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Author Topic:   Compression Testing
skookum point posted 05-29-2002 01:51 PM ET (US)   Profile for skookum point   Send Email to skookum point  
I'm looking at used 60/70hp 3 cylinder OMC (2 stroke) motors. When doing a compression test, what kind of numbers indicate good/fair/poor condition?
JBCornwell posted 05-29-2002 04:50 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Howdy, SP.

Forget Fair and Poor.

All cylinders over 100psi and within 10% of each other is good. Smaller differences between cylinders is better. Anything else is bad.

Red sky at night. . .
JB :)

skookum point posted 05-30-2002 10:23 AM ET (US)     Profile for skookum point  Send Email to skookum point     
Thanks JB
I have looked at 4 different used OMC 60hp this week. Every one of them was a Lake Powell houseboat take-off. They are installed in pairs - when one dies the other is for sale. All appear neglected and abused, probably high hours too.
Tom W Clark posted 05-30-2002 11:29 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
The compression numbers themselves mean little. It is the variation between the cylinders that matters. Generally as long as they are within 25% of one another they are considered normal, but the closer the better.
jimh posted 05-30-2002 02:50 PM ET (US)     Profile for jimh  Send Email to jimh     
If the houseboat chartering on Lake Powell is anything like most of the other houseboat operations I have seen, you probably ought to look at those engines with a jaundiced eye.

For some reason, people who have virtually no boating experience feel perfectly at ease in taking command of a houseboat. Since most of the houseboats are underpowered and have very substantial windage, they are quite difficult to control. Combine the inexperienced pilot with a bulky boat, and you have a forumula for lots of engine abuse like frantic shifting, lugging, poor fuel mixtures, etc.

If they are bargain priced, the engines might be attractive, but I would think they've probably been through some hell.

skookum point posted 05-30-2002 06:18 PM ET (US)     Profile for skookum point  Send Email to skookum point     
jimh -
my thoughts exactly, thanks
Bigshot posted 06-03-2002 03:45 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Never buy anything that is an ex-rental. I bought a jetski once.....once!

Like they say above....anything over 100 and even is good. Reason being I could test that engine with my tester and get 130, 125, 132. That is a great test. You could then use YOUR tester and get 110, 102, 112.

andygere posted 06-05-2002 04:21 PM ET (US)     Profile for andygere  Send Email to andygere     
Test each cylinder a few times and take the average, or throw out any really high or really low numbers. If you are using the type of guage with a tapered rubber inlet, it can sometimes be difficult to get a good seal. The idea is to make sure you have enough data to make a good evaluation.
Bigshot posted 06-06-2002 03:42 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Good point...if you do not have a tester that screws into the port, junk it. Those rubber ones are as useless as......on a bull.
skookum point posted 06-07-2002 01:59 AM ET (US)     Profile for skookum point  Send Email to skookum point     
Thanks for all the helpful feedback! I'll get a screw-on to replace the rubber tipped one.
tlynch posted 06-10-2002 12:05 AM ET (US)     Profile for tlynch  Send Email to tlynch     
Anyone have an online source for a compression tester that they recommend? What size adapters are needed for marine engines? 14mm? 18mm?

Todd Lynch

Bigshot posted 06-10-2002 09:54 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Any autoparts store like Pepboys, etc. They have a decent kit with an adapter that fits most plug holes, like $22-25.

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