1976 850 mercury top cylinder lost compression

[jonsbuell]

I was wondering if I can get the piston out the top cylinder without taking the motor off boat? Or what's the best way of going about it? Thanks Jon

 

[galamb]

You have to pull the powerhead and "split it".

You could probably leave the bracket on the boat, but it could result in some contortions and tight spots trying to get everything unbolted that you need to and in the end the bracket might be the "only thing" left on the boat.

Before you consider going any further I would get a decent manual and have a read. This is not a minor operation. You will need to replace gaskets/seals along the way.

Plus you are going to have to address the "cause" of the toasted cylinder. The lost compression is simply the "result" of an issue further upstream (carb issue would be pretty high on the list).

Also, at the very least, you will need to have the cylinder honed, maybe bored, so the block will have to make it's way to a shop or minimally a bench if you are so equipped yourself.

Do consider all your options. I recently rebuilt a 3 cylinder. The cost of parts, machine work, gaskets, seals, carb kits and a few other items came to about 1000 bucks (not including labour costs). You may not want to drop that kind of money on "soon to be" 40 year old motor.

But until you get it tore down there is no way to guess at all that will be involved. Simply slapping in a new set of rings could leave you in the same predicament in a couple of months..

 

[fastjeff]

Shame you don't have an earlier Merc 850, for I have a perfect power head I'd sell you--and cheap.

If you are mechanically inclined, have some basic tools, and are ambitious, you can repair it yourself. I've fixed a dozen Mercs like yours with such a problem that only needed another (used) piston and a light hone job.

Jeff