Mariner magnum 40hp single piston failure advice.

What I want to know if anyone can help diagnose the issue and point to a solution which may include repair or replacing the existing engine.

It is a 1992 mariner magnum 40hp 2 stroke outboard.

Long story short I've recently been doing up an older outboard and boat owned by family, and got to the point of taking it for a test drive (will post a full list of repairs as a larger paragraph underneath this).

It ran perfectly if a little smokey for the first half hour until it started coughing and lost power, I then limped it back to the launch ramp 700m away and had a look under the cowling.
Unfortunately the bottom pistons had stopped firing and the top of the spark plug is partially mangled. There are visible metal filings in on the piston and some metal damage I think.
I might add the telltale was spitting water like a horse until the loss of power upon which time the telltale slowly decreased slowly to be almost a slow drip by the end of the 700m limp home although the water wasn't hot that was cycling through.

Up until now I have completely relied on my own skills to repair an engine that was seized and hasn't run in 4 years.

A full list of repairs are as follows.
Degreasing and regreasing all grease points,
Replaced spark plugs,
Cleaned carburetors and fuel lines of water dirt and old fuel,
Dismantled cleaned and rebuilt starter motor,
Replaced power cable for tilt trim motor as it was short circuiting,
Replaced fluid in trim tilt system,
Replaced fuel line to fuel tank due to perished plastic,
Drained lower unit and refilled lower gearbox reservoir,
Checked from oil seals and removed debris,
Replaced damaged Impeller,
Replaced battery terminals,
Completly rebuilt wheel bearings on trailer,
Unseized engine using ATF fluid as a penetrating soaking oil in pistons and turning the flywheel by hand with braker bar until easy by hand,
Replaced seized pivot tube and salvaged steering cable from pivot tube,
Started engine with earmuffs with no troubles and decided to take it for a good run to get everything lubricated.

My first post,
Will answer any questions with as much detail as possible.
Cheers for any help.

Mr Timsmith has

Partial piston seizure. Has to come apart. Sorry.

Jeff

You did everything correct. I guess there was too much rust in the affected cylinder, the piston got hot and broke down. You were able to turn it over freely while cold, but once the motor heats up, the piston clearances lessen and it started the "meltdown". With no head to remove and physically clean the rust, you have to do a slower warm up, shut down, relubricate, check compression.......and try to work up adequate piston to cylinder clearance slowly. This will not always work anyway, but try it for next time. It all depends on the reason it got rusty to begin with too......and of course the severity of the rust as well as previous motor condition. Maybe scoring had started already. I've been there brother.....good try.
With regards to water flow, it likely had some old debris come loose and plug up the pee hole. Look in there, or at the fitting on the block. The pump was probably still doing its job.
If you rebuild it, make sure to give adequate piston to bore clearance. If it's too tight you will have problems. On 4 strokes I go .001 per inch of bore. So say a Chev 350 will have .004 clearance at skirt just below bottom ring. The 2 strokes need a bit more clearance.......why? They fire every stroke, creating more focused heat at the piston with no free stroke to dissipate the heat. But what do I know? Listen to a younger machinist with less experience. They remember more from school. I forgot everything they taught me long ago, so I have to base my knowledge on experience now!

Sorry if I loaded you down with too much information. Maybe any other techs that read this will consider me less of a fool than they originally thought I was.

You now have a engine that need to be torn down and inspected. Will likely need cylinders bored.....

Interestingly after starting to dismantle the engine i noticed that the centre of the mangled spark plug has somehow vanished into the engine (see figure 1), whether this has to do with the loss of the piston and the subsequent damage i don't know, however the whole engine still turns over easily by hand and the water pump issue mentioned above was exactly as "timguy" the telltale was simple blocked and after removing the head casing over the spark plugs you can see some serious corrosion build up and salt (figure 2). The only dilemma i face now is to access the cylinder boars and cylinders i need to remove the crank shaft in its entirety simply to inspect the damaged piston and bore. And that of repairing the damage.

Its pretty oxidized from salt. I would look for a different powerhead or perhaps complete motor to use for parts and kinda start over. The plug broke up from trauma happening in the cylinder. The bore might be damaged beyond available piston size. Call Tim's Outboard in Hackensack MN, they might have a good powerhead. Tell them Tim sent you.

I called Tim's Outboard. He doesn't have a good powerhead. Boat Motor Recyclers and Twin Cities Outboard are also good sources. These are all freshwater machines from Minnesota and the Midwest.

Tear your powerhead apart and see what you have. That doesn't look like salt deposits in the pic. It might also be cheaper to replace the motor itself

I think that Tim likes to play around with junkers like I do. My first repair was a Mercury Super 10 that my Dad was so fed up with he was actually loosening it up on the transom out in the middle of the lake because his watch band broke while he was pulling it over. I started to cry and pleaded with him to give it to me instead. I was 9 years old. 2 years later it was repaired, with NO help from Dad and I sold it in 1967 for 150 bucks. I had it running like a RAPED APE. After that I had "outboard fever".