Outboard Cylinder Head - Seized Bolts

[Marqus]

Hi all

Removing the cylinder head from a 15hp outboard engine (alloy engine block), I readily removed several bolts, but the remaining three are very tight.

I went slowly with a socket wrench, and one started to reluctantly turn, to my relief, but alas another 1/4 turn and the bolt head came off !

So for the remaining bolts I plan the usual stuff - soak with penetrant, apply heat, hammer taps and then bring the wrench to bear.

But there's two ways to "apply the heat".

You can heat the BOLT HEAD with a gas torch, or you can heat the ALLOY AROUND THE BOLT HEAD with the gas torch.

If I heat the bolt, I could get it really, really hot.

If I heat the alloy block, I could never get it that hot because heat conducts away from the target area and I'm only using propane. But it should expand the hole size (good thing).

Anyone with relevant experience in the "bolt frozen in alloy block" scenario to suggest the best option?

Of course I can ultimately try both ways, but I'm keen to avoid snapping another head off in my attempts, so want to use the superior option for my first shot.

[Captain Bill]

You need to use something hotter than propane, MAPP might do it, but my instructor in my outboard mechanics course I took at my local community college said that acetylene was the only dependable way to get enough heat fast enough. He demonstrated this several times during the course.

[Marqus]

Thanks Bill. Yes Acetylene would be nice, but it's not in the toolkit for this job.

What did your instructor do with the Acetylene - heat the Bolt or heat the Alloy surrounding the bolt?

[Jammer]

Heating the bolt won't help.


You may find that it is helpful to alternate tightening and loosening forces with the wrench, as this will often break loose a bolt using less torque than is required when just loosening it.


Penetrating oil and time are the best answer.


In some cases, the block may not, as a practical matter, be salvageable.

[a64pilot]

I'd tell you to heat the bolt then let it cool.
Idea is the heat expands the bolt and hopefully will break the corrosion bond. Let time cool of course before trying to remove it, cause you don't want to be removing it when it's expanded.
Ideally you heat, then freeze, but that's probably not too easy here.
I don't think you could heat the block enough to get it to expand much, but he coefficient of expansion of aluminum is greater than steel, so if you could, that would be the way to go.
I have even used a "stick welder" and stuck an electrode to a bolt to get it hot, very effective as your heating the bolt very precisely.

[motion30]

It is not just where the bolt threads into the cylinder block that's corroded. It is also or maybe more so where the bolt passes through the cylinder head that gets corroded and prevents the bolt from turning. That is why the head of the bolt broke off and not where the threads to go into the block

[Marqus]

Originally Posted by Jammer

Helpful to alternate tightening and loosening forces with the wrench, as this will often break loose a bolt using less torque than is required when just loosening it.

Nice one, will do.

Originally Posted by Jammer

In some cases, the block may not, as a practical matter, be salvageable.

Now don't you go bursting my bubble ... so far I've only lost one bolt head from eight.

Originally Posted by a64pilot

Idea is the heat expands the bolt and hopefully will break the corrosion bond. Let time cool of course before trying to remove it

Another arrow to my bow thanks.
Makes me wonder whether using half a dozen blocks of ice from the freezer and rubbing them on the heated bolt head will have any useful effect.

Originally Posted by motion30

where the bolt passes through the cylinder head that gets corroded and prevents the bolt from turning.

Yes agree. The bolts probably need to be released from both the head and the block, but you do imagine that since the shaft only contacts say 1/4" of the head as it goes through, while the bottom end threads contact about 1" of block they screw into, that the threads may be harder to release (provided they are bound to the alu as well).


Impact Wrench ?: I would love to have a go with my cordless impact wrench, but since these bolts are only 1/4" I assume it's looking for trouble.

[RaymondR]

Try greasing up, replacing and torquing up the bolts you have already removed, then have another go at the seized ones. Breaking and re-tightening all the bolts before removing any of them is an old mechanics trick.

[Reefmagnet]

Heat the bolt cherry red. I don't think it's thermal shock that breaks the corrosion bond so much but rather an affect on the corrosion itself from the elevated temperature as it seems to transform it into a dry powder. Sometimes it takes a few hits of the the heat to get the bolt to release. Corrosion release agents are very hit and miss in this application from my experience.