have been cut off (or a shorter bolt was used) to prevent contact between the bolt and the jack plate.
Jack plate pic 1.jpg (62.53 KiB) Viewed 248 times


M
1990 Montauk 17’
1991 Mercury 90 HP outboard