Guys, if you want to see the real interesting (goofy) way Mercruiser recommends doing a compression test look at Mercruiser Gasoline Engine Sevice Bulletin 97-25 (Merc. service bulletins are online). They recommend that the lowest cylinder can be within 70% of the highest cylinder. If the highest is 150 the lowest can be 105.
I am afraid that just wouldn't be good enough for most people!
Sincerely
Dennis Moore
FAMILY AND PERFORMANCE BOATING MAGAZINE

Dennis,
Good to see you back!!
now get back to work on the Mag so I have some good reading in the hot tub this winter:D :D :D

When my 502 scored a piston due to running lean the compression on that cylinder was 140 lbs. The rest were between 153 and 160. The leakdown test showed 50% leakage! You just can't beat a leakdown test when you want to be sure.

A compression test is much easier to do than a leakdown test so I always do that first. I would say that 5% or less difference between cylinders indicates a healthy motor. Anything more than that and I will do a leakdown test on the low cylinders.

Mac Tools has a dual guage (the best type) aircraft quality leakdown tester for $109.99. You can get them even cheaper than that if you look around.

checkmate454mag, Thanks for the info! I did a search on the web and found some instructions, and it looks very easy. If you rotate the crank with a wrench you can even check the condition of the whole cylinder. Very helpfull!
Dennis, I looked at that service bulletin and you are right, it isn't very good info.

Ok, I'm sold. I am going to buy one. One question.
some testers have 2 gauges, and I looks like it is just to monitor the input air pressure and not really needed if your have one on your air hose or compressor. Is this correct?

Thanks

28Eliminator;

You are right. The testers with 2 guages use 1 guage to monitor and regulate line pressure, while the other measures leakdown percentage. If you have a guage on your compressor or the line, you really don't need the 2 guage tester. BUT, personaly, I like the 2 guage tester better.

Dennis can you pls send me a link to bulletins.
thnx

You need to have an actual leakdown tester as it has a regulator on it to control the pressure at the tester.