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Author Topic:   compression testing on 40hp Merc
poker13 posted 09-12-2002 10:51 AM ET (US)   Profile for poker13  
I'll be looking at a boat this weekend and want to test the compression on an old 40hp Merc (the guy says it may be an '85. If I can get a hold of a compression gauge, I'll go and test it myself, but I don't know what the compression should be on that motor. How many lbs./in. should it have had when new? What would be decent compression after 15-17 years of use? Thanks in advance.
rsgwynn1 posted 09-13-2002 08:50 PM ET (US)     Profile for rsgwynn1  Send Email to rsgwynn1     
You can get a good, cheap compression gauge from Harbor Freight Tools. It's not so much what the compression reads as difference between the cylinders. Shoot some WD-40 in the spark plug holes and crank it a few times before you test.
poker13 posted 09-14-2002 06:11 PM ET (US)     Profile for poker13    
Thanks! I thought I wasn't going to get a reply. So what one should be looking out for is one badly worn out cylinder, piston ring, etc. that is way out of line with the other(s). If all cyls. read more or less the same, it should be fine then. Thanks again.
Wayneman posted 09-14-2002 09:39 PM ET (US)     Profile for Wayneman  Send Email to Wayneman     
Hey Poker; be careful with your purchase. I have a Merc 40 on my old Nauset. The motor is of similar vintage. Although it has always been taken care of and rebuilt, I have done nothing but dump a pile of money into it this year just so it could crap out again and so I could be towed in for the 2nd time and humiliated. I think those motors belong on the bottom of the river.
poker13 posted 09-15-2002 11:11 AM ET (US)     Profile for poker13    
Thanks, Wayneman. I ended up not buying it; didn't look very good.

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