Mercury 20 HP Compression

[Knot Working]

Bought a '98 Merc 20 for a couple hundred bucks but the pistons were frozen. Removed power head and soaked in mystery oil for two weeks and broke pistons free. Ran a cold compression test and came back 100 psi on cylinder 1 and 92 psi on cylinder 2. Question is, are these compression readings tolerable? Would comression normally be a little lower since it has not been running or warm or should I plan on a rebuild? Any advice is greatly appreciated.

By the way, what a site! The information posted on this site is truely a wealth of information.

Murray-

 

[Chinewalker]

Re: Mercury 20 HP Compression

I am always leary of any Merc that has been frozen up with any sort of rust. Even if you free it up, chances are there is some bearing damage, too. That will materialize itself with time as the motor will slowly (sometimes quickly) shake and grind itself apart. A complete disassembly is the only way to inspect all the bearings and replace what's damaged. A little emory cloth and some new bearings can ad years to the life of the motor... Being a '98, it is worth putting a bit of in depth TLC into it...
- Scott

 

[Knot Working]

Re: Mercury 20 HP Compression

Thanks Chine,
Should be a good project. Any idea what the compression should be on this motor?

M-

 

[tashasdaddy]

Re: Mercury 20 HP Compression

the mfgr, do not publish that info. it's a secret. why i don't know.

 

[ozziebob]

Re: Mercury 20 HP Compression

Compressions should be higher than that. aprox 120-140. Unfortunatly if the bore is scored may as well use it as a artifitial reef. These engines have a chrome bore. Precision engine rebuild will be the likely cost effective repair.

 

[Knot Working]

Re: Mercury 20 HP Compression

Can chrome be honed?

M-

 

[Laddies]

Re: Mercury 20 HP Compression

Chrome may be cleaned up with a crosshatch hone if it is not scored. Mercury does not list compression on thier engines because it depend on the amount of oil in the cyls (more oil will jump the compression)

 

[Knot Working]

Re: Mercury 20 HP Compression

Thanks for all the help guys. Think I will just buckle down and rebuild the thing. Flounder gigging season is almost over anyway!

Murray-