I am looking at a 2002 ranger 520vx with a 225 Mercury optimax. Everything seems great with the boat and it ran well. The boat though has 735 hours on the engine and compression tested with the lowest cylinder at 81 and the highest at 95. I am concerned about the compression test should I pass on the boat?

Here is the complete test results

87 cylinder 1 top right

93 cylinder 2

95 cylinder 3

85 cylinder 4

95 cylinder 5

81 cylinder 6

5% difference between lowest and highest is OK, 10% between lowest and highest is considered a used up power head.   Based on the numbers you have posted, I would say that one is at the end of it's life and would have to have one seriously good price before I would consider it.  After all, you are probably looking at close to seven grand to have a properly reman'd power head installed.  $4,000 just to have someone rebuild it with boring, new pistons and other parts it will need.  Then throw in the problems the early OPTI's had that earned them the nickname OPTI-POP, and things could get a little expensive for you.

 

I would do a search for average Opti-Max compression, since I don't mess with mercs, and see if those are even in the ball park.  I would think they should have been up in the 120 range.  I know it's hard to pin down a definite number because of difference in gauges, but most of the motors by that time had gotten back up into the 120psi range with the better computers they had to control the fuel over the older late 80's and early 90's. 

 

I prefer leak down test over compression, that takes the difference in gauges other things out of the picture.  A proper leak down test is much more accurate and better indication of motor ring condition than any compression test.