1985 Mercury 35 hp Piston Replacement 1985 Mercury 35 hp SN 0A120528

[2busy2work]

The engine is a 1985 Mercury 35 hp SN 0A120528. I am back to working on the powerhead after it had overheated due to clogged water passages. The top piston is scored and all the advice was to remove it from service and see if it can be fixed. The bottom piston appears to be good.

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I have taken the powerhead apart. It took a few days since I had to collect some 5/16"-24 x 2.5" bolts to use with my flywheel puller. I finally got the two block halves apart this evening.

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I thought the crank would slip out, but it seems to be hung up by the reed assembly. I did not want to pry on it until I know how to remove it. Can someone give me directions?

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I am hoping the powerhead can be repaired with another piston and some honing.

[JOE54]

bump crank with block of wood and mallet , if you try to hone it out it will be too big / bore size out of spec have both bored and replace both pissts ons

[2busy2work]

Thanks for the advice on how to remove it. Today I got the crank out. There is too much damage inside that top cylinder. I agree the cylinder is toast.

Cylinders (Large).jpg Pistons (Large).jpg

The parts are now boxed up for use as parts if ever needed.

Thanks for all the help!

By the way, the replacement powerhead is still running well.

[JOE54]

would clean up nicely with bore job

[2busy2work]

Thanks for the suggestion to have it bored. I did not know if it could be bored since it is sleeved. I will consider that. It looks like the parts alone are available but over $600 for what I need. I will have to find a local machine shop that will bore it for me and see what the cost is.

But then I would have a new engine so it may happen. I will have to consider it for sure. If I do, I will post the rebuild.

Thanks Joe54!

[SilverFox579]

Maybe it's just lighting or angle but crank end for rod #1 (or is it #2?) sure looks as though it got hot compared to the other. Were that mine I'd want to inspect that cylinder's rod bearings and crank journal...

[2busy2work]

SilverFox579 thanks for the headsup. You have a good eye! The connecting rod bearing did get pretty hot. It has discolored the bearing cap and the roller bearings. The crank is still very smooth as is the inside of the rod cap.

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I did measure the crank pin with a caliper (I do not own a micrometer). it measures 1.1375 with the caliper.

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Boats.net shows the roller bearing is no longer available. So I may be back to a parts engine. Can this engine be saved?

[SilverFox579]

I'm a lifelong "maintainer" and have trained eye.

As to parts, Joe or Don will know...

[JOE54]

might try PRO MARINE , crank does look pretty blued

[2busy2work]

First I need to figure out if I can still use the rod and crank. Since the rod and crank are both blued, should they both be replaced? If they have to be replaced I am thinking the cost is going to be too much to consider repairing the powerhead. Or will a new bearing work on the blued parts? Will a machine shop know what should be done?

[JOE54]

just keep the block in case you need it for other mtr

[2DirtyKids]

Thanks for the suggestion to have it bored. I did not know if it could be bored since it is sleeved. I will consider that. It looks like the parts alone are available but over $600 for what I need. I will have to find a local machine shop that will bore it for me and see what the cost is.

But then I would have a new engine so it may happen. I will have to consider it for sure. If I do, I will post the rebuild.

Thanks Joe54!

Dang, I have 1986 50 HP Mariner sitting on a boat and trailer that is waiting for a home. Would most likely cost less.

[2busy2work]

I will put it back in a box and on the shelf for parts. Thanks again for all the help.

[JOE54]