'84 Johnson 70 Damaged Piston (Pics)

[Socak]

I have a 1984 Johnson 70 that was reading a bit low (125, 120, 95) in the lower cylinder so I pulled the cylinder head cover to figure out what might be the problem. The 3rd piston, which had low compression, is slightly damaged, and all the pistons were fairly dirty. The cylinders themselves appear to be in fairly good shape, no scuffs or abrasions. What is the best mode of attack to fix this, just take the piston out and replace it and how should the pistons be cleaned? The carbs appeared to be in good shape, but they are being taken off and cleaned. Also I have ordered a omc manual to help

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[Capt Ken]

Re: '84 Johnson 70 Damaged Piston (Pics)

You are going to have to replace the piston. The indentions in the top is from pieces of its rings that have broken.

 

[dodgeboy1979]

Re: '84 Johnson 70 Damaged Piston (Pics)

Replace all three at the same time, a machine shop can hone the cylinder walls for ya before you put the new ones in.

 

[Socak]

Re: '84 Johnson 70 Damaged Piston (Pics)

You are going to have to replace the piston. The indentions in the top is from pieces of its rings that have broken.

Ok thats what I figured. The hole in the center of the pistons is normal correct? Do the other two pistons look salvageable after a major cleaning?

 

[boobie]

Re: '84 Johnson 70 Damaged Piston (Pics)

I would pull the mtr apart and check the other pistons for any scuffing. If they are okay just put new rings on them. You may have to have the one cyl with the broken ring bored out to oversize if there are gruves in it. You may not need all new pistons. Good job on getting the OMC manual. Lots of good info in it as far as rebuilds.

 

[w.ross]

Re: '84 Johnson 70 Damaged Piston (Pics)

Ok thats what I figured. The hole in the center of the pistons is normal correct? Do the other two pistons look salvageable after a major cleaning?

Are these holes towards the center normal?

Pulled my plugs out today to fog the motor for the winter and noticed the same thing...

Curious.

Thanks guys,

 

[durban]

Re: '84 Johnson 70 Damaged Piston (Pics)

i have striped 3 70hp with worse pitting on top of the piston & in the top of the cylinder head too , this is not caused buy broken pieces of rings as they where all still intack & in good shape after striping , this is caused buy water getting into the port or through cylinder head & spark & water is a conductor of electric .

 

[Socak]

Re: '84 Johnson 70 Damaged Piston (Pics)

i have striped 3 70hp with worse pitting on top of the piston & in the top of the cylinder head too , this is not caused buy broken pieces of rings as they where all still intack & in good shape after striping , this is caused buy water getting into the port or through cylinder head & spark & water is a conductor of electric .

Mine was cause by a blown skirt on the bottom piston.

 

[Chinewalker]

Re: '84 Johnson 70 Damaged Piston (Pics)

Yes, the hole is normal. It's a center point for machining the pistons...

Are these holes towards the center normal?

Pulled my plugs out today to fog the motor for the winter and noticed the same thing...

Curious.

Thanks guys,

 

[Chinewalker]

Re: '84 Johnson 70 Damaged Piston (Pics)

i have striped 3 70hp with worse pitting on top of the piston & in the top of the cylinder head too , this is not caused buy broken pieces of rings as they where all still intack & in good shape after striping , this is caused buy water getting into the port or through cylinder head & spark & water is a conductor of electric .

The sort of damage in the photos is mechanically done by someTHING bouncing around inside the combustion chamber, in this case a piece of the piston skirt. Water doesn't bang into a piston and leave a mark...

 

[w.ross]

Re: '84 Johnson 70 Damaged Piston (Pics)

Yes, the hole is normal. It's a center point for machining the pistons...

Thanks for the information, that's what I needed to know.

 

[durban]

Re: '84 Johnson 70 Damaged Piston (Pics)

The sort of damage in the photos is mechanically done by someTHING bouncing around inside the combustion chamber, in this case a piece of the piston skirt. Water doesn't bang into a piston and leave a mark...

show me the scars in the sleeve then i will be-leave you .

 

[Faztbullet]

Re: '84 Johnson 70 Damaged Piston (Pics)

show me the scars in the sleeve then i will be-leave you .

If water was entering cylinder the piston tops would be steamed cleaned and display no carbon, the electrodes would have a slight rust film on them when removed and set to dry and excessive water would hydro-lock the engine as water will not compress. It will also wash the cylinders of lube and scuff the sides.... and a spark plug will not fire in water or steam.

 

[Faztbullet]

Re: '84 Johnson 70 Damaged Piston (Pics)

show me the scars in the sleeve then i will be-leave you .

If water was entering cylinder the piston tops would be steamed cleaned and display no carbon, the electrodes would have a slight rust film on them when removed and set to dry and excessive water would hydro-lock the engine as water will not compress. It will also wash the cylinders of lube and scuff the sides.... and a spark plug will not fire in water or steam. Below is blow up of piston...

 

[Socak]

Re: '84 Johnson 70 Damaged Piston (Pics)

Sleeves were scarred on all three cylinders, one skirt was blown (seems to be the route of the problem), and heavy carbon build up on all pistons.

 

[V153]

Re: '84 Johnson 70 Damaged Piston (Pics)

If water was entering cylinder the piston tops would be steamed cleaned and display no carbon, the electrodes would have a slight rust film on them when removed and set to dry and excessive water would hydro-lock the engine as water will not compress. It will also wash the cylinders of lube and scuff the sides.... and a spark plug will not fire in water or steam. Below is blow up of piston...

View attachment 184398

Agreed. Looks more like a wrist pin bearing failure to me. Or connecting rod bearing, I've had one let go'n cause similar damage.