300/440/550 89 550 lost compression!

[87sheerips]

Well guys I have been riding the piss out of my JS 550 and something finally let go. It has 145psi in the forward-most cylinder, and 30 in the other. I took the head of yesterday to find everything in great shape. Head domes look new, pistons look good, head gasket looks good.

However, the cylinder with 30psi has some cylinder wall scoring. Also, when the motor died on me in the water, I inspected when i got it back on the lift to find all the head bolts loose with 3 of them in the bottom of the hall.

I torqued everything down again and found the same results. 145psi and 30psi. I want to find out what had happened here as this top end is only a couple years old I believe.

Looks like I have to put a top end in her but what do u guys think happened here?

 

[ger87410]

Your cylinder's toast.

Get a new cylinder and piston combo.

 

[rollin5150]

pics will help diagnose what happened to the rear cylinder

 

[87sheerips]

Your cylinder's toast.

Get a new cylinder and piston combo.

why get a new cylinder? Cant I bore and hone with over-sized pistons/rings?


I don't even know if pics will do justice, the only thing I see is a couple scuff marks on the cylinder walls.

 

[ger87410]

I think U could. I normally take the route of keeping stock.

1 gouge is all it will take to lose compression.

 

[87sheerips]

Sounds expensive lol... Where can u find cylinder piston combos? I heard the WSM pistons are just fine? I'm all for some knowledge here

I wonder what could have caused the scoring? The tops of the pistons and combustion chamber look great... The head popping off has to be part of this, still dont understand root cause tho

 

[ger87410]

Sounds expensive lol... Where can u find cylinder piston combos? I heard the WSM pistons are just fine? I'm all for some knowledge here

Dunno. I got mine from AssJoel.

I wonder what could have caused the scoring? The tops of the pistons and combustion chamber look great... The head popping off has to be part of this, still dont understand root cause tho

U need to find that out to keep it from happening again. Your engine got hungry and ate something. Look in the cases real good for debris. The head popping off could be a result of the ingestion. Look at your intake as well for any lose or missing screws.

 

[87sheerips]

Dunno. I got mine from AssJoel.

U need to find that out to keep it from happening again. Your engine got hungry and ate something. Look in the cases real good for debris. The head popping off could be a result of the ingestion. Look at your intake as well for any lose or missing screws.

Yep, it just quit on me. No destruction on the inside besides the scoring. Might be some sand or something.

 

[87sheerips]

Well, I got a complete cylinder with pistons, used, 150psi in both, for $180 shipped. Ontop of a nice crank for $125 I should have both of my 550's back in action soon. Thx for the input fellas Im sure Ill be back with more questions soon. :icon16:

 

[87sheerips]

Making progress, these little motors are awesome! I have 3 more sets of cases Im going to try and fill because Im having a blast building this one. I have two sets of cylinders/pistons that need bored. Anyone have a good source for oversized pistons?