Broken engine mounting bolt

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Joined: Tue Nov 15, 2022 4:05 pm

Broken engine mounting bolt

Postby msteinkampf » Wed Nov 23, 2022 10:09 pm

I was pulling the 1990 90HP Mercury outboard off my 1990 Montauk 17 today in preparation for installing a hydraulic jack plate. Three of the mounting bolts loosened up smoothly, but the starboard lower bolt head initially felt very tight and then rotated quite easily as if broken. Here is a picture of the broken bolt.

Outboard motor bolt fracture 2.jpg
Broken engine mounting bolt (with intact bolt to compare)
Outboard motor bolt fracture 2.jpg (63.08 KiB) Viewed 130 times

A few things look unusual to me:
  • The break is on the unthreaded part of the bolt. Isn't this supposed to be stronger than the threaded part?
  • The break is quite oblique, not like what I typically see when I break off a rusty bolt trying to extract it.
  • The fracture surfaces are rusty. I suspect this break occurred long ago, perhaps even during installation of the motor.

The bolts all appeared normal externally before extraction, with 1.5-inch-diameter washers supporting the bolt heads on the inside of the boat. The gel coat wasn't depressed by any of the washers. Note that the break was located 0.5 to 1.25 inches from the bolt head, which places it completely within the 2-inch-thick transom (rather than at the interface between the transom and the motor bracket), so I doubt that the bolt was fractured due to operating in rough seas.

Q1: could this have occurred from the dealer using an air impact wrench to install the bolt?

Q2: is there another explanation?

I will be replacing all four bolts with a jack plate bolt set from Bob's Machine.

Q3: how can I prevent this from happening in the future?


User avatar
Phil T
Posts: 2291
Joined: Thu Oct 22, 2015 6:08 pm
Location: Was Maine. Temporarily Kentucky

Re: Broken engine mounting bolt

Postby Phil T » Thu Nov 24, 2022 2:28 pm

I would suggest there was a defect in the material.

In 20 years on the forums, this is the first incident I have seen.

Don't need to dwell on it.

Please post before and after install and on the water photos so we can drool.
1992 Outrage 17
2019 E-TEC 90
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Joined: Thu Oct 15, 2015 10:01 pm

Re: Broken engine mounting bolt

Postby fno » Sun Nov 27, 2022 3:53 pm

I agree with Phil. Looks like if anything was awry it was the amount of sealant used on the bolt. All things being said, it is stain less not rust less steel. It may have been 308 SS(stainless steel) instead of 316 which has much better corrosion resistance and should always be used in any watery environment. Keep in mind that most SS bolts sold in hardware stores and Lowes and Home Depot are 308 at best and from a third or fourth world country. Use McMaster-Carr for the underwater and likewise important stuff. At least you know what you are getting.

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Re: Broken engine mounting bolt

Postby jimh » Sun Nov 27, 2022 9:42 pm

Regarding corrosion, I found this comment:

Does Stainless Steel Corrode?
Even though stainless steel is much more resistant to corrosion than other forms of metal, it can corrode in some circumstances. Stainless steel will not corrode in normal water-based environments. However, in aggressive conditions with low oxygen, high salinity, or poor circulation, corrosion may occur. For example, this could occur in the marine industry where stainless steel is exposed to salt water. Typically, coating the steel with epoxy can help prevent this corrosion.
Source: ... less-steel