Jonsered 820 need your help

[JohnT34]

I got this jonsered I am rebuilding it is in really clean and original condition
I compression tested it at 160 psi which I felt was low as my 920 has 180 psi
So I pulled the exhaust and there was some minor scoring on the piston
Pressure and vac tested it with no leaks detected
I pulled the cylinder to properly assess, the piston and cylinder both have visible marks as per the pics but both feel completely smooth
Anyone have any ideas what may have caused this?

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[brad ruch]

I got this jonsered I am rebuilding it is in really clean and original condition
I compression tested it at 160 psi which I felt was low as my 920 has 180 psi
So I pulled the exhaust and there was some minor scoring on the piston
Pressure and vac tested it with no leaks detected
I pulled the cylinder to properly assess, the piston and cylinder both have visible marks as per the pics but both feel completely smooth
Anyone have any ideas what may have caused this?View attachment 770349

View attachment 770348


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Some dirt or dust got threw the air filter.clean the piston with 600 grit and cylinder with some scotch Brite or little 320 grit.i use flex hone 320 works great.

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[JohnT34]

An update on this 820 cylinder
The piston is actually a golf piston so is a replacement of an unknown failure at some point in its life
The marks on the piston are actually from an incomplete clean up of the cylinder post whatever failure occurred
There are 2 distinct wear patterns on both sides of the piston that indicate to me that wrist bearing is worn. Confirmed by checking the play against a new 281xp wrist bearing which is a match
Next steps will be clean up piston with scotch brite pad and ball hone cylinder with 320 hone
Replace wrist bearing and new caber rings and reassemble


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[JohnT34]

Update jonsered 820 all back together with new rings and wrist bearing
Beautiful saw even if not quite as good as the 920


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