Recent engine rebuild with questions about compression

[m_steiger]

I recently rebuilt my 1995 Mercruiser 5.7......Only about 5 hours total on the engine. Just yesterday went to the lake and motor was hydrolocked. The exhaust manifolds finally gave out after being cracked and JB welded. I have new ones on the way ordered from here. While I has the manifolds off I decided to do a compression test just to see where I was at after the rebuild. I used chrome-moly rings for the rebuild which I have since learned is not a good idea on marine engines, as I should have used standard iron rings. Since the rebuild I have been having very excessive blow-by, to the point where it was pressurizing the crankcase and forcing the oil to some up and drip out of the dipstick tube. I took one side of the valve cover breather off and felt the pressure release. I was thinking I had a cylinder or 2 which the rings weren't completely sealing due to the chrome-moly rings. Now that I have done a compression test I have found all my cylinders to be between 175psi at the low cylinder and 215psi at the highest cylinder. I had not used any oil in the cylinders when taking this test and it was done using a quality Snap-on gauge. Ive always heard you should have all cylinders within 10% compression of each other however I am at 20% difference between my high and low side. Do you guys think I have anything to be alarmed about or give it more time for the low cylinders piston rings to seat?

 

[Bondo]

Ayuh,..... Did the motor Run, 'tween the hydro-lock, 'n yer compression test,..??

 

[m_steiger]

Yes the motor ran great, I had it running on the water hose tuesday of this week actually, Thats why I was pretty stunned to find out come 2 days later when I took it to the lake to find out that it had hydro-locked.

 

[m_steiger]

It has been running great, besides the blow-by issue I should add.

 

[bfjou812]

If there is only about 5 hrs. on it , I wouldn't worry about it ,I'd change the oil and filter and run it awhile longer then go back and check the compression. The highest and lowest weren't adjacent cylinders were they?

 

[Bondo]

Yes the motor ran great, I had it running on the water hose tuesday of this week actually, Thats why I was pretty stunned to find out come 2 days later when I took it to the lake to find out that it had hydro-locked.

Ayuh,.... This statement says it ran tuesday, ya went boatin' thursday, it was hydro-locked, then ya did the compression test,..??

So the motor Hasn't been run, Since the hydro-lock, 'n before the compression test,..??

 

[ricohman]

About the compression. Was the block bored oversize and then fitted with the appropriate sized pistons (rebuilt)?
Or was it deglazed and the rings replaced?
If just the rings were replaced, then you have put round rings into not very round holes. Yes you will have blow by and yes the compression can vary.

 

[m_steiger]

I had the motor running Tuesday, and then shut it off and put the boat back in the garage believing everything would be ready for the lake. Then Thursday morning I haul it to the local lake and it was already hydro locked by this point. I backed it into the water and went to hit the starter and it was a solid clunk. I could tell it was hydro-locked, so I had to bring the boat back home, removed the exhaust manifolds and spark plugs, and cleared the cylinders of water. After the cylinders were clear of water I did the compression test. Ricohman, That could very well be the issue. The motor is already bored over .030.......When I rebuilt I just had the machine shop hone the cylinders and installed new .030 over rings.

 

[Walt T]

Did I read right... You spent all that time and money rebuilding a motor and then used old manifolds and risers?

 

[m_steiger]

Yes I did. My rebuild budget was tight, and considering everything I've got done, I think I did pretty well. Sometimes sacrifices have to be made, and skipping out on replacing manifolds at the time were a decision I made knowing all to well they would need to be replaced eventually. That time finally came.

 

[m_steiger]

UPDATE: Got the new manifolds installed and done a driveway water test. The ole 5.7 is running great. I will be taking it out on Taylorsville Lake tomorrow for a full test.

 

[NHGuy]

Fingers crossed. Wishing you smooth running and long wakes.