After finding the motor having low compression of 115psi and the tear down showing scored pistons and cylinders. I rebuilt the top end with .25 over cylinders, new .25 pistons and rings. I checked compression before running getting 120psi in both. I ran on the hose for 5 mins after cool down I got 120 in both again. Is the compression going to jump up to the 150 range after some real running or should I be looking for something else wrong with the engine?
2004 Sea Doo GTI 720 rebuild compression
- Thread starter TOMZILLA
- Start date
I'd say you got a bum compression tester. When I install WSM pistons and rings new, with fresh bores the ring gap is at the top of spec and sometimes over. I've seen (.021") and had no compression issues. I never see less than (.018") on the end gap. I have not had a problem and I doubt that a few thousandths is gonna drop your compression 30psi.
I've had scoured cylinder walls and broken/missing ring parts and still get 130psi, which is why when I see 130psi on a compression test I know the engine has wear. I'm old school 2-stroke. Keep the top end happy and the rest of the engine will remain happy. The metal from worn parts is gonna get into the crank bearings at some point.
You may have a bore size that is at the top of the limit but as long as it is in spec it will work fine. Good Luck !!
I've had scoured cylinder walls and broken/missing ring parts and still get 130psi, which is why when I see 130psi on a compression test I know the engine has wear. I'm old school 2-stroke. Keep the top end happy and the rest of the engine will remain happy. The metal from worn parts is gonna get into the crank bearings at some point.
You may have a bore size that is at the top of the limit but as long as it is in spec it will work fine. Good Luck !!
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Are those cast or forged pistons?
I don't like doing extra work but I'd like to try compression with no rings at all on a new bore. I think it wouldn't be far from 120psi. I'd mic the cylinder bores and pistons.
That ring gap alone isn't causing all that compression loss. You stated you had scoured pistons and cylinders and had 115psi.
That ring gap alone isn't causing all that compression loss. You stated you had scoured pistons and cylinders and had 115psi.
Stevie96xp
Active Member
I had a 720 ski that had a brand new .25 over top end and my compression tester said 120 psi ski ran great had full speed and started with no throttle In the water. So I just chalked it up to my cheap compression tester. But it reads 150 on my other skis so I don’t know 
I had a 720 ski that had a brand new .25 over top end and my compression tester said 120 psi ski ran great had full speed and started with no throttle In the water. So I just chalked it up to my cheap compression tester. But it reads 150 on my other skis so I don’t know
Engines can't read. LOL Never know
Stevie96xp
Active Member
lol
If you can find some good standard size or 1 or 2 sizes over cylinders I would just order WSM platinum pistons from OSD Marine and find a local machine shop to do your boring and honing if you can.
OSD SEA DOO 717 TOP END KIT (PISTONS/PIN BEARINGS/GASKET KIT) [OSD3706] - $199.99
OSD SEA DOO 717 TOP END KIT (PISTONS/PIN BEARINGS/GASKET KIT) [OSD3706] - $199.99
Stevie96xp
Active Member
Some people don’t like sbt but I like them
I’m not sure but I thought factory cylinders were long gone. I would buy a good set of used cylinders if you can get a set at a reasonable price and have them bored out. I agree, I would not buy a set that says bored. You could also look up Peter Rucinski in Illinois. He does real good work, you could trust him.
I may have an extra set of 720 cylinders but they'll need to be bored. Reboring is the safest thing that way you know what you have. I have (3) 720's to build and at least one 787 and all need cylinders bored. I never care what size I need to go to as long as the bore cleans up but I have never gone over (1mm). I don't buy the coated pistons from WSM, just the regular ones.
I find it strange they vendor is taking back the cylinders based on ring "end gap." When I bore cylinders I don't like going (.25mm) over. Not enough metal to remove if the bore is scoured or even out of round. I go (.50mm).
I find it strange they vendor is taking back the cylinders based on ring "end gap." When I bore cylinders I don't like going (.25mm) over. Not enough metal to remove if the bore is scoured or even out of round. I go (.50mm).
