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Please comment on these Pistons (Photo)

M

Mz3bo

New member
Hello,

I have a 1979 Johnny 85 HP. The engine is running fine except à high idle event at 4-6deg and everything liked and synced. However, I have on piston showing 90psi while all other 3 are showing 120psi. Today, I decided to open the top portion and took some pictures. I am not engine expert but willing to learn. The top piston shows some marks as well as the head.

My questions:

1- can I change only 1 piston ?
2- What about the mark on the head ?
3- If engine is running strong, can I live with it like that ?
4- Do you think the low compression is because of the mark on the piston or something else ? I can wiggle the piston 1/32 left and right
5- Those thiny hair lines on the walls, are they ok?

6- Any other opinions are greatly appreciated.


https://goo.gl/photos/DjaPzcLsjWdxEbJs8
 
Ring failure...common issue on these V4 engines. 1979 was year pistons had the high ring groove style and if this motor has them you will need to buy 4 pistons...
 
​Opinions vary on replacing one piston. However, when it involves a new, improved design, it doesn't make sense to put the old ones back in (and wait for another one to break)
 
Ok Thanks for the reply... but by the pictures, you think it's broken ring ? Caus the pistons slided up and down nicely... it's just like the piston top is chipped as well as the head...

Bo
 
Mz3bo said:
Hello, I have a 1979 Johnny 85 HP. The engine is running fine except à high idle event at 4-6deg and everything liked and synced. However, I have on piston showing 90psi while all other 3 are showing 120psi. Today, I decided to open the top portion and took some pictures. I am not engine expert but willing to learn. The top piston shows some marks as well as the head. My questions:

1- can I change only 1 piston?.... 2- What about the mark on the head? .... 3- If engine is running strong, can I live with it like that? .... 4- Do you think the low compression is because of the mark on the piston or something else? 5- Those thiny hair lines on the walls, are they ok? 6- Any other opinions are greatly appreciated.
https://goo.gl/photos/DjaPzcLsjWdxEbJs8
Click to expand...

1 = No.... 2 = The heads are shot!.... 3 = That engine can never run strong as it is. 4 = Compression is bad due to blown engine. 5 = Those hair lines mean nothing now.

6 = That engine is a 1979 model, going on 38 years old. The overhaul cost will no doubt hit between $4000 to $5000 and obviously it's not worth that. Check it up to experience and get another (good) engine.
 
Mz3bo said:
Ok Thanks for the reply... but by the pictures, you think it's broken ring ? Caus the pistons slided up and down nicely... it's just like the piston top is chipped as well as the head...

Bo
Click to expand...
Definitely a munched ring. The notches on the side of the piston is where the ring was caught between the edge of the port and the piston, denting the aluminum piston.. The shot peened divots on the head are where the pieces of ring bounced around and squished against the aluminum before they made there way out the exhaust port.

As JoeReeves stated, that's a really old motor that needs a total rebuild. Unless you have a specific emotional attachment to that motor it's best to shop around for a newer 85HP. If you want to stay simple look for an 88SPL from the '90's. That was basically a 90HP motor without VRO. Also check your hull for max rated HP. If the 85HP was under your rating now might be the time to upgrade to a few more ponies.

KJ
 
Have seen lots of those V-4 motors from the 1990's with broken rings too.-------If ( a big if ) the owner can do his own work about $1000 on parts may well make the 79 model run like new again.
 
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