Mercruiser 5.7 Stuck Spark Plug

[CLi87]

Hi there all, I have a Mercruiser 5.7 and what I thought would be an easy spark plug replacement has turned into an absolute nightmare. I'm an experienced auto mechanic but don't have quite as much experience in marine engines. I went into the job thinking it would be the same as replacing the plugs on a typical GM 5.7 but had no idea the incredible stupidity of Mercury engineers. I got plugs 2,4, and 8 out fairly "easily" without much hassle, but plug #6 under the exhaust manifold has put me in quite the bind. All the plugs on the engine were severely corroded and took a bit of force to get them out, but I didn't realize my thin wall spark plug socket had gone at a slight angle when trying to take #6 out, the socket had come into contact with the exhaust manifold and had angled just slightly, I didn't notice as first thinking it was just seized badly like the others, by the time I realized what had happened I stopped immediately. The spark plug is still in there, but I'm thinking I've stripped at least a few threads either on the plug itself or the head. At this point, I can still apply a considerable amount of force and the spark plug seems to be coming out, but I don't want to risk further damage if possible. So my question is what is the best course of action at this point?

1) Keep loosening the spark plug and hope it comes out, then assess the damage and decide what to do
2) Remove the exhaust manifold entirely which is a royal pain in the ass on a 1997 Bayliner Capri, there is no room at all to work with, that engine bay was really designed for the 3.0L I4, not a 5.7 V8. And even after removal, I'm not sure it would help me much to straighten the plug out in this situation?
3) Completely removal of the head? Obviously an absolute last resort, but if need be I can do it.
4) Other ideas??

 

[Rick Stephens]

I'd keep going..... I don't know how bad it is, but if thrashed threads, the head will need pulling probably.

There are marine rated plugs. They are plated to prevent corrosion. I also always use a dab of never seize.

The exhaust may not have been made by Merc. I can see where some of them may get in the way.

 

[Bondo]

I'm an experienced auto mechanic but don't have quite as much experience in marine engines.

Ayuh,..... It may take every trick ya know, 'n more to get 'em out,.....

That boat motor is basically a pickup motor, so the removal is the same, Hopefully, without pullin' the head,.......

I run AC Delco, MR43T, or MR43LTS in the Vortec heads,......
The M, of course stands for Marine, but the trick is, the steel bodies are a high alloy content, 'n won't rust for years,.....

Everybody else's plugs are just plain ole steel, or at best, plated steel,....

 

[tank1949]

Pull exhaust manifold. I feel your pain. Old Sea Rays lack engine room too. Always go back with anti-seize on plugs. Use 6 pt plug wrench. 12 pt may round plug's corners. . With exhaust manifold out of the way, hopefully you can see what needs to be done. There is also a tool to rethread spark plug threads, if not too damaged. Been there.

 

[Scott Danforth]

most of the time you need to take your thin-wall socket and grind a taper at the end to get it onto the plug to access plugs on a wet exhaust motor

I think I have 2 or 3 modified plug sockets.

however if the plugs are corroded, they were not marine plugs (marine plugs are either coated or stainless to prevent corrosion) that is not an engineering issue, that is a maintenance gone awry issue

if you screw up the threads, pull the exhaust manifold and use a tap to clean them up. if that doesnt work, then a threaded insert. no need to pull a head unless you really FUBAR it.

most of the time you must spend about an hour removing bits of boat to get to the motor. remember, boats are toys, and ease of maintenance does not sell toys..... lower price than other boat, shiny gel coat and stickers and blue tooth stereos sell toys.

 

[tank1949]

most of the time you need to take your thin-wall socket and grind a taper at the end to get it onto the plug to access plugs on a wet exhaust motor

I think I have 2 or 3 modified plug sockets.

however if the plugs are corroded, they were not marine plugs (marine plugs are either coated or stainless to prevent corrosion) that is not an engineering issue, that is a maintenance gone awry issue

if you screw up the threads, pull the exhaust manifold and use a tap to clean them up. if that doesnt work, then a threaded insert. no need to pull a head unless you really FUBAR it.

most of the time you must spend about an hour removing bits of boat to get to the motor. remember, boats are toys, and ease of maintenance does not sell toys..... lower price than other boat, shiny gel coat and stickers and blue tooth stereos sell toys.

Indeed 100%%%%!!!!! People buy "pretty" and will eventually get shocked at repair bills after gelcoat has faded. And then, many cut corners! I always have extra manifold and riser gaskets in stock, and lots of anti-seize because something that is going to screw up will indeed and right where I cannot easily access it. Try replacing a Chevy V8 starter w/o removing exhaust manifold. It ain't fun. If plug corners are too rounded, boater maybe able to drive thin walled metric over plug into existing metal, just enough to remove it. Been there too!

 

[kvkon]

If you take that manifold off also take a long hard look at your starter. If it looks sketchy now's the time to replace. Every fastener I pull gets cleaned and either antiseize or locktite depending on what it does.