mercury Thunderbolt 85 hp (1976 approx) Seized engine.

[brexitsmuggler]

Hi,
i have a Mercury Thunderbolt 85hp outboard that came with a boat i bought. The outboard was was partly dismantled and had not run for approx 2 yrs.. I fitted a new starter and did some carb and wiring repairs and was delighted when it actually started and ran very well. I used it throughout the summer 2019 without any problems. It always started and ran very well. Its a great engine for its age. It was previously used on fresh water and this was first time running in salt water.


It was parked up under cover for winter 2019...then when I put it back in the water June 2020 it would not turn over.
I put a breaker bar on the fly wheel nut and it will not move a millimetre in any direction. Its Locked solid.
I pulled the plugs and went through various routines of filling the cylinders with various mixes of wd40/diesel and anything else i could think of. after 4 weeks it still wont budge in the slightest.

I removed the lower unit and gearbox /drive shaft. Everything fine there. I removed the two side panels on the cylinder head and can see the 4 pistons and lower cam shaft brackets. Cannot see any corrosion or anything else wrong.

when using the breaker bar i can feel the camshaft and piston arms and pistons trying to move ever so slightly.

I am a total amateur in this field and it has me perplexed how the engine could seize like that after a few months?
its almost as if there is a locking mechanism engaged preventing the engine turning over!

Anyone have this problem? Could the salt water have seized the engine that fast? or am i missing something simple?

 

[The Force power]

How did you prepare the engine for storage? winterize? (if applicable)

Check if the fly-wheel is jamming on something

 

[GA_Boater]

Welcome aboard.

Where are you seeing a camshaft? Merc didn't make 4 strokes until 1996, 20 years after your 1976 motor.

 

[racerone]

If saltwater leaked into the engine and it sat for a week it would turn no problem.----If saltwater sat on bearings over the winter then it is stuck solid.----Put the breaker bar and hammers away.----Take the motor apart find / fix the problems.

 

[brexitsmuggler]

Welcome aboard.

Where are you seeing a camshaft? Merc didn't make 4 strokes until 1996, 20 years after your 1976 motor.

Hi my engine is Mercury 85 hp Thunderbolt. 4 cyl. with Belgium serial number 9011783 which i believe puts it about 1973.
On the starter side of engine there are two small plates which can be removed. The fuel pump is fitted onto one. When these are removed one side of the 4 pistons can be seen and also down into the crank case. I used a small endoscope camera in the crank case and cannot see any corrosion .

 

[brexitsmuggler]

How did you prepare the engine for storage? winterize? (if applicable)

Check if the fly-wheel is jamming on something

Unfortunately "winterize" was not something i was aware of. When i said I was an amateur I really meant it!!
Also i removed the fly wheel and stator and it made no difference,

 

[Chris1956]

It doesn't take much rust to lock 'em up. Also, the pistons you see thru the transfer covers are aluminum, so you will not see any rust. The rust will be on the cast iron rings and cylinder liner, or the bearings.

Old merc motors like yours often have corrosion on the inner water jacket, allowing water into the cylinders. If the motor sits, this can cause your issue.

You will need to pull the powerhead, and remove the crankcase cover and crankshaft to get to the pistons. The cylinder head does not come off. Make sure to capture and isolate the connecting rod roller bearings and cages. I used numbered dixie cups. Overhaul of the motor is pretty standard, but a shop manual is always a good idea.

 

[Scott Danforth]

first, is is rust, or did a critter crawl up the exhaust and build a nest.

is it the lower blew a seal and the gear case is frozen or is it the power head

drop the lower first. especially if you see minute amounts of movement when putting a breaker bar on the crank

 

[The Force power]

first, is is rust, or did a critter crawl up the exhaust and build a nest.

is it the lower blew a seal and the gear case is frozen or is it the power head

drop the lower first. especially if you see minute amounts of movement when putting a breaker bar on the crank

He said that he dropped the LU and did not make difference

 

[brexitsmuggler]

It doesn't take much rust to lock 'em up. Also, the pistons you see thru the transfer covers are aluminum, so you will not see any rust. The rust will be on the cast iron rings and cylinder liner, or the bearings.

Old merc motors like yours often have corrosion on the inner water jacket, allowing water into the cylinders. If the motor sits, this can cause your issue.

You will need to pull the powerhead, and remove the crankcase cover and crankshaft to get to the pistons. The cylinder head does not come off. Make sure to capture and isolate the connecting rod roller bearings and cages. I used numbered dixie cups. Overhaul of the motor is pretty standard, but a shop manual is always a good idea.

Unfortunately your diagnosis seems to fit the scenario. I was reluctant to attempt an overhaul and will have to consider
if its a viable option.. I can take it apart but couldnt be sure i would get it back together again. just a shame to scrap a classic
engine, especially as it was running so well beforehand..

 

[The Force power]

In my opinion........ You can sell the LU it can still fetch some coin (as they can fit on some 65 HP models)
Maybe you can find "Good used" power head to stick on
You will run into big dollars to rebuilt the head due to shortage of supplies (I would not recommend)

Just my two cents