How often does leak down reveal something that compression doesn't?

**pcrussell50** · 08-23-2019, 10:47 AM

Pro engine builders....

I know what a compression test is: I do them often
I know what a leak down test is: I don't to them often
I know what the difference is.

But I'm still fairly new to two stroke outboarding. For you guys who build engines for a living...

How often do you find a bad motor with good compression, that you have to perform a leak down test to find out? Half the time? A quarter of the time? Only one in ten?

-Peter

**eli** · 08-23-2019, 12:12 PM

First thing I do before any work is done is leak down ,will tell the whole story of health of rings/and or ring seal ,compression to me gives false findings of what I need to know except for timing and fuel type that should be used,I have had many motors that had 100 110 120 psi compression merc and Omc that failed leakdown 8 10 12 %or higher you can have scoring below port above port ,cracked ring ,out of round tapered cylinders,compression gage holds air from every stroke so if you spin it long enough you might think motor is ok especially with oil on the cylinders ,general joe who don’t know dick will do it this way and sell it as perfect compression when it’s tired or junk .

**pcrussell50** · 08-23-2019, 01:09 PM

Originally Posted by eli

First thing I do before any work is done is leak down ,will tell the whole story of health of rings/and or ring seal ,compression to me gives false findings of what I need to know except for timing and fuel type that should be used,I have had many motors that had 100 110 120 psi compression merc and Omc that failed leakdown 8 10 12 %or higher you can have scoring below port above port ,cracked ring ,out of round tapered cylinders,compression gage holds air from every stroke so if you spin it long enough you might think motor is ok especially with oil on the cylinders ,general joe who don’t know dick will do it this way and sell it as perfect compression when it’s tired or junk .

Great info. Love to hear it. But getting back to my question... What percentage of the time do you find good compression, but bad leak down?

-Peter

**nitro_rat** · 08-23-2019, 04:16 PM

Fairly often. I have seen one cylinder down 7 psi with 40% leakdown. I only use compression test on 2 strokes to be able to say it has "x" cranking compression because that's the number everyone wants to hear...

**RSWORDS** · 08-23-2019, 06:17 PM

We do compression tests on just about everything that comes through the shop. Mostly for our records and also as a quick check for major issues. If it's slow to come up (cranks forever to get to spec) or has a 5% spread or more I'll do a leak down.

If it comes in with a running issue it gets compression and leakdown before tearing into what's going on. Even if it's a fuel or ignition issue we do it. No point at throwing money at a worn out engine.

**flabum1017** · 08-23-2019, 08:15 PM

with the "slow coming up" if you got a cylinder that takes 20 revolutions to come up close to the others that take 6 revolutions.... that cylinder has a problem. I come across that, I stop the compression test and make a call to the customer.

IMO, at that point a leakdown is a waste of time and if it's a four stroke, I'll squirt some oil in that cylinder to see if it's the rings or valves.

**RSWORDS** · 08-23-2019, 08:38 PM

Originally Posted by flabum1017

with the "slow coming up" if you got a cylinder that takes 20 revolutions to come up close to the others that take 6 revolutions.... that cylinder has a problem. I come across that, I stop the compression test and make a call to the customer.

IMO, at that point a leakdown is a waste of time and if it's a four stroke, I'll squirt some oil in that cylinder to see if it's the rings or valves.

Yeah 20 revolutions I wouldnt bother with a leakdown. Lol I always use the first three "pumps" and compare cyl to cyl.

**Motv18** · 08-23-2019, 10:13 PM

Leak down checks pressure holding ability, or power stroke, a lot of leak down at 100psi is exponentially more loss at combustion pressure upwards of 10k psi. (That’s a et grata number I forget the real average peak psi).

I find leakdows bad on many that make spec compression. Pined rings in a 2s wear faster then rings that rotate in a 4s. Also theirs no maze lock (gaps in same spot)

what you occasionally see is debates over x% being a bad leakdown on a 2s vs definitely bad on a 4s

with bore cameras today leakdows are becoming rare unless a blowby condition or other symptoms of low running compression are present since you can inspect the bore fore scuffs and such.

**flabum1017** · 08-23-2019, 10:26 PM

Originally Posted by Motv18

Leak down checks pressure holding ability, or power stroke, a lot of leak down at 100psi is exponentially more loss at combustion pressure upwards of 10k psi. (That’s a et grata number I forget the real average peak psi).

I find leakdows bad on many that make spec compression. Pined rings in a 2s wear faster then rings that rotate in a 4s. Also theirs no maze lock (gaps in same spot)

what you occasionally see is debates over x% being a bad leakdown on a 2s vs definitely bad on a 4s

with bore cameras today leakdows are becoming rare unless a blowby condition or other symptoms of low running compression are present since you can inspect the bore fore scuffs and such.

boats run salt water down here, you get cylinder on a 4 stroke that's slow to come up, that's automatic valve job and exhaust elbows. On a two stroke, usually means a lean condition or eggshaped cylinders from overheat.

Salt water is cruel on motors down here.

**pcrussell50** · 08-24-2019, 02:15 PM

So this has me convinced to leak down test my otherwise pretty good running 1987 150hp 2.0l. In compression, it's 120 all around except the top two, which are 110. I read in another thread that sometimes people just replace or re ring a piston or two instead of a full rebuild... If I find that I have one or two with too much leak down, is there anything wrong with re-pistoning them? This is for a non-race-only, fish/recreation motor.

-Peter

**Greg G** · 08-24-2019, 02:33 PM

As everyone has said, leak down is your real friend and cheap to do. You can do "partial" work to that little fella and feel fine about that approach.

**eli** · 08-24-2019, 05:40 PM

On my 245’s before I had the nic done I just wanted to get thru the summer I would pull a head and take off carb intake and un fasten rod bolts for that cylinder push out rod piston put a new moly ring on top put it back in ,the trick is to get the rod cap back and lined up ,put it back together and run it till end of year then tear it down, but I only put 10 hrs a year

**gfinch** · 08-24-2019, 06:52 PM

Years ago I wanted to know how much leak down at different places in the cylinder. I took 3 pieces of aluminum, center one I rough cut teeth in to sorta match the flywheel ring gear and it was also a little thinner then the ring gear. 2 bolts thru all pieces. This made a tool that clamped to the flywheel and when butted against the starter stopped the crankshaft from turning when air was applied to the leak down tester. Forgot exact leakage numbers somewhere about (Pistons were top pinned) 7% to 10% TDC and 30% before the exhaust port was uncovered. Cylinder walls were very scored.

**racer** · 08-24-2019, 07:24 PM

I agree with Gary doing a leak down at only TDC does not always tell the whole story.

**baja200merk** · 08-24-2019, 10:25 PM

There’s gotta be a spot in the bore that gets hammered harder than the rest of it. I’ve got a bunch of engines with carbon and wear in the bore by the pins and some .003-4 out in random spots. They are really easy to see after u run a 4 stone through it.