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Author Topic:   Mercury 90-HP FOURSTROKE: Removing Broken Bolt Stud
howlingdogsteve posted 04-29-2015 07:10 AM ET (US)   Profile for howlingdogsteve   Send Email to howlingdogsteve  
I am looking for advice on the best way to remove a 6-mm stainless steel bolt that holds the zinc plate to the transom mount of a Mercury 90-HP FourStroke outboard engine. The head broke off leaving about 1/2-inch of stud left. I already tried the jamb nut method with no success. [Give me your] ideas. Thanks much--Steve
jimh posted 04-29-2015 08:42 AM ET (US)     Profile for jimh  Send Email to jimh     
I am not familiar with the method of removing broken bolts called "the jamb nut" method. Perhaps you could describe it or point to a description of the method.

I presume the stainless steel broken bolt is threaded into aluminum.

Most suggested methods of freeing a broken stud of a stainless steel bolt that is frozen in an aluminum threaded hole include application of heat. If the outboard engine is still mounted on a fiberglass boat, the amount of heat that can be applied will be limited by concerns for damage of the boat hull.

A procedure is demonstrated in this presentation:

In the above, a new nut is welded to the broken stud. The aluminum is heated. A novel method shown is the application of candle wax, which is claimed to help release the frozen stud.

The general principles shown above are:

--attachment of some nut or other hexagonal shape to the broken stud to permit use of a wrench in applying torque to the frozen stud;

--application of heat to the area to expand the metals;

--application of some sort of liquid that will tend to flow into the threaded area and break down any corrosion between the steel and aluminum

tedious posted 04-29-2015 09:18 AM ET (US)     Profile for tedious  Send Email to tedious     
With 1/2 inch of stud still sticking out, you have a lot of options. I'd be inclined to start with clamping some visegrips on there as hard as you can, and then applying moderate pressure while someone else delivers sharp taps to the end of the stud with a hammer and punch. Be sure to use a steel hammer and punch, not a rubber or wooden mallet - the sharpness of the blows matters. Steady pressure combined with sharp blows to break up the initial stiction usually does the trick.

If you do break off the rest of the stud, use a left hand drill bit followed by an easy-out.

Other alternatives would be to just take it to a machine shop, or to leave it as is, if there's more than one bolt holding the zinc in place.

Good luck!


Whaler_bob posted 04-30-2015 11:07 AM ET (US)     Profile for Whaler_bob    
You'll probably have the best chance of success by heating the area around the stud [with a] MAPP gas torch, then trying to back it out with ViseGrips. If you can't get enough bite [with the] ViseGrip, you'll have to file the stud flat, center punch it, and drill it out enough for an easz-out bit. Use heat with that, too.

The last resort is drilling out and retaping. If the thread area has deteriorated enough that a new 6mm bolt won't hold, you'll have to get a thread repair kit for it.

Jerry Townsend posted 04-30-2015 03:05 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Be very careful when using heat - as aluminum deterioates and melts at relatively low temperatures, depending on the alloy. A little heat goes a long way.

First use vise-grips and a metal hammer and punch - which often will do the job. --- Jerry/Idaho

howlingdogsteve posted 04-30-2015 09:40 PM ET (US)     Profile for howlingdogsteve  Send Email to howlingdogsteve     
Well you guys pegged it right! Vice grips and sharp taps did the trick. I installed new stainless bolts with anti-seize on the tips. Would the anti-seize affect the continuity to the aluminum anode? Thanks much guys.--Steve
tedious posted 05-01-2015 07:50 AM ET (US)     Profile for tedious  Send Email to tedious     
Steve, glad it came out OK - pun intended!

The "moderate pressure while applying sharp blows" trick is the most effective way I have found to remove stuck bolts. You don't always have room to use it, but if you do, it's always the first choice for me--fast and easy.

I would not worry about the anti-seize. And even with it, back the bolts out and re-install yearly, even if you don't install new anodes.


jimh posted 05-01-2015 01:08 PM ET (US)     Profile for jimh  Send Email to jimh     
The mounting bolts should NOT be relied on to provide an electrical circuit. On most well-designed outboard engines, the sacrificial anode material usually has a dedicated electrical conductor attached to it that maintains the electric circuit bonding. If your Mercury engine lacks this refinement, use an Ohmmeter to check that there is a very low resistance between the zinc anode and the engine block. [Forgot the important qualifier "not" in my original version--jimh]
howlingdogsteve posted 05-01-2015 02:49 PM ET (US)     Profile for howlingdogsteve  Send Email to howlingdogsteve     
Jim--it does have a lug connector going from the anode bolt to the case.

Tim--I do remove every year to change the anode bar. This year I removed the anode in November after the boat was hauled out. I replaced the bolts without an anode for the winter lay up. I wonder if that had anything to do with the stuck condition? I usually [repair and replace] the anode in the spring.

tedious posted 05-01-2015 09:21 PM ET (US)     Profile for tedious  Send Email to tedious     
Hard to say Steve, I do pretty much the same thing. I think you just got unlucky. In any event, glad the end result was good.


jimh posted 05-02-2015 05:36 AM ET (US)     Profile for jimh  Send Email to jimh     
Having a dedicated electrical circuit conductor for the sacrificial anode is good design. Don't work about the anti-seize having an effect on conductivity.

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